Table of contents

We show that magnetic susceptibility can reveal spin entanglement between individual constituents of a solid, while magnetization describes their local properties. We then show that magnetization and its variance (equivalent to magnetic susceptibility for a wide class of systems) satisfy complementary relation in the quantum-mechanical sense. It describes sharing of (quantum) information in the solid between spin entanglement and local properties of its individual constituents. Magnetic susceptibility is shown to be a macroscopic (thermodynamical) spin entanglement witness that can be applied without complete knowledge of the specific model (Hamiltonian) of the solid.

We report on first principles self-interaction corrected LSD (SIC-LSD) calculations of electronic structure of LaMnO₃ in the cubic phase. We found a strong tendency to localization of the Mn e_g electron and to orbital ordering. We found the ground state to be orbitally ordered with a staggered order of x² − z² and y² − z² orbits in one plane and this order is repeated along the third direction. The difference in energy with a solution consisting of the ordering of 3x² − r² and 3y² − r² is, however, very small. The system is in the insulating A-type antiferromagnetic ordered state in both cases. The presence of orbital ordering means breaking of the cubic symmetry and without recourse to distortion. The latter may rather be the result of the orbital ordering but the symmetry of this ordering is determined by coupling to the lattice. The strong tendency to localization of the e_g electron in LaMnO₃ accounts for the survival of local distortions above the structural phase transition temperature.

We point out that for an arbitrary number of identical particles, each defined on a Hilbert space of arbitrary dimension, there exists a whole ladder of relations of complementarity between certain local and nonlocal measurements corresponding to every conceivable grouping of the particles, e.g., the more accurately we can know (by a measurement) some joint property of three qubits (projecting the state onto a tripartite-entangled state), the less accurate some other property, local to the three qubits, becomes. We investigate the relation between these complementarity relations and a similar relation based on interference visibilities. We also show that the complementarity relations are particularly tight for particles defined on prime dimensional Hilbert spaces.

Recently, the concept of superlensing has received considerable attention for its unique ability to produce images below the diffraction limit. The theoretical study has predicted a 'superlens' made of materials with negative permittivity and/or permeability, is capable of resolving features much smaller than the working wavelength and a near-perfect image can be obtained through the restoration of lost evanescent waves (Pendry 2000 Phys. Rev. Lett.85 3966–9). We have already demonstrated that a 60 nm half-pitch object can indeed be resolved with λ₀/6 resolution with the implementation of a silver superlens with λ₀ = 365 nm illumination wavelength, which is well below the diffraction limit (Fang et al 2005 Science308 534–7). In order to further support the imaging ability of our silver superlens, a two-dimensional arbitrary object with 40 nm line width was also imaged (Fang et al 2005 Science308 534–7). In this paper, we present experimental and theoretical investigations of optical superlensing through a thin silver slab. Experimental design and procedures as well as theoretical studies are presented in detail. In addition, a new superlens imaging result is presented which shows the image of a 50 nm half-pitch object at λ₀/7 resolution.

We show that the quantum order parameters (QOPs) associated with the transitions between a normal conductor and a superconductor in the BCS and η-pairing models and between a Mott-insulator and a superfluid in theBose–Hubbard model are directly related to the amount of entanglement existent in the ground state of each system. This connection gives a physical meaningful interpretation to these QOP, which shows the intrinsically quantum nature of the phase transitions considered.

Neutron production capabilities of table-top laser accelerated protons based on data from a high-energy single-shot large laser are addressed. Recently, McKenna et al (2005 Phys. Rev. Lett.94 084801) have analysed the energy spectrum for a beam of protons accelerated on the VULCAN laser. In this paper, we present a new analysis of the same experiment which demonstrates, for the first time, production levels in excess of 10⁹ neutrons per laser shot within a nanosecond pulse through (p,xn) reactions on lead targets. We have used this ^natPb(p,xn) conversion analysis approach to make predictions on the neutron production capabilities of table-top laser systems. Neutron spectra for current state-of-the-art table-top lasers have been calculated, and we have estimated that current systems are capable of producing 10⁶ neutrons per second in nanosecond pulses. Furthermore, we have found that nanosecond neutron pulses at a rate of 5 × 10⁹ neutrons per second are possible with the next generation of table-top lasers currently under construction.

We study effects of different network topologies on the noise-induced pattern formation in a two-dimensional model of excitable media with FitzHugh–Nagumo local dynamics. In particular, we show that the introduction of long-range couplings induces decoherence of otherwise coherent noise-induced spatial patterns that can be observed by regular connectivity of spatial units. Importantly, already a small fraction of long-range couplings is sufficient to destroy coherent pattern formation. We argue that the small-world network topology destroys spatial order due to the lack of a precise internal spatial scale, which by regular connectivity is given by the coupling constant and the noise robust excursion time that is characteristic for the local dynamics. Additionally, the importance of spatially versus temporally ordered neural network functioning is discussed.

We analyse the quantum dynamics of a nanomechanical resonator coupled to a normal-state single-electron transistor (SET). Starting from a microscopic description of the system, we derive a master equation for the SET island charge and resonator which is valid in the limit of weak electromechanical coupling. Using this master equation we show that, apart from brief transients, the resonator always behaves like a damped harmonic oscillator with a shifted frequency and relaxes into a thermal-like steady state. Although the behaviour remains qualitatively the same, we find that the magnitude of the resonator damping rate and frequency shift depend very sensitively on the relative magnitudes of the resonator period and the electron tunnelling time. Maximum damping occurs when the electrical and mechanical timescales are the same, but the frequency shift is greatest when the resonator moves much more slowly than the island charge. We then derive reduced master equations which describe just the resonator dynamics. By making slightly different approximations, we obtain two different reduced master equations for the resonator. Apart from minor differences, the two reduced master equations give rise to a consistent picture of the resonator dynamics which matches that obtained from the master equation including the SET island charge.

The resonant transmission of microwaves polarized perpendicular to a single subwavelength slit of finite length is presented in detail. It is shown that the resonant frequency rises monotonically as slit length is reduced. Increasing confinement of the resonant fields within the slit is shown to cause the frequency rise. Angle dependence of the transmission is also presented. The results show clearly Fabry-Perot-like standing waves in the direction of propagation with waveguide mode behaviour in the orthogonal direction.

Using terahertz (THz) time-domain techniques, we measure the relative coupling and scattering characteristics of surface plasmon-polariton waves for rectangular cross-section grooves on a thick metal foil. This is accomplished by using a single subwavelength aperture surrounded by one or more annular grooves. A unique aspect of the measurement technique is that it allows one to discriminate between the contributions of each groove in the transmitted THz waveform. The measurements are obtained by using structures that contain either a single annular groove, for coupling measurements, or two concentric annular grooves, for scattering measurements. The basic approach should be extendable to other spectral ranges and applicable to different forms of surface electromagnetic waves. Using the aforementioned measurements, we demonstrate the ability to alter the pulse shape of the transmitted waveform using a single aperture surrounded by four annular grooves of varying dimensions. In addition to the value of this capability for pulse shaping applications, the basic technique allows for greater control of the transmission resonance lineshape related to enhanced transmission through a single subwavelength aperture.

Phonons in expanding Bose–Einstein condensates with wavelengths much larger than the healing length behave in the same way as quantum fields within a universe undergoing an accelerated expansion. This analogy facilitates the application of many tools and concepts known from general relativity (such as horizons) and the prediction of the corresponding effects such as the freezing of modes after horizon crossing and the associated amplification of quantum fluctuations. Basically the same amplification mechanism is (according to our standard model of cosmology) supposed to be responsible for the generation of the initial inhomogeneities—and hence the seeds for the formation of structures such as our galaxy—during cosmic inflation (i.e., a very early epoch in the evolution of our universe). After a general discussion of the analogy (analogue cosmology), we calculate the frozen and amplified density–density fluctuations for quasi-two-dimensional (Q2D) and three-dimensional (3D) condensates which undergo a free expansion after switching off the (longitudinal) trap.

Around the year 1500, Leonardo da Vinci designed the first mechanical calculator connecting a number of toothed wheels for simple adding operations. Since then, mechanical systems have become a major part of the later industrial revolutions with an abundance of machines in our everyday lives. Only with the advent of semiconductor electronics, however, did microstructuring techniques become available to realize mechanical systems with dimensions below 100 microns. With most recent structure sizes now reaching the limit of a few nanometres, suspended nanostructures that couple mechanical with electronic motion have been constructed. Moreover, novel lithographic techniques have enabled the investigation of transport across hybrid structures such as. suspended carbon nanotubes or flexible molecular bridges connected to mesoscopic leads.

In this invited focus issue of New Journal of Physics some of the leading experts in the field of nano-electromechanical systems (NEMS) describe the latest status and trends, from both an experimental and a theoretical perspective. A multitude of applications for NEMS are now within reach, ranging from high-frequency filters and switches in signal processing circuits, to ultra-sensitive sensors. In particular the development of mass sensors and scanning probe microscopy will be spurred by nano-mechanical systems. Considering that mechanical resonance frequencies of 1 GHz and more have already been achieved, these devices will be extremely sensitive and will offer high data acquisition rates.

On a fundamental level NEMS enable the investigation of electron–phonon coupling in the absolute limit via, for example, single electrons interacting with single (quantized) phonons, the study of single electrons being shuttled via mechanical motion, and the manipulation of single molecules with nano-mechanical tweezers. The future for NEMS research looks certain to be exciting – we can expect it to help us build detectors of virtually any kind, ultra-precise clocks and, at some point, maybe true nanomachines.

Focus on Nano-electromechanical Systems Contents

Thermomechanical noise limits on parametric sensing with nanomechanical resonators A Cleland

Dynamics of a nanomechanical resonator coupled to a superconducting single-electron transistor M Blencowe, J Imbers and A Armour

Simple models suffice for the single dot quantum shuttle A Donarini, T Novotny and A-P Jauho

Quantum nano-electromechanics with electrons, quasiparticles and Cooper pairs: effective bath descriptions and strong feedback effects A Clerk and S Bennett

Nuclear wave function interference in single-molecule electron transport M R Wegewijs and K C Nowack

Self-excitation in nanoelectromechanical charge shuttles below the field emission regime F Rüting, A Erbe and C Weiss

Formation of micro-tubes from strained SiGe/Si heterostructures H Qin, N Shaji, N E Merrill, H S Kim, R C Toonen, R H Blick, M M Roberts, D Savage, M G Lagally and G Celler

Spin-controlled nanoelectromechanics in magnetic NEM-SET systems L Y Gorelik, D Fedorets, R I Shekhter and M Jonson

Coupling between electronic transport and longitudinal phonons in suspended nanotubes S Sapmaz, P Jarillo-Herrero, Ya M Blanter and H van der Zant

Phonon-assisted tunneling in interacting suspended single wall carbon nanotubes W Izumida and M Grifoni

Theoretical and experimental investigations of three-terminal carbon nanotube relays S Axelsson, E E B Campbell, L M Jonsson, J M Kinaret, S W Lee, Y W Park and M Sveningsson

Quantum dots in Si/SiGe 2DEGs with Schottky top-gated leads K A Slinker, K L M Lewis, C C Haselby, S Goswami, L J Klein, J O Chu, S N Coppersmith and R Joynt

VHF, UHF and microwave frequency nanomechanical resonatorsX M H Huang, X L Feng, C A Zorman, M Mehregany and M Roukes

Quantum master equation descriptions of a nanomechanical resonator coupled to a single-electron transistorD A Rodrigues and A D Armour

Electron–vibron coupling in suspended nanotubes Karsten Flensberg

Nonlinear response of a driven vibrating nanobeam in the quantum regime V Peano and M Thorwart

Dynamics and current fluctuations in an ac-driven charge shuttleF Pistolesi and Rosario Fazio

Robert H Blick, University of Wisconsin at Madison, USA Milena Grifoni, Universität Regensburg, Germany

Nanomechanical resonators with fundamental mode resonance frequencies in the very-high frequency (VHF), ultra-high frequency (UHF) and microwave L-band ranges are fabricated from monocystalline silicon carbide (SiC) thin film material, and measured by magnetomotive transduction, combined with a balanced-bridge readout circuit. For resonators made from the same film, we measured the frequency dependence (thus geometry dependence) of the quality factor. We have seen a steady decrease of quality factor as the frequency goes up. This indicates the importance of clamping losses in this regime. To study this source of dissipation, a free-free beam SiC nanomechanical resonator has been co-fabricated on the same chip with a doubly clamped beam resonator operating at similar frequencies. Device testing has been performed to directly compare their characteristics and performance. It is observed that a significant improvement in quality factor is attained from the free-free beam design. In addition, from studies of resonators made from different chips with varying surface roughness, we found a strong correlation between surface roughness of the SiC thin film material and the quality factor of the resonators made from it. Furthermore, we experimentally studied the eddy current damping effect in the context of magnetomotive transduction. A high-aspect ratio SiC nanowire resonator is fabricated and tested for this study. Understanding the dissipation mechanisms, and thus improving the quality factor of these resonators, is important for implementing applications promised by these devices.

We report on the fabrication and characterization of quantum-dot devices in a Schottky-gated silicon/silicon–germanium modulation-doped two-dimensional electron gas (2DEG). The dots are confined laterally inside an etch-defined channel, while their potential is modulated by an etch-defined 2DEG gate in the plane of the dot. For the first time in this material, Schottky top gates are used to define and tune the tunnel barriers of the dot. The leakage current from the gates is reduced by minimizing their active area. Further suppression of the leakage is achieved by increasing the etch depth of the channel. The top gates are used to put the dot into the Coulomb-blockade regime, and conductance oscillations are observed as the voltage on the side gate is varied.

We present theoretical and experimental investigations of three-terminal nanoelectromechanical relays based on suspended carbon nanotubes. A charge is induced in the nanotube by applying a voltage to an underlying gate electrode thus inducing the nanotube to bend and make contact with a drain electrode. Such devices have potential applications as fast switches, logic devices, memory elements and pulse generators. We describe two modes of operation: a contact mode where the nanotube makes physical contact with the drain electrode and a non-contact mode where electrical contact between the nanotube and the drain electrode is made via a field emission current.

Transport in suspended metallic single-wall carbon nanotubes in the presence of strong electron–electron interaction is investigated. We consider a tube of finite length and discuss the effects of the coupling of the electrons to the deformation potential associated to the acoustic stretching and breathing modes. Treating the interacting electrons within the framework of the Luttinger liquid model, the low-energy spectrum of the coupled electron–phonon system is evaluated. The discreteness of the spectrum is reflected in the differential conductance which, as a function of the applied bias voltage, exhibits three distinct families of peaks. The height of the phonon-assisted peaks is very sensitive to the parameters. The phonon peaks are best observed when the system is close to the Wentzel–Bardeen singularity.

Current–voltage characteristics of suspended single-wall carbon nanotube (NT) quantum dots show a series of steps equally spaced in voltage. The energy scale of this harmonic, low-energy excitation spectrum is consistent with that of the longitudinal low-k phonon mode in the NT. Agreement is found with a Franck–Condon-based model in which the phonon-assisted tunnelling process is modelled as a coupling of electronic levels to underdamped quantum harmonic oscillators. Comparison with this model indicates a rather strong electron–phonon coupling factor of order unity. We investigate different electron–phonon coupling mechanisms and give estimates of the coupling factor.

We present a theory of the nanoelectromechanical coupling in a magnetic nanoelectromechanical single-electron tunnelling (NEM-SET) device, where a nanometre-sized metallic cluster or 'dot' is suspended between two magnetic leads. In this device, the spin projections of the tunnelling electrons, which can be manipulated by an external magnetic field, control the strength of the tunnel current. The magnitude of the current, in turn, determines the power that can be supplied to the vibrational degree of freedom of the suspended cluster. The electromechanical instability that occurs in the system if the dissipation rate of the mechanical cluster vibration energy is slow enough, is shown to strongly depend on the external magnetic field. As a result different regimes of 'shuttle' vibrations appear and are analysed. The strength of the magnetic field required to control the nanomechanical vibrations decreases as the tunnel resistance of the device increases and can be as low as 10 gauss for gigaohm tunnel structures.

We report the formation of micrometre-sized SiGe/Si tubes by releasing strained SiGe/Si bilayers from substrates in a wet chemical-etching process. In order to explore statistical studies of dynamic formation of microtubes, we fabricated arrays of square bilayers. Due to the dynamic change in curvature of the bilayers, and hence the underlying etch channels, the etching process deviates from a transport-controlled regime to one of kinetic controlled. We identified two distinct modes of etching. A slow etching mode is associated with symmetric surface deformation in which the bilayers mostly retain their initial pattern. In the fast mode, bilayers are asymmetrically deformed while large etch channels are induced and etching becomes kinetically controlled. Etch rate dispersion is directly related to the degree of asymmetry in surface deformation. When the dimensions of the bilayers become significantly larger than the curvature radius, kinetic etching dominates. During the formation of tubes, SiGe/Si bilayers strongly interact with the liquid environment of etchant and solvent. Assisted by the surface tension of evaporating liquids, the tubes are drawn near the substrate and eventually fixed to it because of van der Waals forces. Our study illuminates the dynamic etching and curling processes involved with and provides insight on how a uniform etch rate and consistent curling directions can be maintained.

The behaviour of a nanomechanical electron shuttle for applied dc-bias is investigated below the field emission regime. Simulations of the distribution of the electrical potential between the shuttling island and the leads show that field emission, which has recently been observed in a driven electron shuttle, can also play a role in the self-oscillating shuttle. For realistic experimental parameters of a silicon-based shuttle below the field emission regime, it is shown numerically that only one Coulomb step might be observable.

It is demonstrated that non-equilibrium vibrational effects are enhanced in molecular devices for which the effective potential for vibrations is sensitive to the charge state of the device. We calculate the electron tunnelling current through a molecule accounting for the two simplest qualitative effects of the charging on the nuclear potential for vibrational motion: a shift (change in the equilibrium position) and a distortion (change in the vibrational frequency). The distortion has two important effects: firstly, it breaks the symmetry between the excitation spectra of the two charge states. This gives rise to new transport effects which map out changes in the current-induced non-equilibrium vibrational distribution with increasing bias voltage. Secondly, the distortion modifies the Franck–Condon factors for electron tunnelling. Together with the spectral asymmetry this gives rise to pronounced nuclear wavefunction interference effects on the electron transport. For instance nuclear-parity forbidden transitions lead to differential conductance anti-resonances, which are stronger than those due to allowed transitions. For special distortion and shift combinations a coherent suppression of transport beyond a bias voltage threshold is possible.

Using a quantum-noise approach, we discuss the physics of both normal metal and superconducting single-electron transistors (SSETs) coupled to mechanical resonators. Particular attention is paid to the regime where transport occurs via incoherent Cooper-pair tunnelling (either via the Josephson quasi-particle (JQP) or double JQP (DJQP) process). We show that, surprisingly, the back-action of tunnelling Cooper pairs (or superconducting quasi-particles) can be used to significantly cool the oscillator. We also discuss the physical origin of negative-damping effects in this system and how they can lead to a regime of strong electromechanical feedback, where despite a weak SET–oscillator coupling, the motion of the oscillator strongly effects the tunnelling of the Cooper pairs. We show that in this regime, the oscillator is characterized by an energy-dependent effective temperature. Finally, we discuss the strong analogy between back-action effects of incoherent Cooper-pair tunnelling and ponderomotive effects in an optical cavity with a moveable mirror; in our case, tunnelling Cooper pairs play the role of the cavity photons.

A quantum shuttle is an archetypical nanoelectromechanical device, where the mechanical degree of freedom is quantized. Using a full-scale numerical solution of the generalized Master equation describing the shuttle, we have recently shown (Novotný et al 2004 Phys. Rev. Lett.92 248302) that for certain limits of the shuttle parameters one can distinguish three distinct charge transport mechanisms: (i) an incoherent tunnelling regime, (ii) a shuttling regime, where the charge transport is synchronous with the mechanical motion, and (iii) a coexistence regime, where the device switches between the tunnelling and shuttling regimes. While a study of the crossover between these three regimes requires the full numerics, we show here that by identifying the appropriate timescales it is possible to derive vastly simpler equations for each of the three regimes. The simplified equations allow a clear physical interpretation, are easily solved and are in good agreement with the full numerics in their respective domains of validity.

We present an analysis of the dynamics of a nanomechanical resonator coupled to a superconducting single-electron transistor (SSET) in the vicinity of Josephson quasi-particle (JQP) and double Josephson quasi-particle (DJQP) resonances. For weak coupling and wide separation of dynamical timescales, we find that for either superconducting resonances the dynamics of the resonator are given by a Fokker–Planck equation, i.e. the SSET behaves effectively as an equilibrium heat bath, characterized by an effective temperature, which also damps the resonator and renormalizes its frequency. Depending on the gate and drain–source voltage bias points with respect to the superconducting resonance, the SSET can also give rise to an instability in the mechanical resonator marked by negative damping and temperature within the appropriate Fokker–Planck equation. Furthermore, sufficiently close to a resonance, we find that the Fokker–Planck description breaks down. We also point out that there is a close analogy between coupling of a nanomechanical resonator to an SSET in the vicinity of the JQP resonance and Doppler cooling of atoms by means of lasers.

Measuring and monitoring the dynamic parameters of a nanomechanical resonator, in particular the resonance frequency, has received significant attention recently, in part due to the possibility of very sensitive, fast and precise mass sensing. Added mass can include chemisorbed or physisorbed metals or organic molecules, and if sufficiently high sensitivity, dynamic range and detector speed can be achieved, they could have applications in, e.g., proteomics. Here, I investigate some of the fundamental limits to mass sensing in such resonators, discussing the limits imposed by thermomechanical noise on both the linear operating regime of a simple harmonic oscillator, and the equivalent limits on nonlinear parametric amplifiers used as parametric sensors. The model system is a cantilevered flexural resonator, but the results apply equally well (in most cases) to doubly clamped or torsional resonant structures as well.

The swimming of a pair of spherical bladders that change their volumes and mutual distance is superior to other models of artificial swimmers at low Reynolds numbers. The swimming resembles the wriggling motion known as metaboly of certain protozoa.

We investigate the nonlinear dynamics of a large-amplitude shear Alfvén wave (AW) with circular polarization in collisionless plasmas by using a 2D3V fully relativistic electromagnetic particle-in-cell (PIC) simulation code. We found that when the amplitude, A = δB/B₀, of the shear AW is larger than , the shear AW spontaneously becomes unstable for the modified two-stream instability (MTSI), resulting in the excitation of small-scale quasi-electrostatic waves with electric fields parallel to a uniform magnetic field. We found that the electrons are heated mostly in the direction parallel to the magnetic field due to the quasi-electrostatic lower hybrid waves by the MTSI. Subsequently, the shear AW becomes unstable with strong transverse modulation, resulting in the excitation of ion acoustic waves that can heat ions. About 70% of the AW energy can be converted to plasma heating of both electrons and ions. The plasma heating time is within about 250ω_ci⁻¹, which is shorter than the collision time between protons and neutral hydrogens in the upper chromosphere. The obtained results could be very important for plasma heating of coronal loops in the upper chromosphere.

We report a quantum key distribution experiment based on the differential phase shift keying (DPSK) protocol with a Poissonian photon source, in which secure keys were generated over >100 km fibre for the first time. We analysed the security of the DPSK protocol and showed that it is robust against strong attacks by Eve, including a photon number splitting attack. To implement this protocol, we developed a new detector for the 1.5 μm band based on frequency up-conversion in a periodically poled lithium niobate waveguide followed by an Si avalanche photodiode. The use of detectors increased the sifted key generation rate up to >1 Mbit s⁻¹ over 30 km fibre, which is two orders of magnitude larger than the previous record.

We propose the use of normal and Andreev resonances in normal-superconducting structures to generate divergent beams of nonlocally entangled electrons. Resonant levels are tuned to selectively transmit electrons with specific values of the perpendicular energy, thus fixing the magnitude of the exit angle. When the normal metal is a ballistic two-dimensional electron gas, the proposed scheme guarantees arbitrarily large spatial separation of the entangled electron beams emitted from a finite interface. We perform a quantitative study of the linear and nonlinear transport properties of some suitable structures, taking into account the large mismatch in effective masses and Fermi wavelengths. Numerical estimates confirm the feasibility of the proposed beam separation method.

It is sketched how a monostable rf- or dc-SQUID can mediate an inductive coupling between two adjacent flux qubits. The non-trivial dependence of the SQUID's susceptibility on external flux makes it possible to continuously tune the induced coupling from antiferromagnetic (AF) to ferromagnetic (FM). In particular, for suitable parameters, the induced FM coupling can be sufficiently large to overcome any possible direct AF inductive coupling between the qubits.

The main features follow from a classical analysis of the multi-qubit potential. A fully quantum treatment yields similar results, but with a modified expression for the SQUID susceptibility.

Since the latter is exact, it can also be used to evaluate the susceptibility—or, equivalently, energy-level curvature—of an isolated rf-SQUID for larger shielding and at degenerate flux bias, i.e., a (bistable) qubit. The result is compared to the standard two-level (pseudospin) treatment of the anticrossing, and the ensuing conclusions are verified numerically.

We investigate the presence of multipartite entanglement in macroscopic spin chains. We discuss the Heisenberg and the XY model and derive bounds on the internal energy for systems without multipartite entanglement. Based on this we show that in thermal equilibrium the above-mentioned spin systems contain genuine multipartite entanglement, even at finite modest temperatures.

We prove that universal quantum computation is possible using only (i) the physically natural measurement on two qubits which distinguishes the singlet from the triplet subspace and (ii) qubits prepared in almost any three different (potentially highly mixed) states. In some sense this measurement is a 'more universal' dynamical element than a universal two-qubit unitary gate, since the latter must be supplemented by measurement. Because of the rotational invariance of the measurement used, our scheme is robust to collective decoherence in a manner very different to previous proposals—in effect it is only ever sensitive to the relational properties of the qubits.

A highly efficient coagulation process of microparticles was discovered during experiments that were carried out on the International Space Station. Charged microspheres injected into a low pressure neutral gas environment form mm-sized aggregates, provided the initial number density is high enough. The coagulation occurred at a rate ∼10⁴–10⁵ times faster than that usually observed with uncharged grains. Theoretical estimates show that this process is so fast that it cannot even be explained by a coagulation process with charge enhanced cross-section. The observations can, however, be described by a 'gelation phase transition'. We summarize the salient measurements, outline the theoretical description and show their compatibility.

We propose an experimental implementation of a quantum game algorithm in a hybrid scheme combining the quantum circuit approach and the cluster state model. An economical cluster configuration is suggested to embody a quantum version of the Prisoners' Dilemma. Our proposal is shown to be within the experimental state of the art and can be realized with existing technology.The effects of relevant experimental imperfections are also carefully examined.

The rotation of a cylindrical plasma column in a magnetic field has been studied in the linear section of the new plasma device Mistral. Under suitable conditions we observe a transition to a turbulent regime characterized by strong, bursty fluctuations at the edge of the column. The detection and the study of the spatio-temporal evolution of structures in the turbulent regime have been performed by means of a new enhanced conditional sampling technique. We have collected evidence of the development of a bent tail emanating from the plasma column. The charged particles inside the structure move along a spiral trajectory resulting in a net radial convection of the plasma to the walls. We show experimentally that a poloidal electric field is present inside the structures leading to the observed outwards radial E × B drift, in agreement with the expectations of recent and past theoretical works.

Imaging of surface adsorbed molecules is investigated as a novel detection method for matter wave interferometry with fluorescent particles. Mechanically magnified fluorescence imaging turns out to be an excellent tool for recording quantum interference patterns. It has a good sensitivity and yields patterns of high visibility. The spatial resolution of this technique is only determined by the Talbot gratings and can exceed the optical resolution limit by an order of magnitude. A unique advantage of this approach is its scalability: for certain classes of nano-sized objects, the detection sensitivity will even increase significantly with increasing size of the particle.

In this work, we studied the far-field properties of the microwave radiation from sources embedded inside the split-ring resonator (SRR) metamaterial medium. Our results showed that the emitted power near the resonance frequency of the SRR structure was confined to a narrow angular region in the far field. The measured radiation patterns showed half-power beamwidths around 14°. The highly directive radiation is obtained with a smaller radiation surface area when compared to the previous results obtained by using photonic crystals. The reduction in the surface area is ten-fold in the case of the SRR metamaterial medium when compared to the photonic crystals. Our results provide means to create compact size highly directive antennas.

Through the quantum trajectory approach, we calculate the geometric phase acquired by a bipartite system subjected to decoherence. The subsystems that compose the bipartite system interact with each other and then are entangled in the evolution. The geometric phase due to the quantum jump for both the bipartite system and its subsystems is calculated and analysed. As an example, we present two coupled spin- particles to detail the calculations.

We present a mathematical formalism for the description of un- restricted quantum walks with entangled coins and one walker. The numerical behaviour of such walks is examined when using a Bell state as the initial coin state, with two different coin operators, two different shift operators, and one walker. We compare and contrast the performance of these quantum walks with that of a classical random walk consisting of one walker and two maximally correlated coins as well as quantum walks with coins sharing different degrees of entanglement.

We illustrate that the behaviour of our walk with entangled coins can be very different in comparison to the usual quantum walk with a single coin. We also demonstrate that simply by changing the shift operator, we can generate widely different distributions. We also compare the behaviour of quantum walks with maximally entangled coins with that of quantum walks with non-entangled coins. Finally, we show that the use of different shift operators on two and three qubit coins leads to different position probability distributions in one- and two-dimensional graphs.

We describe the properties of specific non-reflecting birefringent left-handed metamaterials and demonstrate a birefringent perfect lens for vectorial fields. We predict that, in a sharp contrast to the concept of a conventional perfect lens realized at = μ = −1 (where is the dielectric permittivity and μ is the magnetic permeability), the birefringent left-handed slab possesses the property of negative refraction either for TE- or TM-polarized waves or for both of them simultaneously. This allows selective focusing and a spatial separation of the images created at different polarizations. We discuss several applications of the birefringent left-handed lenses such as the beam splitting and near-field diagnostics at the sub-wavelength scale.

In thin superconducting wires, phase-slip by thermal activation near the critical temperature is a well-known effect. It has recently become clear that phase-slip by quantum tunnelling through the energy barrier can also have a significant rate at low temperatures. In this paper, it is suggested that quantum phase-slip can be used to realize a superconducting quantum bit without Josephson junctions. A loop containing very thin nanofabricated wire is biased with an externally applied magnetic flux of half a flux quantum, resulting in two states with opposite circulating current and equal energy. Quantum phase-slip should provide coherent coupling between these two macroscopic states. Numbers are given for a wire of amorphous niobium-silicon that can be fabricated with advanced electron beam lithography.

We investigate the influence of a dipole interaction with a classical radiation field on a qubit during a continuous change of a control parameter. In particular, we explore the non-adiabatic transitions that occur when the qubit is swept with linear speed through resonances with the time-dependent interaction. Two classic problems come together in this model: the Landau–Zener (LZ) and the Rabi problem. The probability of LZ transitions now depends sensitively on the amplitude, the frequency and the phase of the Rabi interaction. The influence of the static phase turns out to be particularly strong, since this parameter controls the time-reversal symmetry of the Hamiltonian. In the limits of large and small frequencies, analytical results obtained within a rotating-wave approximation compare favourably with a numerically exact solution. We discuss physical realizations in microwave optics, quantum dots and molecular nanomagnets.

A detailed numerical analysis of heavily chirped pulses in the positive-dispersion regime (PDR) is presented on the basis of the distributed cubic–quintic generalized complex nonlinear Ginzburg–Landau equation. It is demonstrated that there are three main types of pulse spectra: truncated parabolic-top, Π- and M-shaped profiles. The strong chirp broadens the pulse spectrum up to 100 nm for a Ti:Sa oscillator, which provides compressibility of the picosecond pulse down to sub-30 fs. Since the picosecond pulse has a peak power lower than the self-focusing power inside a Ti:Sa crystal, the microjoule energies become directly available from a femtosecond oscillator. The influence of the third- and fourth-order dispersions on the pulse spectrum and stability is analysed. It is demonstrated that the dynamic gain saturation plays an important role in pulse stabilization. The common action of dynamic gain saturation, self-amplified modulation (SAM) and saturation of the SAM provides pulse stabilization inside the limited range of the positive group-delay dispersions (GDDs). Since the stabilizing action of the SAM cannot be essentially enhanced for a pure Kerr-lens mode-locking regime, a semiconductor saturable absorber is required for pulse energies of >0.7 μJ inside an oscillator. The basic results of the numerical analysis are in an excellent agreement with experimental data obtained from oscillators with repetition rates ranging from 50 to 2 MHz.

Broadening the ultrashort laser pulse in a Kerr-lens mode-locked laser by net positive round-trip group-delay dispersion has proven to be a powerful concept for scaling the pulse energy directly achievable with a femtosecond laser oscillator without external amplification. Drawing on this concept, we demonstrate here Ti : Sa chirped-pulse oscillators delivering sub-40 fs pulses of 0.5 μJ and 50 nJ energy at average power levels of 1 and 2.5 W (repetition rate: 2 and 50 MHz), respectively, which to the best of our knowledge constitute the highest pulse energy and average power achieved with a femtosecond (<100 fs) laser oscillator to date. The 0.5 μJ pulses have a peak power in excess of 10 MW and reach a peak intensity >10¹⁵ W cm⁻² (when focused down to ∼1 μm²), both of which represent record values from a laser oscillator. These pulse parameters appear to be limited merely by the pump power available, affording promise of scaling chirped-pulse femtosecond Ti : Sa oscillators to microjoule pulse energies and—by simultaneous spectral broadening—towards peak power levels of several hundred megawatts.

The influence of background light on satellite-to-ground free-space quantum key distribution (QKD) is investigated. By comparing the number of noise photons to that of the signal photons per pulse, the technical requirements for a practical system are evaluated. We show that satellite-to-ground QKD is feasible with currently available technology even under full moonlight.

Multifractal fluctuations in the time dynamics of geoelectrical data, recorded in a seismic area of southern Italy, have been analysed. We find that the multifractality of the signal depends mostly on the different long-range properties for small and large fluctuations. Furthermore, we quantitatively characterized the multifractality of the geoelectrical time series, on the basis of the characteristic parameters derived from the multifractal spectrum (maximum α₀, asymmetry B and width W) (Shimizu Y, Thurner S and Ehrenberger K 2002 Fractals10 103). We then analysed the time evolution of the multifractal parameters and we found that the multifractal degree of the signal, revealed by the variation of the width of the multifractal spectrum, is enhanced in association with the occurrence of the largest seismic event. This study aims to suggest another approach to investigate the complex dynamics of geoelectrical signals.

The physical origins of negative refractive index are derived from a dilute microscopic model, producing a result that is generalized to the dense condensed phase limit. In particular, scattering from a thin sheet of electric and magnetic dipoles driven above resonance is used to form a fundamental description for negative refraction. Of practical significance, loss and dispersion are implicit in the microscopic model. While naturally occurring negative index materials are unavailable, ferromagnetic and ferroelectric materials provide device design opportunities.

An erratum page was added to the end of the published paper on 11 November 2005.

We investigated the influences of field orientation with respect to transport current, magnitude of transport current, temperature and field-sweep rate (dH/dt) on the evolution of magnetovoltage (V–H) curves in polycrystalline superconducting bulk sample of Y₁Ba₂Cu₃O_7−δ. In well-defined magnetic field and temperature ranges, it was found that a relative decrease in dissipation could be obtained depending on the current as the field-sweep rate decreases, so that it underlines the importance of time spent to plot the whole cycle of V–H curves. This physical observation was correlated to more effective field and less relaxation evolving in the grains and more return flux and less effective field at the grain boundaries. On the other hand, an enhancement in hysteresis effects in V–H curves which manifests itself as an increase in the area enclosed by the hysteresis loop was observed at low currents and at low temperatures. These behaviours were attributed to the increase in effective trapped field originated from the relative increase in the height of pinning barriers due to the low currents, and low-thermal fluctuations together with flux creep appearing at low temperatures. Finally, the strong clockwise hysteresis effects in V–H curves were interpreted within the granularity of sample mainly in terms of flux trapped in the grains returning through the grain boundaries.

There are several examples of bipartite entangled states of continuous variables for which the existing criteria for entanglement using the inequalities involving the second-order moments are insufficient. We derive new inequalities involving higher order correlation, for testing entanglement in non-Gaussian states. In this context, we study an example of a non-Gaussian state, which is a bipartite entangled state of the form . Our results open up an avenue to search for new inequalities to test entanglement in non-Gaussian states.

A one-dimensional (1D) subwavelength resonator utilizing a left-handed material (LHM) is first chosen as an example to show that for a reliable analysis, one should use the wave-field theory instead of the ray-trace theory. The resonant modes of a 2D subwavelength open resonator utilizing an LHM is calculated with wave-field simulation. An open resonator formed by a photonic crystal with negative effective index is also studied.

We examine the collective behaviour of coupled Duffing Hamiltonian systems (HS) and show the existence of measure synchronization (MS) in the quasi-periodic (QP) and -chaotic states. We show that the dynamics of coupled Duffing Hamiltonians exhibit a transition to coherent invariant measure, their orbits sharing the same phase space as the coupling strength is increased. Transitions from QP measure desynchronous to QP MS state and QP measure desynchronous to chaotic measure synchronous (CMS) state were both identified. Moreover, a transition from partial measure synchronization state to complete CMS state was found for three coupled subsystems.

We identify the SiCl₃ bonding geometry as the most desorption-active site in x-ray photon-stimulated desorption (XPSD) of positive chlorine ions from chlorinated Si(1 1 1) surfaces due to their much higher desorption cross-section in comparison to the on-top majority site. The novel combination of x-ray standing waves (XSW), XPSD and density functional theory (DFT) allows to quantitatively determine this trichloride adsorption geometry with high spatial resolution and to deduce the site-specific desorption cross-sections for Cl⁺ and Cl²⁺ ions, respectively. Additionally, these SiCl₃ groups are shown to exhibit a preferential crystallographic orientation which is confirmed by both XSW and DFT. Moreover, the analoguous identification of desorption-active SiBr₃ species for Br/Si(1 1 1) suggests a common physical origin of the site-specific desorption cross-sections.

The influence of an inhomogeneous magnetic field, ion focusing effect and equilibrium charge gradient on the propagation of dust-lattice (DL) modes in a one-dimensional string formed by paramagnetic particles is considered. In typical discharge conditions, all three anisotropic factors can compete with each other even at moderate magnetic fields (∼0.1–0.2 T), modifying the DL waves and leading to mode coupling. The characteristics of the mode coupling can be controlled externally by varying the magnetic-field regime, thus opening new possibilities for studies of collective effects in strongly coupled systems.

In the standard minority game (MG), players use historical minority choices as the sole public information to pick one out of the two alternatives. However, publishing historical minority choices is not the only way to present global system information to players when more than two alternatives are available. Thus, it is instructive to study the dynamics and co-operative behaviours of this extended game as a function of the global information provided. We numerically find that, although the system dynamics depend on the kind of public information given to the players, the degree of co-operation follows the same trend as that of the standard MG. We also explain most of our findings by the crowd–anticrowd theory.

Among the remarkable new ideas that Einstein introduced into physics in 1905 was the revision of our concept of space and time. His special theory of relativity quickly became incorporated into all of physics. This cannot be said of his later general relativity, the ultimate outgrowth of his original insight 100 years ago. In the general theory, curvature of Einsteinian spacetime was introduced to represent gravitation. For many years the general theory remained an intellectual and aesthetic triumph with little connection to the rest of physics. This has now all changed dramatically, and the broader physics community may not be aware of the extent of the change. The general theory is at the centre of new modes of astronomical observation; it is being tested in laboratories and on large scales; it is crucial to understanding some of the most exciting astrophysical discoveries; it presents a frustrating challenge to computational scientists; it is indispensable for constructing a new understanding of cosmology from our present quandary.

This celebratory Focus Issue in New Journal of Physics will attempt to give a glimpse of the current status of what Einstein wrought in 1905, as well as a historical perspective on how ideas about spacetime have evolved since then.

Focus on Spacetime 100 Years Later Contents

Gravity and the quantum A Ashtekar

Black holes in astrophysics R Narayan

A most useful manifestation of relativity: gravitional lenses E E Falco

Spacetime in string theory G T Horowitz

Gravitational wave detectors P Aufmuth and K Danzmann

Hawking radiation and black hole thermodynamics D N Page

The basics of gravitational wave theory E E Flanagan and S A Hughes

Laboratory tests of gravity J H Gundlach

Richard Price, University of Texas at Brownsville, USA Jorge Pullin, Louisiana State University, Baton Rouge, USA

Experimental tests of gravity are probes for new physics. Theoretical speculations ranging from string theory scenarios to explanations of cosmological phenomena have led to many new experiments. Recent laboratory searches for equivalence principle violations and tests of Newton's 1/r²-law at short distances are reviewed.

Einstein's special theory of relativity revolutionized physics by teaching us that space and time are not separate entities, but join as 'spacetime'. His general theory of relativity further taught us that spacetime is not just a stage on which dynamics takes place, but is a participant: the field equation of general relativity connects matter dynamics to the curvature of spacetime. Curvature is responsible for gravity, carrying us beyond the Newtonian conception of gravity that had been in place for the previous two and a half centuries. Much research in gravitation since then has explored and clarified the consequences of this revolution; the notion of dynamical spacetime is now firmly established in the toolkit of modern physics. Indeed, this notion is so well established that we may now contemplate using spacetime as a tool for other sciences. One aspect of dynamical spacetime—its radiative character, 'gravitational radiation'—will inaugurate entirely new techniques for observing violent astrophysical processes. Over the next 100 years, much of this subject's excitement will come from learning how to exploit spacetime as a tool for astronomy. This paper is intended as a tutorial in the basics of gravitational radiation physics.

An inexhaustive review of Hawking radiation and black hole thermodynamics is given, focusing especially upon some of the historical aspects as seen from the biased viewpoint of a minor player in the field on and off for the past 30 years.

The existence of gravitational radiation is a prediction of Einstein's general theory of relativity. Gravitational waves are perturbations in the curvature of spacetime caused by accelerated masses. Since the 1960s gravitational wave detectors have been built and constantly improved. The present-day generation of resonant mass antennas and laser interferometers has reached the necessary sensitivity to detect gravitational waves from sources in the Milky Way. Within a few years, the next generation of detectors will open the field of gravitational wave astronomy.

We give a brief overview of the nature of spacetime emerging from string theory. This is radically different from the familiar spacetime of Einstein's relativity. At a perturbative level, the spacetime metric appears as 'coupling constants' in a two-dimensional quantum field theory. Nonperturbatively (with certain boundary conditions), spacetime is not fundamental but must be reconstructed from a holographic, dual theory.

Gravitational lenses are scarce but extraordinary phenomena that yield a very high rate of return on observational investment. Given their scarcity, it is very impressive that since their discovery in the extragalactic realm in 1979, they have had such an enormous impact on our knowledge of the universe. Gravitational lensing is a manifestation of general relativity that has contributed to a great variety of astrophysical and cosmological studies. In the weak-field limit, lensing studies are based on well-established physics and thus offer a direct approach to study many of the currently pressing problems of astrophysics. Examples of these are the significance of dark matter and the age and size of the universe. I present a brief history of gravitational lensing and describe recent developments in fields such as searches for dark matter and studies of galaxy evolution and cosmology. The approach is non-specialized and emphasizes observational results, to reach the widest possible audience.

This paper reviews the current status of black hole (BH) astrophysics, focusing on topics of interest to a physics audience. Astronomers have discovered dozens of compact objects with masses greater than 3M_⊙, the likely maximum mass of a neutron star. These objects are identified as BH candidates. Some of the candidates have masses ∼5M_⊙–20M_⊙ and are found in x-ray binaries, while the rest have masses ∼10⁶M_⊙–10^9.5M_⊙ and are found in galactic nuclei. A variety of methods are being tried to estimate the spin parameters of the candidate BHs. There is strong circumstantial evidence that many of the objects have event horizons, so there is good reason to believe that the candidates are true BHs. Recent MHD simulations of magnetized plasma accreting on rotating BHs seem to hint that relativistic jets may be produced by a magnetic analogue of the Penrose process.

The goal of this review is to present a broad perspective on quantum gravity for non-experts. After a historical introduction, key physical problems of quantum gravity are illustrated. While there are a number of interesting and insightful approaches to address these issues, over the past two decades sustained progress has primarily occurred in two programs: string theory and loop quantum gravity. The first program is described in Horowitz's contribution to this Focus Issue while my article will focus on the second. The emphasis is on underlying ideas, conceptual issues and overall status of the program rather than mathematical details and associated technical subtleties.

We demonstrate that subsequent on- and off-times of the luminescence blinking of semiconductor quantum dots (QDs) are correlated, indicating that the process behind is not memoryless. A residual memory, which has been overlooked in previous investigations of the blinking, is found to last for several (∼40) detected on/off cycles. No influence of the substrate nature or the excitation intensity is observed, pointing to a process intrinsic to the QDs. These results should encourage re-analysis of existing data and may represent the key to understand the underlying physical mechanism of QD luminescence blinking.

Vibrational calculations for H₃⁺ are performed using an accurate global ab initio potential energy surface. Fourteen bound states close to dissociation are found to have interesting long-range dynamics. These asymptotic vibrational states (AVS) are studied graphically by cuts through their wave functions and by calculating a rotational constant. These AVS, which overlap open system classical trajectories that form half-tori, should lead to an increased density of states near dissociation. Their influence on the infrared near-dissociation spectrum of H₃⁺ remains to be determined.

It has recently been shown that the work extractable from correlated bipartite quantum systems under an appropriate protocol can be used to distinguish entanglement from classical correlation. A natural question is now whether it can be generalized to multipartite systems. In this paper, we devise a protocol to distinguish the GHZ, the W, and separable states in terms of the thermodynamically extractable work under local operations and classical communication, and compare the results with those obtained from Mermin's inequalities.

We present a scheme for rapidly entangling matter qubits in order to create graph states for one-way quantum computing. The qubits can be simple three-level systems in separate cavities. Coupling involves only local fields and a static (unswitched) linear optics network. Fusion of graph-state sections occurs with, in principle, zero probability of damaging the nascent graph state. We avoid the finite thresholds of other schemes by operating on two entangled pairs, so that each generates exactly one photon. We do not require the relatively slow single qubit local flips to be applied during the growth phase: growth of the graph state can then become a purely optical process. The scheme naturally generates graph states with vertices of high degree and so is easily able to construct minimal graph states, with consequent resource savings. The most efficient approach will be to create new graph-state edges even as qubits elsewhere are measured, in a 'just in time' approach. An error analysis indicates that the scheme is relatively robust against imperfections in the apparatus.

The evolution of the Si(1 1 1) surface after submonolayer deposition of Ga has been observed in situ by low-energy electron microscopy and scanning tunnelling microscopy. A phase separation of Ga-terminated -R 30° reconstructed areas and bare Si(1 1 1)-7 × 7 regions leads to the formation of a two-dimensional nanopattern. The shape of this pattern can be controlled by the choice of the surface miscut direction, which is explained in terms of the anisotropy of the domain boundary line energy and a high kink-formation energy. A general scheme for the nanopattern formation, based on intrinsic properties of the Si(1 1 1) surface, is presented. Experiments performed with In instead of Ga support this scheme.

We solve the two-particle s-wave scattering problem for ultracold atom gases confined in arbitrary quasi-one-dimensional (1D) trapping potentials, allowing for two different atom species. As a consequence, the centre-of-mass and relative degrees of freedom do not factorize. We derive bound-state solutions and obtain the general scattering solution, which exhibits several resonances in the 1D scattering length induced by the confinement. We apply our formalism to two experimentally relevant cases: (i) interspecies scattering in a two-species mixture, and (ii) the two-body problem for a single species in a non-parabolic trap.

Surface wave propagation at the interface between different types of gyrotropic materials and an isotropic negatively refracting medium, in which the relative permittivity and relative permeability are, simultaneously, negative is investigated. A general approach is taken that embraces both gyroelectric and gyromagnetic materials, permitting the possibility of operating in either the low GHz, THz or the optical frequency regimes. The classical transverse Voigt configuration is adopted and a complete analysis of non-reciprocal surface wave dispersion is presented. The impact of the surface polariton modes upon the reflection of both plane waves and beams is discussed in terms of resonances and an example of the influence upon the Goos–Hänchen shift is given.

We present the exact analysis of a spatially restricted one-dimensional diffusion process with a time-dependent, spatially linear potential and a reflecting boundary. Using matching conditions together with the known solution for the unbounded diffusion in the time modulated potential, we have derived a new integral equation, whose solution yields the Green function for the restricted diffusion. Applying the general scheme, we give the numerical analysis of the diffusion in a symmetrically oscillating force field superimposed on the time-independent component. The latter component alone guarantees the approach to Gibbs equilibrium with exponential probability density. We calculate both the kinetic and the energetic characteristics of the emerging non-equilibrium isothermal process and discuss their dependence on the model parameters.

The effect of the exchange interaction in the photoionization continuum is investigated, using N 1s photoionization of NO into the 1s⁻¹2π (¹Π) and (³Π) final states as an example. The separation in energy of these two final states is 1.41 eV. Significant differences in their partial photoionization cross-sections are observed over a wide range of energies and cannot be accounted for by the different multiplicity of the states. We suggest that the deviation of the ³Π/¹Π cross-section ratio from the statistical weighting at intermediate energies is dominated by the difference in the final-state potential experienced by the photoelectron and at asymptotically high energies by the multiplet-dependent amount of intensity going into multi-electron (shake-up) processes. Calculations underpinning this point are presented. We also show supporting measurements of the ³Π/¹Π cross-section ratio for O 1s ionization and the absolute photoabsorption cross-section for NO over a wide energy range covering the core level region.

In the series of 3d(t_2g)¹ perovskites, SrVO₃–CaVO₃–LaTiO₃–YTiO₃, the transition-metal d electron becomes increasingly localized and undergoes a Mott transition between CaVO₃ and LaTiO₃. By defining a low-energy Hubbard Hamiltonian in the basis of Wannier functions for the t_2g LDA band and solving it in the single-site dynamical mean-field (DMFT) approximation, it was recently shown (Pavarini et al (2004) Phys. Rev. Lett.92 176403) that simultaneously with the Mott transition there occurs a strong suppression of orbital fluctuations due to splitting of the t_2g levels. The present paper reviews and expands this work, in particular in the direction of exposing the underlying chemical mechanisms by means of ab initio LDA Wannier functions generated with the Nth order muffin-tin orbital (NMTO) method. The Wannier functions for the occupied oxygen-p band illustrate the importance of oxygen-p to large cation-d covalency for the progressive GdFeO₃-type distortion along the series. The oxygen-p orbitals which pdσ-bond to the cations are the same as those which pdπ-bond to the transition-metal t_2g orbitals. As a consequence, the Wannier functions for the t_2g band exhibit residual covalency between the transition-metal t_2g, the large cation-d, and the oxygen-p states. This residual covalency, which increases along the series, turns out to be responsible not only for the splittings, Δ, of the t_2g levels, but also for non-cubic perturbations of the hopping integrals, both of which are decisive for the Mott transition. We find good agreement with the optical and photoemission spectra for all four materials, with the crystal-field splittings and orbital polarizations recently measured for the titanates, and with the metallization volume (pressure) for LaTiO₃. The metallization volume for YTiO₃ is predicted and the role of the Jahn–Teller (JT) distortion is discussed. For use in future many-body calculations, we tabulate the t_2g on-site and hopping matrix elements for all four materials and give an analytical expression for the orthorhombic Hamiltonian in the k + Q representation. Using conventional super-exchange theory, our on-site and hopping matrix elements reproduce the observed magnetic orders in LaTiO₃ and YTiO₃, but the results are sensitive to detail, in particular for YTiO₃ where, without the JT distortion, the magnetic order would be antiferromagnetic C- or A-type, rather than ferromagnetic. It is decisive that upon increasing the GdFeO₃-type distortion, the nearest-neighbour hopping between the lowest and the upper-level Wannier functions becomes stronger than the hopping between the lowest-level Wannier functions. Finally, we show that the non-cubic perturbations responsible for this behaviour make it possible to unfold the orthorhombic t_2g LDA bandstructure to a pseudo-cubic zone. In this zone, the lowest band is separated from the two others by a direct gap and has a width, W_I, which is significantly smaller than that, W, of the entire t_2g band. The progressive GdFeO₃-type distortion thus favours electron localization by decreasing W, by increasing Δ/W, and by decreasing W_I/W. Our conclusions concerning the roles of GdFeO₃-type and JT distortions agree with those of Mochizuki and Imada (2003 Phys. Rev. Lett.91 167203).

In the last few years quantum information has played an increasingly central role in the research activities of many scientists within wide ranging areas of physics, mathematics and computer science as it known to be more efficient than its classical counterpart. The impact and advantages of quantum information protocols emerge in numerous situations. In the case of cryptography, quantum dynamics guarantees secure protocols, and in quantum computation, factorization of large numbers, intractable with classical algorithms, can be solved much faster with a quantum computer. It is now widely believed that quantum information will play a leading role in future technologies.

Together with the ongoing development of more efficient schemes to solve both new and old tasks in information science, a great deal of interest has been devoted to selecting suitable physical systems where one could realize these ideas. In particular, the quest for large scale integrability has stimulated an increasing interest in the field of solid state physics. Nanoelectronics seems the natural arena to realize physical implementation of quantum hardware. Qubits made out of solid-state devices, such as spins/charges in quantum dots or superconducting nanocircuits, may offer a greater advantage in this respect because fabrication techniques allow for scalability to a large number of coupled qubits. Recent experimental breakthroughs in semiconducting and superconducting nanostructures constitute the first important steps towards the realization of a solid-state quantum computer. In addition the interest in solid state quantum computation has stimulated a large body of research aimed at understanding the properties of entanglement in solid state systems.

This Focus issue of New Journal of Physics gathers together contributions from leading experts in the area of solid state quantum information with the aim of providing a panorama of the most exciting scientific questions currently being investigated in the field.

Focus on Solid State Quantum Information Contents

Quasiparticle entanglement: redfinition of the vacuum and reduced density matrix approach P Samuelsson, E Sukhorukov and M Buttiker

Pseudospin quantum computation in semiconductor nanostructures V W Scarola, K Park and S Das Sarma

Superconducting qubit network with controllable nearest-neighbour coupling M Wallquist, J Lantz, V S Shumeiko and G Wendin

Weak coupling Josephson junction as a current probe: effect of dissipation on escape dynamics J M Kivioja, T E Nieminen, J Claudon, O Buisson, F Hekking and J P Pekola

The single Cooper-pair box as a charge qubit K Bladh, T Duty, D Gunnarsson and P Delsing

Quantum state transfer in arrays of flux qubits A O Lyakhov and C Bruder

Spin filling of a quantum dot derived from excited-state spectroscopy L H Willems van Beveren, R Hanson, I T Vink, F H L Koppens, L P Kouwenhoven and L M K Vandersypen

Clauser–Horne inequality for the full counting statistics F Taddei, R Fazio and E Prada

Recent advances in exciton based quantum information processing in quantum dot nanostructures H J Krenner, S Stufler, M Sabathil, E C Clark, P Ester, M Bichler, G Abstreiter and J J Finley

Spatially highly resolved study of AFM scanning tip-quantum dot local interaction S Kicin, A Pioda, T Ihn, M Sigrist, A Fuhrer, K Ensslin, M Reinwald and W Wegscheider

Transfer of entanglement from electrons to photons by optical selection rules M Titov, B Trauzette, B Michealis and C W J Beenakker

Non-Abelian Chern-Simons models with discrete gauge groups on a lattice B Doucot and L B Ioffe

Landau–Zener transitions in qubits controlled by electromagnetic fields Martijn Wubs, Keiji Saito, Sigmund Kohler, Yosuke Kayanuma and Peter Hänggi

Phase-slip flux qubits J E Mooij and C J P M Harmans

Mediated tunable coupling of flux qubits Alec Maassen van den Brink, A J Berkley and M Yalowsky

Divergent beams of nonlocally entangled electrons emitted from hybrid normal-superconducting structures Elsa Prada and Fernando Sols

Decoherence from ensembles of two-level fluctuators Josef Schriefl, Yuriy Makhlin, Alexander Shnirman and Gerd Schön

Semiconductor quantum dots for electron spin qubitsW G van der Wiel, M Stopa, T Kodera, T Hatano and S Tarucha

Rosario Fazio, NEST-INFM and Scuola Normale Superiore, Pisa, Italy

We construct the local Hamiltonian description of the Chern–Simons theory with discrete non-Abelian gauge group on a lattice. We show that the theory is fully determined by the phase factors associated with gauge transformations and classify all possible non-equivalent phase factors. We also construct the gauge invariant electric-field operators that move fluxons around and create/anihilate them. We compute the resulting braiding properties of the fluxons. We apply our general results to the simplest class of non-Abelian groups, the dihedral groups D_n.

The entanglement transfer from electrons localized in a pair of quantum dots to circularly polarized photons is governed by optical selection rules, enforced by conservation of angular momentum. We point out that the transfer cannot be achieved by means of unitary evolution unless the angular momentum of the two initial qubit states differs by 2 units of ℏ. In particular, for spin-entangled electrons, the difference in angular momentum is 1 unit—so the transfer fails. Nevertheless, the transfer can be successfully completed if the unitary evolution is followed by a measurement of the angular momentum of each quantum dot and post-processing of the photons using the measured values as input.

Scanning-gate imaging of semiconductor quantum dots (QDs) promises access to probability distributions of quantum states. It could therefore be a novel tool for designing and optimizing tailored quantum states in such systems. A detailed study of a lithographically defined semiconductor QD in the Coulomb-blockade regime is presented, making use of the scanning-gate technique at a base temperature of 300 mK. The method allows a one-by-one manipulation of electrons in the structure. The obtained images interpreted with a suitable QD model guide the way to a local investigation of the electronic interior of the QD. Future perspectives of scanning-gate experiments on QDs are discussed.

Recent experimental developments in the field of semiconductor quantum dot (QD) spectroscopy are discussed. Firstly, we report about single QD exciton two-level systems and their coherent properties in terms of single-qubit manipulations. In the second part, we report on coherent quantum coupling in a prototype 'two-qubit' system consisting of a vertically stacked pair of QDs. The interaction can be tuned in such QD molecule devices using an applied voltage as external parameter.

We discuss the Clauser–Horne (CH) inequality for electrons in mesoscopic systems formulated in terms of the full counting statistics (FCS). A derivation of such an inequality is provided for a prototype setup consisting of an entangler attached to two conducting wires, that brings pairs of spin-entangled electrons into spatially separated counters, where detection takes place. Violation of the CH inequality is analysed as a function of the various parameters characterizing the system. The effect of dephasing, which can occur in realistic wires, is also addressed. As expected, the extent of violation is monotonically suppressed by increasing dephasing strength. The CH inequality is finally applied to a three-arm normal-metallic beam splitter. Our results show that violation takes place even in the absence of interaction.

We study the spin filling of a semiconductor quantum dot using excited-state spectroscopy in a strong magnetic field. The field is oriented in the plane of the two-dimensional electron gas in which the dot is electrostatically defined. By combining the observation of Zeeman splitting with our knowledge of the absolute number of electrons, we are able to determine the ground state spin configuration for one to five electrons occupying the dot. For four electrons, we find a ground state spin configuration with total spin S = 1, in agreement with Hund's first rule. The electron g-factor is observed to be independent of magnetic field and electron number.

In this paper, we describe a possible experimental realization of Bose's idea to use spin chains for short-distance quantum communication (Bose 2003 Phys. Rev. Lett.91 207901). Josephson arrays have been proposed and analysed as transmission channels for systems of superconducting charge qubits. Here, we consider a chain of persistent-current qubits, that is appropriate for state transfer with high fidelity in systems containing flux qubits. We calculate the fidelity of state transfer for this system. In general, the Hamiltonian of this system is not of XXZ-type, and we analyse the magnitude and the effect of the terms that do not conserve the z-component of the total spin.

We present a series of measurements on nine single Cooper-pair boxes (SCBs), where the charging energy, E_C, and the Josephson coupling energy, E_J, have been varied. We have investigated both the ground state properties of the SCBs and their quantum coherent properties. The state of the SCBs could be manipulated by an external gate voltage and the charge was measured by coupling it capacitatively to a radio-frequency single-electron-transistor (RF-SET). By ramping the gate voltage and simultaneously measuring the charge of the SCBs using the RF-SET, we could measure the Coulomb staircases of the SCBs. For sufficiently low E_C the SCBs showed a fully 2e periodic Coulomb staircase. For samples with higher E_C the staircase showed a short step for odd number of charges indicating quasi-particle 'poisoning'. However, if E_C was not too large, the short step could be removed by applying a parallel magnetic field. We attribute this effect to a stronger suppression of the superconducting energy gap in the reservoir than in the box. Using microwave spectroscopy we have determined E_C and E_J for the SCBs. These values agree well with the shape of the Coulomb staircases which we measure. For a limited range of gate voltage, the SCBs were found to behave as model two-level quantum-mechanical systems. A non-adiabatic change in the induced island charge was used to bring two charge states into resonance. The resulting time evolution showed clear charge oscillations between the ground and excited state, with an amplitude above 70% and a frequency given by the energy level separation divided by Planck's constant. These oscillations had a longest coherence time of T₂ = 9 ns, at a point where the pure charge states are degenerate. The coherence time at this point was found to be limited by the relaxation rate. Away from the charge degeneracy point, the coherence time was limited by the pure dephasing rate. The dependence of T₂ on gate charge suggested that low frequency fluctuators were the main source of dephasing away from the degeneracy point.

We have studied the temperature dependence of escape phenomena in various underdamped Josephson junctions (JJs). The junctions had different Josephson coupling energies E_J which were relatively small, but larger than the charging energy E_C. Upon increasing the temperature T, we first observe the usual cross-over between macroscopic quantum tunnelling and thermally activated (TA) behaviour at temperatures k_BT ∼ ℏω_p, where ω_p is the plasma frequency of the junction. Increasing T further, the width of the switching current distribution has, counterintuitively, a non-monotonic temperature dependence. This can be explained by the novel cross-over from TA behaviour to underdamped phase diffusion. We show that this cross-over is expected to occur at temperatures such that k_BT ∼ E_J(1 − 4/πQ)³/2, where Q is the quality factor of the junction at the plasma frequency, in agreement with experiment. Our findings can be compared with detailed model calculations which take into account dissipation and level quantization in a metastable well.

Particular attention is paid to the sample with the smallest E_J, which shows extensive phase diffusion even at the lowest temperatures. This sample consists of a dc-SQUID and a single JJ close to each other, such that the SQUID acts as a tunable inductive protection for the single junction from fluctuations of a dissipative environment. By varying the flux through the dc-SQUID, we present, for the first time, experimental evidence of the escape of a JJ from the phase diffusion regime to the free running state in a tunable environment. We also show that in the zero voltage state the losses mainly occur at frequencies near the plasma resonance.

We investigate the design and functionality of a network of loop-shaped charge qubits with switchable nearest-neighbour coupling. The qubit coupling is achieved by placing large Josephson junctions (JJs) at the intersections of the qubit loops and selectively applying bias currents. The network is scalable and makes it possible to perform a universal set of quantum gates. The coupling scheme allows gate operation at the charge degeneracy point of each qubit, and also applies to charge-phase qubits. Additional JJs included in the qubit loops for qubit readout can also be employed for qubit coupling.

We review the theoretical aspects of pseudo-spin quantum computation using vertically coupled quantum dots in the quantum Hall regime. We discuss the robustness and addressability of these collective, charge-based qubits. The low-energy Hilbert space of a coupled set of qubits yields an effective quantum Ising model tunable through external gates. An experimental prediction of an even–odd effect in the Coulomb-blockade spectra of the coupled quantum dot system allows for a probe of the parameter regime necessary for realization of these qubits.

A scattering approach to entanglement in mesoscopic conductors with independent fermionic quasi-particles is discussed. We focus on conductors in the tunnelling limit, where a redefinition of the quasi-particle vacuum transforms the wavefunction from a many-body product state of non-interacting particles to a state describing entangled two-particle excitations out of the new vacuum (Samuelsson, Sukhorukov and Büttiker 2003 Phys. Rev. Lett.91 157002). The approach is illustrated with two examples: (i) a normal–superconducting system, where the transformation is made between Bogoliubov–de Gennes quasi-particles and Cooper pairs, and (ii) a normal system, where the transformation is made between electron quasi-particles and electron–hole pairs. This is compared to a scheme where an effective two-particle state is derived from the manybody scattering state by a reduced density matrix approach.

Image formation in focusing mirrors for surface plasmon polaritons (SPPs) is studied numerically. The transition from diffractive to geometrical optics is traced as a function of mirror size and surface polariton wavelength. The role of the mirror shape and the SPP propagation length in the image formation is also investigated. The small losses are shown to be helpful for imaging with surface waves, resulting in the increased contrast and reduced background of the images. Surface polariton focusing mirrors have recently found important applications in high-resolution optical microscopy based on short-wavelength SPP.

We present a qutrit quantum computer design using trapped ions in the presence of a magnetic field gradient. The magnetic field gradient induces a 'spin–spin' type coupling, similar to the J-coupling observed in molecules, between the qutrits which allows conditional quantum logic to take place. We describe in some detail how one can execute specific one and two qutrit quantum gates, required for universal qutrit quantum computing.

Plasma polymerization is a versatile and important technique for depositing uniform, pinhole-free and flawless thin films of organic materials. So plasma-polymerized N,N,3,5-tetramethylaniline (PPTMA) thin films were deposited onto glass substrates at room temperature by a capacitively coupled plasma polymerization system using TMA as a precursor. Infrared spectroscopy, elemental analysis and UV–visible (UV–vis) spectroscopy reveal that there are conjugations in the matrix of the PPTMA thin films. From UV–vis spectroscopy it is found that indirect energy gap varies from 1.49 to 1.86 eV with film thickness. Current density–voltage characteristics indicate that the conduction mechanism in PPTMA thin films is space charge-limited conduction (SCLC). The activation energies in the SCLC region are 0.21 ± 0.05 and 0.93 ± 0.08 eV at lower and higher temperature regions respectively. Electrical and optical measurements suggest that the top of valance band and the bottom of the conduction band may have gap states and the middle of the energy gap may be equal to the high-temperature activation energy.

A scheme is proposed to teleport charge qubits via super-radiance. Reservoir-induced entanglement is generated between two semiconductor dots in a microcavity where a quantum state encoded in a third quantum dot is then tuned into collective decay with one of the entangled dots. Teleportation is achieved automatically in our scheme which we also extend to quantum wires.

We report on the occurrence of negative-phase-velocity (NPV) planewave propagation in the ergosphere of a rotating black hole. By implementing the Kerr metric, it is demonstrated that regions of NPV propagation are concentrated at the equator of the ergosphere, while NPV propagation is less common towards the polar regions. Increasing the angular velocity of the black hole exaggerates the NPV concentration at the equator. NPV propagation is not observed outside the stationary limit surface.

The stabilizer formalism allows the efficient description of a sizeable class of pure as well as mixed quantum states of n-qubit systems. That same formalism has important applications in the field of quantum error correcting codes, where mixed stabilizer states correspond to projectors on subspaces associated with stabilizer codes. In this paper, we derive efficient reduction procedures to obtain various useful normal forms for stabilizer states. We explicitly prove that these procedures will always converge to the correct result and that these procedures are efficient in that they only require a polynomial number of operations on the generators of the stabilizers. On one hand, we obtain two single-party normal forms. The first, the row-reduced echelon form, is obtained using only permutations and multiplications of generators. This form is useful to calculate partial traces of stabilizer states. The second is the fully reduced form, where the reduction procedure invokes single-qubit operations and CNOT operations as well. This normal form allows for the efficient calculation of the overlap between two stabilizer states, as well as of the Uhlmann fidelity between them, and their Bures distance. On the other hand, we also find a reduction procedure of bipartite stabilizer states, where the operations involved are restricted to be local ones. The two-party normal form thus obtained lays bare a very simple bipartite entanglement structure of stabilizer states. To wit, we prove that every bipartite mixed stabilizer state is locally equivalent to a direct product of a number of maximally entangled states and, potentially, a separable state. As a consequence, using this normal form we can efficiently calculate every reasonable bipartite entanglement measure of mixed stabilizer states.

A full electromagnetic calculation is performed to investigate two-dimensional near-field imaging using a 'Pendry' lens. We demonstrate that in spite of a positive magnetic permeability satisfactory resolution is possible for a thin silver slab, or a multilayer structure, at certain optical frequencies. These analytical results are confirmed by using a novel finite-element numerical simulation, which makes use of linear superpositions of one-dimensional field components. The simulation displays clearly the excited surface plasmons. The results in the electrostatic limit are also given for comparison purposes.

Negative refraction occurs when a beam of light is refracted at an interface, somewhat unexpectedly at first glance, not into the usual quarter-space seen in diagrams in textbooks on electromagnetics and optics, but into the other quarter-space left blank in those diagrams. This phenomenon is perfectly in accord with the principle normally referred to as Snell's Law (usually attributed to Willebrord van Royen van Snel), and had been forecast in 1968 by the Russian physicist Victor Veselago. It was first demonstrated experimentally as recently as 2000 by Sheldon Schultz and David Smith of the University of California at San Diego. At that time John Pendry of Imperial College London showed that one of the remarkable properties of a negatively refracting planar slab is to act as a 'perfect lens', focusing beyond the text book limits set by diffraction of propagating waves, at least within limited frequency ranges.

During the last five years, research on negative refraction has increased exponentially, and on several fronts. Negative refraction by homogeneous materials has been shown to be engendered by the opposition of the phase velocity vector and the time-averaged Poynting vector. Such materials, although fabricated with a microstructure, are effectively homogenous in the frequency regimes wherein negative refraction is observed. The frequency regimes can lie anywhere from ~10 GHz to ~ 100 THz today, and efforts to push to the visible part of the electromagnetic spectrum continue at a fast and furious pace.

Negative refraction by periodically inhomogeneous substances such as photonic crystals has also been demonstrated, theoretically as well as experimentally. There also exists a case of positive refraction by certain crystals that masquerades as negative refraction. Negative refraction of acoustic waves is also being studied.

The experimental confirmation of negative refraction breathed new life in the electromagnetics research community. A host of observable and exploitable effects have been identified, as may be readily noted from the papers included in this Focus Issue on Negative Refraction.

Focus on Negative Refraction Contents

Diffraction by a grating made of an uniaxial dielectric–magnetic medium exhibiting negative refraction R A Depine and A Lakhtakia

Negative refraction in periodic and random photonic crystals D Felbacq and G Bouchitte

Imaging of extended objects by a negative refractive index slab J L Garcia Polmer and M Nieto-Vesperinas

Minimization of losses in a structure having a negative index of refractionG Dewar

Sub-diffraction imaging with compensating bilayers D Schurig and D R Smith

A two-dimensional uniplanar transmission-line metamaterial with a negative index of refraction F Elek and G V Eleftheriades

Negative refraction in 2-D checkerboards by mirror anti-symmetry and 3-D corner lenses S Guenneau, A C Vutha and S Anantha Ramakrishna

Negative phase velocity in a material with simultaneous mirror-conjugated and racemic chirality characteristics T G Mackay and A Lakhtakia

Reducing losses and dispersion effects in multilayer metamaterial tunnelling devices J D Baena, L Jelinek and R Marqués

The challenge of homogenization in metamaterials C Caloz, A Lai and T Itoh

Investigation of magnetic resonances for different split ring resonator parameters and designs K Aydin, I Bulu, K Guven, M Kafesaki, C M Soukoulis and E Ozbay

Gyrotropic impact upon negatively refracting surfaces Allan Boardman, Neil King, Yuriy Rapoport and Larry Velasco

On subwavelength and open resonators involving metamaterials of negative refraction index Sailing He, Yi Jin, Zhichao Ruan and Jinguo Kuang

Birefringent left-handed metamaterials and perfect lenses for vectorial fields Alexander A Zharov, Nina A Zharova, Roman E Noskov, Ilya V Shadrivov and Yuri S Kivshar

Compact size highly directive antennas based on the SRR metamaterial medium Irfan Bulu, Humeyra Caglayan, Koray Aydin and Ekmel Ozbay

Realization of optical superlens imaging below the diffraction limit Hyesog Lee, Yi Xiong, Nicholas Fang, Werayut Srituravanich, Stephane Durant, Muralidhar Ambati, Cheng Sun and Xiang Zhang

Akhlesh Lakhtakia, Pennsylvania State University, USA and Imperial College, London, UK Martin McCall, Imperial College, London, UK

We investigate the magnetic resonance of split-ring resonators (SRR) experimentally and numerically. The dependence of the geometrical parameters on the magnetic resonance frequency of SRR is studied. We further investigate the effect of lumped capacitors integrated to the SRR on the magnetic resonance frequency for tunable SRR designs. Different resonator structures are shown to exhibit magnetic resonances at various frequencies depending on the number of rings and splits used in the resonators.

The problem of homogenization, i.e. substantial reduction of the electrical size of the average lattice constant, of electromagnetic left-handed (LH), or more generally composite right/left-handed (CRLH) metamaterials (MTMs), is presented. The undesirable effects of an electrical very large structural unit cell are explained and illustrated by full-wave simulations and experiments with CRLH mushroom structures. These effects are limited bandwidth, anisotropy, poor refraction due to coupling of the fundamental wave with space harmonics and diffraction at interfaces with other media. Transmission line method (TLM) simulations demonstrate that homogenization may mitigate or suppress these parasitic effects. In addition, the challenging necessity of drastically increasing inductances and capacitances in the homogenization process is demonstrated for CRLH MTMs.

This paper focuses on reduction of losses and dispersion effects on tunnelling through waveguides filled with metamaterial. It will be shown that these unwanted effects could be reduced by dividing the metamaterial into several regions separated by air slabs. In the first part, these effects will be studied for isotropic left-handed media (LHM). Later this will be substituted by an anisotropic magnetic medium which will lead to a practical realization with broadside coupled split ring resonators (BC-SRRs). Finally, it is shown that quasi-perfect tunnelling is possible, even in the presence of unavoidable losses and dispersion in the metamaterial.

The propagation of electromagnetic plane waves in a material with simultaneous mirror-conjugated and racemic chirality (SMCRC) characteristics is investigated. General conditions for negative phase velocity (NPV) (i.e., phase velocity directed opposite to the time-averaged Poynting vector) are derived for both unirefringent and birefringent propagation. Through numerical studies, it is demonstrated that NPV propagation arises provided that the magnitude of the magnetoelectric constitutive parameter is sufficiently large compared with the magnitudes of dielectric and magnetic constitutive parameters. However, the relative magnitude of the magnetoelectric constitutive parameter has little bearing upon the directions, which support NPV propagation. The propensity for NPV propagation is much enhanced through incorporating dielectric and magnetic constitutive parameters, which are negative-real. A wide range of constitutive parameter values are considered in order to accommodate the possibilities offered by the fabrication of artificial SMCRC materials.

We investigate the electromagnetic response of a periodic checker- board consisting of alternating rectangular cells of positive refractive index (ε = +1, μ = +1) and negative refractive index (ε = −1, μ = −1). We show that the system has peculiar imaging properties in that it reproduces images of a source in one cell in every other cell. Using coordinate transformations, we map this system into a class of imaging systems in three dimensions consisting of three orthogonal planes delimiting eight alternating cubical regions of positive and negative index media sharing the same vertex. We also generalize these results to more general checkerboards that are inhomogeneous and anisotropic that can then be used to generate a class of three-dimensional (3D) corner imaging systems.

A uniplanar transmission-line (TL) network has been loaded with lumped elements (chip or printed), enabling one to achieve a two-dimensional (2D) uniplanar negative-refractive-index (NRI) metamaterial. The metamaterial consists of a 2D array of unit cells, composed of TL sections connected in series and loaded in a specified manner. The unit cell dimensions can be designed to be much smaller than the operating wavelength, enabling one to identify the structure as an effective medium, with a negative index of refraction. This NRI metamaterial supports transverse electric (TE) waves, as opposed to related previous work on NRI-TL media that supported transverse magnetic (TM) waves. The dispersion characteristics are calculated using a simple, fast 2D loaded TL model with periodic (Bloch) boundary conditions. Subsequently the dispersion relation is simplified in the homogeneous limit, thus allowing one to identify effective permittivities and permeabilities, which are shown to be simultaneously negative. Simulations demonstrating the negative refraction of a plane wave on an interface between such a NRI uniplanar metamaterial and a commensurate positive-refractive-index (PRI) metamaterial verify the validity of the proposed concept and theory. A fully printed unit cell is presented at microwave frequencies (∼10 GHz) along with a prescription for synthesizing an isotropic 3D transmission line NRI metamaterial based on this unit cell.

We derive a general expression for the material properties of a compensating bilayer, which is a pair of material layers which transfer the field distribution from one side of the bilayer to the other with resolution limited only by the deviation of the material properties from specified values. One of the layers can be free space, a special case of which is the perfect lens, but the layers need not have equal thickness. Compensating a thick layer of free space with a thin layer creates a focusing device with increased working distance, and employs an anisotropic material. It is also possible to achieve compensation of materials with property tensors that are neither positive nor negative definite. In this case, we refer to such media as indefinite, and we analyse, in detail, bilayers of these media which support coupling of internal propagating waves to incident waves of any transverse wave vector. In this case, we find that the enhanced spatial resolution provided by large transverse wave vectors is far less sensitive to loss than that of the perfect lens.

A structure consisting of an array of wires cladded with a nonmagnetic dielectric and embedded in a ferrimagnetic host has been calculated to have a negative index of refraction. The structure has moderate losses over a bandwidth of a few GHz. The calculation takes into account the skin effect within the wires and is valid provided the wavelength of electromagnetic waves in the structure is long compared to the radius of the cladded wires. The structure's electromagnetic response is accurately described by the ferrimagnet's permeability and a permittivity derived in the long wavelength limit. Losses can be minimized by choosing the pass band to be between 30 and 80% of the plasma frequency and by choosing wires to be of the highest possible conductivity and largest radius compatible with the required plasma frequency.

Using a finite element method, we numerically study the imaging of an extended object by slabs of media with negative refractive index (within an effective medium theory). We analyse the consequences of possible deviations of the refractive index of the slab from the archetypal value of n = −1. These variations are obtained by introducing losses in the material and also by changing the real part of n. In this way, we show how slight changes in the refractive index from n = −1 affect the resolution of the image of the extended object.

We present a theory of the artificial magnetic activity in a two-dimensional (2D) dielectric photonic crystal. We show, by using a multiple-scale approach, that if the rods constituting the crystal have Mie resonances at large enough wavelengths, the crystal can be characterized by an effective permeability exhibiting anomalous dispersion. The theoretical results are checked numerically for periodic and random photonic crystals.

Diffraction of linearly polarized plane electromagnetic waves at the periodically corrugated boundary of vacuum and a linear, homogeneous, uniaxial, dielectric–magnetic medium is formulated as a boundary-value problem and solved using the Rayleigh method. The focus is on situations where the diffracted fields maintain the same polarization state as the s- or p-polarized incident plane wave. Attention is paid to two classes of diffracting media: those with negative definite permittivity and permeability tensors, and those with indefinite permittivity and permeability tensors. For the situations investigated, whereas the dispersion equations in the diffracting medium turn out to be elliptic for the first class of diffracting media, they are hyperbolic for the second class. Examples are reported with the first class of diffracting media of instances when the grating acts either as a positively refracting interface or as a negatively refracting interface. For the second class of diffracting media, hyperbolic dispersion equations imply the possibility of an infinite number of refraction channels.

Water flow through quasi two-dimensional percolation model objects was studied with the aid of NMR velocity and acceleration mapping techniques. The model objects were fabricated based on computer generated templates of Ising-correlated percolation clusters of different growth/nucleation ratios for the occupation of the base lattice sites. The same pore networks were used for computational fluid dynamics simulations of hydrodynamic transport properties including hydrodynamic dispersion of tracer particles. The percolation threshold turned out to be lower than that in the uncorrelated case and adopts a minimum if cluster growth is about 100 times more likely than nucleation of the new clusters. The experimental and simulated flow velocity and acceleration maps coincide in great detail. The data have been analysed in terms of histograms and spatial autocorrelation functions. Furthermore, the travelling time of a tracer particle across percolation clusters was evaluated as a function of the Péclet number.

Quantum walks, both discrete (coined) and continuous time, form the basis of several recent quantum algorithms. Here we use numerical simulations to study the properties of discrete, coined quantum walks. We investigate the variation in the entanglement between the coin and the position of the particle by calculating the entropy of the reduced density matrix of the coin. We consider both dynamical evolution and asymptotic limits for coins of dimensions from two to eight on regular graphs. For low coin dimensions, quantum walks which spread faster (as measured by the mean square deviation of their distribution from uniform) also exhibit faster convergence towards the asymptotic value of the entanglement between the coin and particle's position. For high-dimensional coins, the DFT coin operator is more efficient at spreading than the Grover coin. We study the entanglement of the coin on regular finite graphs such as cycles, and also show that on complete bipartite graphs, a quantum walk with a Grover coin is always periodic with period four. We generalize the 'glued trees' graph used by Childs et al (2003 Proc. STOC, pp 59–68) to higher branching rate (fan out) and verify that the scaling with branching rate and with tree depth is polynomial.

The Hong–Ou–Mandel (HOM) dip has played an important role in recent linear optics experiments. It is crucial for quantum computing with photons and can be used to characterize the quality of single photon sources and linear optics setups. In this paper, we consider generalized HOM experiments with N bosons or fermions passing simultaneously, i.e. within their coherence time, through a symmetric Bell multiport beam splitter. It is shown that for an even number of bosons, the HOM dip occurs naturally in the coincidence detection in the output ports. In contrast, fermions always leave the setup separately, exhibiting perfect coincidence detection. Our results can be used to verify or employ the quantum statistics of particles experimentally.

We present an experimental study of second-harmonic generation in the light reflected from a flat silver surface. It is discussed that the harmonic generation from such a surface may be expressed in terms of the three unique elements of its effective surface susceptibility tensor. A method is proposed to determine the susceptibilities by measuring the second-harmonic power with different polarization conditions. By employing a picosecond light source and photon-counting techniques, we determine the susceptibilities and compare our results with previous work.

The nonlinear coupling between two perpendicularly propagating (with respect to the external magnetic field direction) upper-hybrid (UH) waves in a uniform magnetoplasma is considered, taking into account quasi-stationary density perturbations which are driven by the UH wave ponderomotive force. This interaction is governed by a pair of coupled nonlinear Schrödinger equations (CNLSEs) for the UH wave envelopes. The CNLSEs are used to investigate the occurrence of modulational instability. Waves in the vicinity of the UH resonance are considered, so that the group dispersion terms for both waves are approximately equal, but the UH wave group velocities may be different. It is found that a pair of unstable UH waves (obeying anomalous group dispersion) yields an increased instability growth rate, while a pair of stable UH waves (individually obeying normal group dispersion) remains stable for equal group velocities, although it is destabilized by a finite group velocity mismatch. Stationary nonlinear solutions of the CNLSEs are presented.

The concept of fidelity decay is discussed from the point of view of the scattering matrix, and the 'scattering fidelity' is introduced as the parametric cross-correlation of a given S-matrix element, taken in the time domain, normalized by the corresponding autocorrelation function. We show that for chaotic systems, this quantity represents the usual fidelity amplitude, if appropriate ensemble and/or energy averages are taken. We present a microwave experiment where the scattering fidelity is measured for an ensemble of chaotic systems. The results are in excellent agreement with random matrix theory for the standard fidelity amplitude. The only parameter, namely the perturbation strength, could be determined independently from level dynamics of the system, thus providing agreement between theory and experiment without any free fit parameter.

Passively mode-locked lasers are extended one-dimensional dynamical systems subject to noise, with a nonlinear instability and a global power constraint. We use the recent understanding of the importance of entropy in these systems to study mode locking thermodynamically. We show that this class of problems is solvable by a mean field-like theory, where the nonlinear pulse free energy and entropic continuum free energy compete on the available power, and calculate explicitly the pulse power and mode locking, which occurs when the dimensionless scaled interaction strength γ = 9. A transfer matrix calculation shows that the mean field theory is exact in the thermodynamic limit, where the number of active laser modes tends to infinity.

Up-to-date estimates of proton–proton total cross-sections, σ_tot^pp, at very high energies in the literature were obtained from cosmic rays (>10¹⁷ eV) by approximations using the measured proton–air cross-section at these energies. As σ_tot^pp are measured with present day high energy colliders up to nearly 2 TeV in the centre of mass (∼10¹⁵ eV in the laboratory), several proven theoretical, empirical and semi-empirical parametrizations for interpolation at accelerator energies were used to extrapolate these measured values to get reasonable estimates of cross-sections at higher cosmic ray energies (∼10¹⁷ eV). The cross-section estimates from these two methods disagree by a discrepancy beyond statistical error. Here we use a phenomenological model based on the 'multiple diffraction' approach to successfully describe data at accelerator energies. Using this model, we then estimate σ_tot^pp at cosmic ray energies. The model free-parameters used in the fit depend on only two physical observables: the differential cross-section and the parameter ρ. The model estimates of σ_tot^pp are then compared with total cross-section data. Using regression analysis, we determine confidence error bands, analysing the sensitivity of our predictions to the data used in the extrapolations. This work reduces the width of the confidence band around 'multiple diffraction' model fits of accelerator data. With the data at 546 GeV and 1.8 TeV, our extrapolations are compatible with only the Akeno cosmic ray data, predicting a slower rise with energy than do other cosmic ray results and other extrapolation methods. We discuss our results within the context of constraints expected from future accelerator and cosmic ray experimental results.

By using the abstract linear-optical network derived by Scheel and Lütkenhaus (2004 New J. Phys.6 51) we show that for the lowest possible ancilla photon numbers the probability of success of realizing a (single-shot) generalized nonlinear sign-shift gate on an (N + 1)-dimensional signal state scales as 1/N². We limit ourselves to single-shot gates without conditional feed-forward. We derive our results by using determinants of Vandermonde-type over a polynomial basis which is closely related to the well-known Jacobi polynomials.

Measurement of lung ventilation is one of the most reliable techniques in diagnosing pulmonary diseases. The time-consuming and bias-prone traditional methods using hyperpolarized H³He and ¹H magnetic resonance imageries have recently been improved by an automated technique based on 'multiple active contour evolution'. This method involves a simultaneous evolution of multiple initial conditions, called 'snakes', eventually leading to their 'merging' and is entirely independent of the shapes and sizes of snakes or other parametric details. The objective of this paper is to show, through a theoretical analysis, that the functional dynamics of merging as depicted in the active contour method has a direct analogue in statistical physics and this explains its 'universality'. We show that the multiple active contour method has an universal scaling behaviour akin to that of classical nucleation in two spatial dimensions. We prove our point by comparing the numerically evaluated exponents with an equivalent thermodynamic model.

We investigated optical excitations of transition-metal (TM) oxides with metal oxygen octahedra taking account of the orbital multiplicity effects. We predicted excitation energies of intersite d–d transitions and p–d transitions of TM oxides. We compared the evaluated excitation energies with reported experimental data, and found that they are in good agreement with each other. Moreover, we could demonstrate possible answers for a few long-standing problems of the low-frequency spectral features in some early 3d TM oxides: (i) the broad and multi-peak structures of the d–d transitions, (ii) the low values (around 2 eV) of the d–d transition energies for some t_2g¹ and t_2g² systems, and (iii) the lack of the d–d transition below 4.0 eV region for LaCrO₃, one of the t_2g³ systems. These indicate that our approach considering the orbital multiplicity effects could provide good explanations of intriguing features in the optical spectra of some early TM oxides. In addition, we showed that optical spectroscopy can be useful as a powerful tool to investigate spin and/or orbital correlations in the TM ions. Finally, we discussed the implications of the orbital multiplicity in the Zannen–Sawatzky–Allen scheme, which has been used successfully to classify correlated electron systems.

We provide an exact analytic description of decelerating, stopping and reaccelerating optical solitons in atomic media in the non-adiabatic regime. Dynamical control over slow-light pulses is realized via a nonlinear interplay between the solitons and the controlling field generated by an auxiliary laser. This leads to recovery of optical information. We discuss physically interesting features of our solution, which are in good agreement with recent experiments.

A population of complete subgraphs or cliques in a protein network model is studied. The network evolves via duplication and divergence supplemented with linking a certain fraction of target–replica vertex pairs. We derive a clique population distribution, which scales linearly with the size of the network and is in perfect agreement with numerical simulations. Fixing both parameters of the model so that the number of links and abundance of triangles are equal to those observed in the fruitfly protein-binding network, we precisely predict the 4- and 5-clique abundance. In addition, we show that such features as fat-tail degree distribution, various rates of average degree growth and non-averaging, revealed recently for a particular case of a completely asymmetric divergence, are present in a general case of arbitrary divergence.

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO₃ (R = La, Sm and Y), LaMnO₃, Y₂BaNiO₅, CaCu₂O₃ and K₄Cu₄OCl₁₀, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO₃, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yields good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g. the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular, in case of the orbital excitations at ≈0.25 eV in RTiO₃. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.

When initially introduced, a Hamiltonian that realizes perfect transfer of a quantum state was found to be analogous to an x-rotation of a large spin. In this paper, we extend the analogy further to demonstrate geometric effects by performing rotations on the spin. Such effects can be used to determine properties of the chain, such as its length, in a robust manner. Alternatively, they can form the basis of a spin network quantum computer. We demonstrate a universal set of gates in such a system by both dynamical and geometrical means.

We present an experimental test of Corrsin's conjecture against experimental data obtained by a particle tracking technique in approximately homogeneous and isotropic turbulent flow at Reynolds numbers R_λ ≈ 100. The conjecture states that where R_L(t − t') = ⟨v(t)·v(t')⟩ is the Lagrangian velocity covariance function, G is the single particle mean Green's function, and R_E(x − x', t − t') ≡ ⟨u(x, t)·u(x', t')⟩ is the Eulerian two-point, two-time velocity covariance function. All terms in the relation have been measured in the experiment. The equation is exact if a conditional Lagrangian velocity function R_L(t|x) is inserted in place of R_E(x, t) on the right-hand side. R_L(t|x) is obtained by restricting sampling of the two velocities to situations where both belong to the same fluid particle trajectory. The experimental data show that the R_E(x, t) and R_L(t|x) behave fundamentally differently, thereby seriously questioning the rationale of the conjecture. The estimate of R_L(t), based on Corrsin's conjecture and the experimentally determined R_E and G, is found to decrease too fast compared to the directly measured R_L, thus underestimating the Lagrangian timescale by about 40%. Even asymptotically (t → ∞) the estimate is considerably lower than the measured Lagrangian correlation function. The simpler relation R_L(t) = R_E(0, t), which has also been attributed to Corrsin, appears to agree much better with data. Various simple physical models of the spatiotemporal Eulerian correlation function have been compared with data, and models inspired by eddy sweeping seem to perform well.

The 2s^2 1S^e autoionization resonance state of the hydrogen negative ion embedded in Debye plasmas is determined by calculating the density of resonance states using the stabilization method. The electron affinity of the hydrogen atom is also estimated for various Debye lengths. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the charged particles. A correlated wave function consisting of a generalized exponential expansion has been used to represent the correlation effect between the three charged particles. The screening effect is taken care of for all pairs of the charged particles. The calculated resonance energies and widths for various Debye parameters ranging from infinity to a small value along with the electron affinity are reported.

We have carried out extensive first-principles computations of angle-resolved photoemission (ARPES) spectra from the cuprate superconductors within the general framework of the local density approximation (LDA). Selected results on Bi₂Sr₂CaCu₂O₈ (Bi2212), La_2−xSr_xCuO₄ (LSCO) and Nd_2−xCe_xCuO₄ (NCCO) are presented and discussed. Our focus is on understanding how the underlying electronic structure is mapped via the complex process of photoexcitation into the observed ARPES intensities. Effects of the ARPES matrix element and its remarkable selectivity properties with respect to the energy and polarization of the incident photons in exciting a specific state and/or electrons from a particular site in the lattice are clarified. The importance of deviations from perfect two-dimensionality and the associated interlayer couplings in shaping the ARPES spectra of the cuprates is delineated. Our computations explain many salient features of the experimental spectra. Surprisingly, this agreement extends in some cases to the underdoped regime where strong electron correlations are obviously important. We discuss how the LDA-inspired tight-binding parameters can serve as a useful starting point for the treatment of strong coupling effects in the cuprates.

Formation and motion of copper adatoms and addimers on Ag(1 1 1) are investigated with low-temperature scanning tunnelling microscopy between 6 and 25 K. Adatoms move between fcc and hcp sites with a strong preference for the fcc site. Adatom motion and dimer rotation change due to the presence of other adatoms or dimers. Furthermore, rotating dimers influence other rotating dimers. These changes are attributed to changes in the diffusion or rotation potential, which are mediated by the electrons in the two-dimensional surface state band.

The classical dynamics of the hydrogen atom in parallel static and microwave electric fields are analysed. This work is motivated by recent experiments on excited hydrogen atoms in such fields, which show enhanced resonant ionization at certain combinations of field strengths.

By analysing the dynamics using an appropriate representation and averaging approximations, a simple picture of the ionization process is obtained. This shows how the resonant dynamics are controlled by a separatrix that develops and moves through phase space as the fields are switched on and provides necessary conditions for a dynamical resonance to affect the ionization probability. In addition, these methods yield a simple approximate Hamiltonian that facilitates quantal calculations.

Using high-order perturbation theory, we obtain a series expansion for the position of the dynamical resonance and an estimate for its radius of convergence. Because, unusually, the resonance island moves through the phase space, the position of the dynamical resonance does not coincide precisely with the ionization maxima. Moreover, there are circumstances in which the field switch-on time dramatically affects the classical ionization probability; for long switch times, it reflects the shape of the incipient homoclinic tangle of the initial state, making it impossible to predict the resonance shape. Additionally, for a similar reason, the resonance ionization time can reflect the timescale of the motion near the separatrix, which is therefore much longer than conventional static field Stark ionization. All these effects are confirmed using accurate Monte Carlo calculations using the exact Hamiltonian.

The dynamical structures producing these effects are present in the quantum dynamics; so we conclude that, for sufficiently large principal quantum numbers, the effects seen here will also be observed in the quantum dynamics.

Quantum information processing (QIP) offers the promise of being able to do things that we cannot do with conventional technology. Here we present a new route for distributed optical QIP, based on generalized quantum non-demolition measurements, providing a unified approach for quantum communication and computing. Interactions between photons are generated using weak nonlinearities and intense laser fields—the use of such fields provides for robust distribution of quantum information. Our approach only requires a practical set of resources, and it uses these very efficiently. Thus it promises to be extremely useful for the first quantum technologies, based on scarce resources. Furthermore, in the longer term this approach provides both options and scalability for efficient many-qubit QIP.

We present a numerical study of the surfing mechanism in which electrons are trapped in Bernstein–Greene–Kruskal (BGK) modes, and are accelerated across the magnetic field direction by the Lorentz force in magnetized space plasmas. The BGK modes are the product of an ion-beam Buneman instability that excites large-amplitude electrostatic upper-hybrid waves in the plasma. Our study, which is performed with particle-in-cell (PIC) and Vlasov codes, reveals the stability of the BGK mode as a function of the magnetic field strength and the ion beam speed. It is found that the surfing acceleration is more effective for a weaker magnetic field owing to the longer lifetime of the BGK modes. The importance of our investigation to electron acceleration in astrophysical environments has been emphasized.

We suggest a scheme that allows arbitrarily perfect state transfer even in the presence of random fluctuations in the couplings of a quantum chain. The scheme performs well for both spatially correlated and uncorrelated fluctuations if they are relatively weak (say 5%). Furthermore, we show that given a quite arbitrary pair of quantum chains, one can check whether it is capable of perfect transfer by only local operations at the ends of the chains, and the system in the middle being a 'black box'. We argue that unless some specific symmetries are present in the system, it will be capable of perfect transfer when used with dual-rail encoding. Therefore our scheme puts minimal demand not only on the control of the chains when using them, but also on the design when building them.

We study the point-spread and optical transfer-function (OTF) of a stimulated emission depletion (STED)-4Pi fluorescence microscope that provides diffraction unlimited resolution along the optic axis. Our calculations take into account the orientation of the linear transition dipole moment of the fluorescent molecules with respect to that of the focal field. We demonstrate a subdiffraction axial resolution of 44–48 nm for water-immersion lenses, corresponding to a 7–8-fold expansion of the OTF beyond the diffraction barrier of a single lens confocal microscope, which is in excellent agreement with theoretical predictions for the conditions applied. Furthermore, we study phase modifications of the wavefront of the stimulating beam that strengthen weakly transferred frequencies within the OTF support. The enlarged bandwidth enables the separation of objects at 76 nm axial distance.

We have built a model organic field-effect transistor that is basically composed of a single layer of pentacene crystal in interaction with an oxide surface. Drain and source contacts are ohmic so that the pentacene layer can carry a current density as high as 3000 A cm⁻² at a gate voltage of –60 V. Four-probe and two-probe transport measurements as a function of temperature and fields are presented in relation with structural near-field observations. The experimental results suggest a simple two-dimensional model where the equilibrium between free and trapped carriers at the oxide interface determines the OFET characteristics and performance.

Ion transport through single-cation selective nanopores in thin polymer foils is examined experimentally and described by a diffusional model based on the reduction of the three-dimensional Smoluchowski equation into a one-dimensional equation of Fick–Jacobs type. The model enables semi-quantitative predictions of the transport properties of nanopores of various shapes and surface charge properties even when bulk electrolyte values of various parameters are used. The experimental conductivity data clearly indicate the presence of a surface current component not described by the bulk-type diffusion. The values of the measured surface conductivities depend, among others, on the properties of the channel's internal surface. These surface currents play a substantial role in the rectification processes and are partially responsible for the high-cation selectivity of nanopores.

Both theory and electrolytic conductivity measurements show that asymmetric nanopores partially rectify the current with a preferential direction of cation flow from a pore of high surface charge density towards a pore of low surface charge density and/or from the narrow towards the wide opening of the pore.

Room-temperature carrier dynamics as functions of heavy-ion implantation and subsequent thermal annealing were investigated for technologically important InGaAs/GaAs quantum wells (QWs) by means of a time-resolved up-conversion method. Sub-picosecond lifetimes were achieved at 10 MeV Ni⁺ doses of (20–50) × 10¹⁰ ions cm⁻². The decay rates reached a maximum at the highest irradiation dose, yielding the shortest lifetime of the confined QW states of 600 fs. A simple theoretical model is proposed for the photodynamics of the carriers. The relaxation rate depended on the irradiation dose according to a power law of 1.2, while the irradiated and subsequently annealed samples exhibited a power law of 0.35. The results are qualitatively interpreted.

The possibilities of detecting high-energy neutrinos through inclined showers produced in the atmosphere are addressed with an emphasis on the detection of air showers by arrays of particle detectors. Rates of inclined showers produced by both down-going neutrino interactions and by up-coming τ decays from Earth-skimming neutrinos as a function of shower energy are calculated with analytical methods using two sample neutrino fluxes with different spectral indices. The relative contributions from different flavours and charged, neutral current and resonant interactions are compared for down-going neutrinos interacting in the atmosphere. No detailed description of detectors is attempted but rough energy thresholds are implemented to establish the ranges of energies which are more suitable for neutrino detection through inclined showers. Down-going and up-coming rates are compared.

By applying the technique of uniform asymptotic approximation to the oscillatory integrals representing tsunami wave profiles, the form of the travelling wave far from the source is calculated for arbitrary initial disturbances. The approximations reproduce the entire profiles very accurately, from the front to the tail, and their numerical computation is much faster than that of the oscillatory integrals. For one-dimensional propagation, the uniform asymptotics involve Airy functions and their derivatives; for two-dimensional propagation, the uniform asymptotics involve products of these functions. Separate analyses are required when the initial disturbance is specified as surface elevation or surface velocity as functions of position, and when these functions are even or odd.

'There was an awful rainbow once in heaven' (John Keats, 1820)

Ellipsometric microscopy is a technique for simultaneous measurement of thin film thickness and index of refraction at a lateral resolution of approximately 1 μm. Up to now this technique has been used on silicon–air interfaces. However, biological processes take place often in aqueous solution and are studied at the glass–water interface. Due to the very low reflectivity of this interface we had to improve ellipsometric microscopy substantially. Here we present our approach to suppress the intensity of internal stray light by several orders of magnitude and show quantitative and laterally resolved ellipsometric measurements at the glass–water interface. When instrumental polarization was taken into account, an accuracy of δΨ = 0.41° and δΔ = 4.3° was achieved.

Recently, spectroscopic and calorimetric observations of hydrogen plasmas and chemical reactions with them have been interpreted as evidence for the existence of electronic states of the hydrogen atom with a binding energy of more than 13.6 eV. The theoretical basis for such states, which have been dubbed hydrinos, is investigated. We discuss both the novel deterministic model of the hydrogen atom, in which the existence of hydrinos was predicted, and standard quantum mechanics. Severe inconsistencies in the deterministic model are pointed out and the incompatibility of hydrino states with quantum mechanics is reviewed.

We report quasi-particle energy calculations of the electronic bandstructure as measured by valence-band photoemission for selected II–VI compounds and group III nitrides. By applying GW as perturbation to the ground state of the fictitious, non-interacting Kohn–Sham electrons of density-functional theory (DFT), we systematically study the electronic structure of zinc-blende GaN, ZnO, ZnS and CdS. Special emphasis is put on analysing the role played by the cation semicore d-electrons that are explicitly included as valence electrons in our pseudo-potential approach. Unlike in the majority of previous GW studies, which are almost exclusively based on ground state calculations in the local-density approximation (LDA), we combine GW with exact-exchange DFT calculations in the optimized-effective potential approach (OEPx). This is a much more elaborate and computationally expensive approach. However, we show that applying the OEPx approach leads to an improved description of the d-electron hybridization compared to the LDA. Moreover, we find that it is essential to use OEPx pseudo-potentials in order to treat core–valence exchange consistently. Our OEPx-based quasi-particle valence bandstructures are in good agreement with available photoemission data in contrast to the ones based on the LDA. We therefore conclude that for these materials, OEPx constitutes the better starting point for subsequent GW calculations.

A controllable excitation of fibre modes is examined theoretically and experimentally. We verify that the modes of a few-mode fibre can be selectively excited by means of a spatial light modulation of a laser beam focused to the fibre. The proposed method is demonstrated on a dynamical switching, phase coupling and weighted mixing of LP₀₁, LP₁₁ and LP₂₁ modes, achieved by an exchange of computer-generated holograms sent to the spatial light modulator. The possibility to excite a vortex fibre mode with a well-defined azimuthal phase dependence is also verified. It is promising for an encoding and transfer of information by vortices in fibre communications.

We present an entanglement generation scheme which allows arbitrary graph states to be efficiently created in a linear quantum register via an auxiliary entangling bus (EB). The dynamical evolution of the EB is described by an effective non-interacting fermionic system undergoing mirror-inversion in which qubits, encoded as local fermionic modes, become entangled purely by Fermi statistics. We discuss a possible implementation using two species of neutral atoms stored in an optical lattice and find that the scheme is realistic in its requirements even in the presence of noise.

A mean-field theory of long-range frustration is constructed for spin glass systems with quenched randomness of vertex–vertex connections and of spin–spin coupling strengths. This theory is applied to a spin glass model of the random K-satisfiability (K-SAT) problem (K=2 or K=3). The satisfiability transition in a random 2-SAT formula occurs when the clauses-to-variables ratio α approaches α_c(2)=1. However, long-range frustration among unfrozen variable nodes builds up only when α>α_R(2)=4.4588. For the random 3-SAT problem, we find a long-range frustrated mean-field solution when α>α_R(3)=4.1897. The long-range frustration order parameter R of this solution jumps from zero to a finite positive value at α_R(3), while the energy density increases only gradually from zero as a function of α. The SAT–UNSAT transition point of this solution is lower than the value of α_c(3)=4.267 obtained by the survey propagation algorithm. Two possible reasons for this discrepancy are suggested. The zero-temperature phase diagram of the ±J Viana–Bray model is also determined, which is identical to that of the random 2-SAT problem. The predicted phase transition between a non-frustrated and a long-range frustrated spin glass phase might also be observable in real materials at a finite temperature.

We study the morphological evolution of surfaces during ion sputtering and we compare their dynamical corrugation with aeolian ripple formation in sandy deserts. We show that, although the two phenomena are physically different, they must obey similar geometrical constraints and therefore they can be described within the same theoretical framework. The present theory distinguishes between atoms that stay bounded in the bulk and others that are mobile on the surface. We describe the excavation mechanisms, the adsorption and the surface mobility by means of a continuous equation derived from the study of dune formation on sand. We explore the spontaneous development of ordered nanostructures and explain the different dynamical behaviours experimentally observed in metals or in semiconductors or in amorphous systems. We also show that this novel approach can describe the occurrence of rotation in the ripple direction and the formation of other kinds of self-organized patterns induced by changes in the sputtering incidence angle.

We derive a spin–orbital Hamiltonian for a triangular lattice of e_g orbital degenerate (Ni³⁺) transition-metal ions interacting via 90° superexchange involving (O²⁻) anions, taking into account the onsite Coulomb interactions on both the anions and the transition metal ions. The derived interactions in the spin–orbital model are strongly frustrated, with the strongest orbital interactions selecting different orbitals for pairs of Ni ions along the three different lattice directions. In the orbital-ordered phase, favoured in mean field theory, the spin–orbital interaction can play an important role by breaking the U(1) symmetry generated by the much stronger orbital interaction and restoring the three-fold symmetry of the lattice. As a result, the effective magnetic exchange is non-uniform and includes both ferromagnetic and antiferromagnetic spin interactions. Since ferromagnetic interactions still dominate, this offers yet insufficient explanation for the absence of magnetic order and the low-temperature behaviour of the magnetic susceptibility of stoichiometric LiNiO₂. The scenario proposed to explain the observed difference in the physical properties of LiNiO₂ and NaNiO₂ includes small covalency of Ni–O–Li–O–Ni bonds inducing weaker interplane superexchange in LiNiO₂, insufficient to stabilize orbital long-range order in the presence of stronger intraplane competition between superexchange and Jahn–Teller coupling.

Single-spin measurement represents a major challenge for spin-based quantum computation. In this paper, we propose a new method for measuring the spin of a single electron confined in a quantum dot (QD). Our strategy is based on entangling (using unitary gates) the spin and orbital degrees of freedom. An 'orbital qubit', defined by a second, empty QD, is used as an ancilla and is prepared in a known initial state. Measuring the orbital qubit will reveal the state of the (unknown) initial spin qubit, hence reducing the problem to the easier task of single charge measurement. Since spin-charge conversion is done with unit probability, single-shot measurement of an electronic spin can be, in principle, achieved. We evaluate the robustness of our method against various sources of error and discuss possible implementations.

Orbital excitations in orbital-ordered transition metal oxides are investigated theoretically and experimentally. Characteristic differences in the collective orbital excitations, termed orbital waves, between the e_g and t_2g systems are clarified. As a probe to detect the orbital excitations, the scattering processes for the Raman scattering and the resonant and non-resonant inelastic x-ray scatterings are proposed and their scattering cross-sections are calculated. The orbital excitations in hole-doped and undoped manganites are examined experimentally by using resonant inelastic x-ray scattering. The obtained experimental results are compared with the theoretical calculations.

The application of both a strong magnetic field and magnetic field gradient to a diamagnetic body can produce a vertical force which is sufficient to counteract its weight due to gravity. By immersing the body in a paramagnetic fluid, an additional adjustable magneto-buoyancy force is generated which enhances the levitation effect. Here we show that cryogenic oxygen and oxygen–nitrogen mixtures in both gaseous and liquid form provide sufficient buoyancy to permit the levitation and flotation of a wide range of materials. These fluids may provide an alternative to synthetic ferrofluids for the separation of minerals. We also report the dynamics of corrugation instabilities on the surface of magnetized liquid oxygen.

We use a three-dimensional Gerchberg–Saxton algorithm (Shabtay (2003) Opt. Commun.226 33) to calculate the Fourier-space representation of physically realizable light beams with arbitrarily shaped three-dimensional intensity distributions. From this representation we extract a phase-hologram pattern that allows us to create such light beams experimentally. We show several examples of experimentally shaped light beams.

A new, monolithic scheme for stabilizing the phase between the carrier wave and the envelope (CE phase) in a train of few-cycle laser pulses is demonstrated. Self-phase modulation and second-harmonic generation or difference-frequency generation in a single periodically poled lithium niobate crystal, that transmits the main laser beam, allows for the CE-phase locking directly in the usable output. The monolithic scheme obviates the need for splitting off a fraction of the laser output for CE-phase control, coupling into microstructured fibre, as well as separation and recombination of spectral components. As a result, the CE-phase error integrated over the spectral range of 0.2 mHz–35 MHz is as small as 0.016 × 2π rad. This implies that the phase of the field oscillations (λ ∼ 830 nm) with respect to the pulse peak is locked to within 44 attoseconds, resulting in optical waveform control with subhundred attosecond fidelity for the first time.

Around the turn of the last century the scientific community was puzzled by experiments on the photoelectric effect reported in 1887 by Hertz. In 1905 Einstein gave the astonishing explanation in one blow: the light is quantized. This was the beginning of quantum theory and resulted in Einstein being awarded the Nobel Prize in 1921. It was however not until the 1960s that the photoelectric effect started to become a tool in research laboratories for basic science.

The access to new, sensitive electronic equipment and ultrahigh vacuum, together with more powerful computational physics, quickly saw photoelectron spectroscopy develop into a useful scientific technique. Soon sophisticated variants of the method appeared. In particular the use of synchrotron radiation led to the tunability of the light wavelength and the use of polarized radiation. In addition an enormous increase in brightness was obtained by using electron storage rings with undulators. Many new extensions have now been added, including detection of the spin and emission angle of the electrons.

Today photoemission is one of the major tools for detailed investigations of the electronic structure of matter and contributes heavily to our understanding of the properties of matter. For example it provides the complete set of quantum numbers for electrons in a solid and has been referred to as the `smoking gun' for solving difficult puzzles in condensed matter physics.

This celebratory Focus Issue on the application of the photoelectric effect in science shows examples of the rich variety of applications of the phenomenon explained by Einstein one hundred years ago.

Focus on Photoemission and Electronic Structure Contents

Photoemission spectroscopy—from early days to recent applications Friedrich Reinert and Stefan Hüfner

On the extraction of the self-energy from angle-resolved photoemission spectroscopy Adam Kaminski and Helen M Fretwell

Self-energy determination and electron–phonon coupling on Bi(110) C Kirkegaard, T K Kim and Ph Hofmann

Metal–insulator transition in one-dimensional In-chains on Si(111): combination of a soft shear distortion and a double-band Peierls instability C González, J Ortega and F Flores

One-dimensional versus two-dimensional electronic states in vicinal surfaces J E Ortega, M Ruiz-Osés, J Cordón, A Mugarza, J Kuntze and F Schiller

Correlation in low-dimensional electronic states on metal surfaces A Menzel, Zh Zhang, M Minca, Th Loerting, C Deisl and E Bertel

Momentum-resolved dynamics of Ar/Cu(1 0 0) interface states probed by time-resolved two-photon photoemission M Rohleder, K Duncker, W Berthold, J Güdde and U Höfer

Site-specific electronic structure of an oligo-ethylenedioxythiophene derivative probed by resonant photoemission W Osikowicz, R Friedlein, M P de Jong, S L Sorensen, L Groenendaal and W R Salaneck

High-resolution ARPES study of quasi-particles in high-T_c superconductors T Takahashi, T Sato, H Matsui and K Terashima

Photoemission as a probe of coexisting and conflicting periodicities in low-dimensional solids M Grioni, Ch R Ast, D Pacilé, M Papagno, H Berger and L Perfetti

Activated adsorption of methane on Pt(1 1 1) —an in situ XPS study T Fuhrmann, M Kinne, B Tränkenschuh, C Papp, J F Zhu, R Denecke and H-P Steinrück

Electronic structure of the Si(1 1 1):GaSe van der Waals-like surface termination Reiner Rudolph, Christian Pettenkofer, Aaron A Bostwick, Jonathan A Adams, Fumio Ohuchi, Marjorie A Olmstead, Bengt Jaeckel, Andreas Klein and Wolfram Jaegermann

Can circular dichroism in core-level photoemission provide a spectral fingerprint of adsorbed chiral molecules? F Allegretti, M Polcik, D I Sayago, F Demirors, S O'Brien, G Nisbet, C L A Lamont and D P Woodruff

Elastic scattering in image-potential bands observed by two-photon photoemission K Boger, Th Fauster and M Weinelt

Spin-polarized surface state of MnSb(0 0 0 1) O Rader, M Lezaic, S Blügel, A Fujimori, A Kimura, N Kamakura, A Kakizaki, S Miyanishi and H Akinaga

Evolution of electronic structure in Ca_2-xSr_xRuO₄ observed by photoemission Shancai Wang and Hong Ding

Ultrafast electron dynamics studied with time-resolved two-photon photoemission: intra- and interband scattering in C₆F₆/Cu(1 1 1) P S Kirchmann, P A Loukakos, U Bovensiepen and M Wolf

Electron states and the spin density wave phase diagram in Cr(1 1 0) films Eli Rotenberg, B K Freelon, H Koh, A Bostwick, K Rossnagel, Andreas Schmid and S D Kevan

Photoemission study of S adsorption on GaAs (0 0 1) T Strasser, L Kipp, M Skibowski and W Schattke

Combining GW calculations with exact-exchange density-functional theory: an analysis of valence-band photoemission for compound semiconductors Patrick Rinke, Abdallah Qteish, Jörg Neugebauer, Christoph Freysoldt and Matthias Scheffler

Role of site selectivity, dimensionality, and strong correlations in angle-resolved photoemission from cuprate superconductorsA Bansil, M Lindroos, S Sahrakorpi and R S Markiewicz

Franz Himpsel, University of Wisconsin, Madison, USA Per-Olof Nilsson, Chalmers University of Technology, Gothenburg, Sweden

Angle-resolved photoemission spectra have been calculated with the one-step model for S/GaAs(0 0 1) and compared with experimental distributions. The data are analysed in terms of the ideal 1 × 1 and, furthermore, of the reconstructed 2 × 6 surface which is assumed to be closest to the experimentally realized structure. Emissions are characterized by electronic structure terms such as energy bands and orbital composition, though partly also by geometric properties. In particular, the determination of the second layer as consisting of Ga atoms has been achieved because of the distinct differences in the theoretical spectra with S–Ga and those with S–As bonds.

Chromium films offer an excellent system to study the impact of dimensional confinement on physical properties associated with the spin-density-wave (SDW) ground state observed in bulk materials. These properties are also of some technological importance since chromium is a common component of thin film magnetic structures. We prepared chromium (1 1 0) films of high crystalline quality on a W(1 1 0) substrate with a wedge-shaped thickness profile so that the impact of confinement can be systematically studied. We have characterized these films using a combination of low-energy electron diffraction and microscopy as well as high-resolution angle-resolved photoemission spectroscopy. We have probed the Fermi surface and the nesting vectors therein that are relevant to the SDW ground state. We find these to predict accurately the observed bulk SDW periodicity. We have also characterized the SDW periodicity in the film directly by measuring the splitting between backfolded bands, and we find that this periodicity deviates markedly from the bulk periodicity for thinner films at higher temperatures. We have systematically mapped the SDW incommensurability and phase diagram as a function of both film thickness and temperature. We find commensurate and incommensurate phases that are separated by nearly continuous transitions. Our results suggest a simple model to explain the delicate interplay between commensurate and incommensurate phases that involves a balance between SDW stabilization energy and surface and interface energetics.

The advances in femtosecond laser techniques facilitate the investigation of ultrafast electron dynamics at surfaces directly in the time-domain. We employ time-resolved two-photon-photoemission (2PPE) spectroscopy to study the electron dynamics of the unoccupied electronic states in hexafluorobenzene (C₆F₆) on Cu(1 1 1) serving as a model system for charge transfer across organic–metal interfaces. Our coverage-dependent study reveals a lifetime of the lowest unoccupied molecular resonance of 7 fs for a single monolayer (ML) which increases to 37 fs above 3 ML coverage. We find that the population build-up of the excited state is delayed by a characteristic time of about 10 fs with respect to the exciting laser pulse. By angle-resolved 2PPE spectroscopy, the mechanism of the delayed population rise is identified as intraband relaxation in the adsorbate band structure. The actual electron-transfer to the metal substrate occurs through interband scattering between the molecular resonance and substrate states on comparable timescales. Therefore the present study demonstrates that relaxation of hot electrons at molecule–metal interfaces include—even in the presence of strong electronic molecule–substrate interaction—also decay channels within the adlayer.

We report a systematic angle-resolved photoemission study in Ca_2−xSr_xRuO₄ for a wide range of Sr concentrations (0.3 ≤ x ≤ 2.0), with a focus on the Fermi surface (FS) topology at x = 0.5, which is believed to be a quantum critical point. While the dispersion of the valence bands formed by the t_2g orbitals is observed to be similar for all Sr levels, the low-energy quasi-particle (QP) weight changes significantly with x. At x_c = 0.5, all the three t_2g bands, γ (d_xy), α, β (mixing of d_yz and d_zx) remain metallic, with no major electron transfer among them. This is in contrast to the scenario of orbital-selective Mott transition proposed to explain the critical behaviour at x_c = 0.5. There is a mild change on the γ FS, which changes to hole-like in Ca_1.5Sr_0.5RuO₄ from electron-like in Sr₂RuO₄. The doping dependence of the QP weight has a general trend of decreasing with decreasing x, but there is a local maximum at x = 0.5. This suggests that the QP weight is controlled by the competition between Mott localization and disorder-driven localization.

Knowledge of the spin-dependent electronic structure at surfaces and interfaces plays an increasingly important role when assessing possible use of novel magnetic materials for spintronic applications. It is shown that spin- and angle-resolved photoelectron spectroscopy together with ab initio electronic structure methods provides a full characterization of the surface electronic structure of ferromagnetic MnSb(0 0 0 1). Two different surface reconstructions have been compared in spin- and angle-resolved valence-band photoemission. For annealing at elevated temperatures, the (1 × 1)-structure transforms into 2 × 2 and a majority-spin peak appears at −1.7 eV inside a majority-spin bulk band gap at the surface Brillouin zone centre. Its sensitivity to oxygen supports an interpretation as magnetic compound surface state. Local spin density calculations predict at the same energy (−1.75 eV) a prominent d surface state of majority spin for (1 × 1)-Mn terminated MnSb(0 0 0 1) but no such feature for (1 × 1)-Sb termination. The calculation shows that neither the bulk nor the surface is half-metallic, in agreement with the expectation for the hexagonal NiAs structure.

Adsorbate atoms on surfaces cause considerable scattering of electrons. In image-potential bands, elastic scattering rates can be determined by measuring the linewidth in angle-, time- and energy-resolved two-photon photoelectron spectroscopy. From these data, the total cross section for elastic scattering of electrons in the first image-potential band on the Cu(0 0 1) surface by statistically distributed Cu adatoms has been determined. For large parallel momenta of the electron, the scattering by an adatom can be described by a dipole potential.

The results of experimental measurements and theoretical simulations of circular dichroism in the angular distribution (CDAD) of photoemission from atomic core levels of each of the enantiomers of a chiral molecule, alanine, adsorbed on Cu(1 1 0) are presented. Measurements in, and out of, substrate mirror planes allow one to distinguish the CDAD due to the chirality of the sample from that due to a chiral experimental geometry. For these studies of oriented chiral molecules, the CDAD is seen not only in photoemission from the molecular chiral centre, but also from other atoms which have chiral geometries as a result of the adsorption. The magnitude of the CDAD due to the sample chirality differs for different adsorption phases of alanine, and for different emission angles and energies, but is generally small compared with CDAD out of the substrate mirror planes which is largely unrelated to the molecular chirality. While similar measurements of other molecules may reveal larger CDAD due to molecular chirality, the fact that the results for one chiral molecule show weak effects means that such CDAD is unlikely to provide a simple and routine general spectral fingerprint of adsorbed molecular chirality.

The electronic structure of the Si(1 1 1):GaSe van der Waals-like surface termination has been determined by angle-resolved photoelectron spectro- scopy using photons in the energy range hν = 12–170 eV supplied by the BESSY and ALS synchrotron light sources. The Si(1 1 1):GaSe surface is isoelectronic to the passivated Si(1 1 1):H and Si(1 1 1):As surfaces, and also reflects the principal building block of layered chalcogenide GaSe single crystals. The electronic structure is discussed in relation to these systems. The chemical bond between the Si and Ga surface atoms is non-polar and therefore similar to the Ga–Ga bond in GaSe single crystals and also to the Si–Si bond in bulk silicon. This explains both the absence of a surface core-level shift in Si 2p photoelectron spectra of the terminated surface and the striking similarity between its observed band structure and that of bulk GaSe.

We have investigated the activated adsorption of methane on Pt(1 1 1) by the combination of a supersonic molecular beam and in situ high-resolution X-ray photoelectron spectroscopy at the German synchrotron radiation facility BESSY II. On exposing the surface to a methane beam with kinetic energies between 0.30 and 0.83 eV, CH₃ is formed as a stable species at 120 K; upon heating, at around 260 K the adsorbed methyl partly dehydrogenates to CH and partly recombines to methane, which desorbs. Upon adsorption at 300 K, CH is directly formed as a stable surface species. To verify the chemical identity of CH as an intermediate, we have also investigated the thermal evolution of a saturated ethylene layer. Upon heating, at ∼290 K partial ethylene desorption and the formation of ethylidyne is clearly observed in the spectra, as expected from the literature. From the binding energies and also from the vibrational signature of the C 1s spectra, an unequivocal assignment of the various surface species is possible. Measurements of the sticking coefficients of methane show that the saturation coverage at 120 K depends on the kinetic energy of the molecule; furthermore, the sticking coefficient for vibrationally excited molecules is strongly enhanced.

When two different periodic potentials are present at the same time in a solid, the electron wavefunctions must conform to the resulting overall periodicity. It is the case of the broken-symmetry phases which are often observed in low-dimensional systems. The rearrangement of the electronic states has some interesting and perhaps unexpected consequences on the momentum distribution of the spectral weight, which can be measured in an ARPES experiment.

We have performed systematic and comprehensive high-resolution angle-resolved photoemission (ARPES) measurements on Bi-system high-T_c superconductors to study the quasi-particles related to and/or responsible for the occurrence of the superconductivity. We have experimentally determined the full energy dispersion and the coherence factors of the Bogoliubov quasi-particles, which show a good quantitative agreement with the prediction from the BCS theory. This proves the basic validity of the BCS theory in the broad sense to describe the high-T_c superconductivity. We have experimentally identified two different bosonic modes in Bi-system high-T_c superconductors. One produces a 'small' kink in the limited momentum region around the nodal cut and another is related to a 'large' kink which exists in a relatively wide momentum region with the stronger magnitude closer to the (π, 0) point. The observed momentum and temperature dependence of the kinks as well as the impurity effect show that the large kink is of magnetic origin and closely related to the high-T_c superconductivity.

A combination of conventional and resonant photoemission spectroscopy, x-ray absorption spectroscopy and ground-state quantum-chemical calculations has been used to study the valence electronic structure of a phenyl-capped 3,4-ethylenedioxythiophene oligomer, in polycrystalline thin films. The photon energy-dependent intensities of specific resonant decay channels are interpreted in terms of the spatial overlap of the excitation site and the ground-state molecular orbital involved in the decay. By making use of chemical shifts, excitations on different atomic sites are distinguished. It is demonstrated that site-specific information on the electronic structure of relatively large and complex organic systems may be obtained experimentally from non-radiative resonant decay spectra. In addition, these spectra provide relevant insight into the interpretation of near-edge x-ray absorption fine structure spectra.

The electron dynamics of buried Ar/Cu(1 0 0) image-potential states was investigated by time-resolved two-photon photoemission (2PPE) as a function of parallel momentum. The first interface state shows a parabolic dispersion with an effective mass of 0.6. Its lifetime of 110 fs at the -point decreases with increasing parallel momentum. The momentum dependence of the decay can be understood by intra- and inter-band decay processes mediated by Cu electrons, just as the decay of image-potential states on the clean Cu(1 0 0) surface.

We investigate quasi-one-dimensional (quasi-1D) surface states on metals as a well-defined model system for the study of correlation effects by angle-resolved photoemission. Both dimensionally constrained Shockley and Tamm states are examined, the former on the striped O/Cu(1 1 0) phase, the latter on Pt(1 1 0) with and without adsorbates. We observe an unusual change in photoemission intensity of quasi-particle peaks as a function of temperature or adsorbate coverage, which is very similar to ARUPS results on layered systems, Kondo systems, Mott-insulator systems and high-T_c superconductors. The intensity change of the quasi-particle peak is interpreted in terms of a coherent–incoherent transition of the quasi-1D states. For the Tamm states on Pt(1 1 0), we also find other typical fingerprints of correlation such as a kink in the dispersion and a significant mass renormalization close to E_F. A saddle point at the Fermi level provides a large density of states. Therefore, it is reasonable to expect that this quasi-1D surface resonance is involved in surface phase transitions. The results support our previous report about a surface charge-density-wave-induced phase transition on Br/Pt(1 1 0).

Vicinal surfaces with periodic arrays of steps are among the simplest lateral nanostructures. In particular, noble metal surfaces vicinal to the (1 1 1) plane are excellent test systems to explore the basic electronic properties in one-dimensional superlattices by means of angular photoemission. These surfaces are characterized by strong emissions from free-electron-like surface states that scatter at step edges. Thereby, the two-dimensional surface state displays superlattice band folding and, depending on the step lattice constant d, it splits into one-dimensional quantum well levels. Here we use high-resolution, angle-resolved photoemission to analyse surface states in a variety of samples, in trying to illustrate the changes in surface state bands as a function of d.

The most stable geometries for the (4 × 1) and (4 × 2) phases of the In/Si(111) surface have been reanalysed using DFT-annealing. A very efficient local orbital method (FIREBALL) has allowed us to explore the geometrical phase space and look for new absolute minima. The most stable theoretical structure is a new In/Si(111)-(4 × 2) geometry that presents a semiconducting band structure. This (4 × 2) surface, qualitatively very similar to the structural model proposed by Kumpf et al (2000 Phys. Rev. Lett.85 4916), is found to be the result of the combination of a soft shear distortion, whereby the two zigzag In-rows are displaced in opposite directions along the In-chains, and a pairing of the outer In atoms associated with a double-band Peierls distortion.

The electron–phonon coupling strength for two different surface states on Bi(110) close to the Fermi level is determined by measuring the energy- and temperature-dependent imaginary part of the self-energy and fitting it to different models of the phonon spectrum. For the hole pockets near the and points, the electron–phonon mass enhancement parameter λ is found to be 0.19(3) and 0.27(2), respectively.

We examine a technique commonly used to extract the self-energy from angle-resolved photoemission spectroscopy data. We find that this technique works extremely well for the marginal Fermi liquid model where it follows the analytical model perfectly. Surprisingly, we find a similarly good performance in the case of a Fermi liquid. Finally, we test the influence of the energy and momentum resolution on the results. We find that while the momentum resolution plays only a minor role in the analysis (it slightly inflates the values of the imaginary part of the self-energy), the energy resolution changes the extracted functional form of the self-energy in a significant way at low binding energy.

In this review we describe the development of photoemission spectroscopy (PES) from the first historic observations of the photoelectric effect by Hertz and Hallwachs to state-of-the-art experiments. We present several examples for the application of PES for chemical analysis of solids (ESCA), the determination of the valence band structure by angle-resolved photoemission (ARUPS), and the investigation of many-body effects, in particular by high-resolution PES on the meV-scale. Furthermore, we give a brief overview about the wide spectrum of experimental methods based on PES.

We propose a mechanism for the collective cooling of a large number N of trapped particles to very low temperatures by applying red-detuned laser fields and coupling them to the quantized field inside an optical resonator. The dynamics is described by what appears to be rate equations, but where some of the major quantities are coherences and not populations. The cooperative behaviour of the system provides cooling rates of the same order of magnitude as the cavity decay rate κ. This constitutes a significant speed-up compared to other cooling mechanisms since κ can, in principle, be as large as times the single-particle cavity or laser coupling constant.

We present theoretical and simulation studies of the linear self-focusing of frequency modulated electron whistlers in plasmas. A comparison has been made between analytical results of a wave-kinematic model and a direct simulation of the whistler wave equation. The self-focusing of whistlers comes about due to the specific form of the whistler mode dispersion, which allows the whistler wave energy to be transported at different speeds for waves with different wavelengths, and where the waves with larger group speeds overtake waves with lower speeds. Our numerical results are also compared with recent experiments, which report the frequency-modulated whistlers and their rapid localization in plasmas.

The equal charge-to-mass ratio for both species in pair plasmas induces a decoupling of the linear eigenmodes between waves that are charge neutral or non-neutral, also at oblique propagation with respect to a static magnetic field. While the charge-neutral linear modes have been studied in greater detail, including their weakly and strongly nonlinear counterparts, the non-neutral mode has received less attention. Here the nonlinear evolution of a solitary non-neutral mode at oblique propagation is investigated in an electron–positron plasma. Employing the framework of reductive perturbation analysis, a modified Korteweg–de Vries equation (with cubic nonlinearity) for the lowest-order wave magnetic field is obtained. In the linear approximation, the non-neutral mode has its magnetic component orthogonal to the plane spanned by the directions of wave propagation and of the static magnetic field. The linear polarization is not maintained at higher orders. The results may be relevant to the microstructure in pulsar radiation or to the subpulses.

We consider the additivity of the minimal output entropy and the classical information capacity of a class of quantum channels. For this class of channels, the norm of the output is maximized for the output being a normalized projection. We prove the additivity of the minimal output Renyi entropies with entropic parameters α ∊ [0, 2], generalizing an argument by Alicki and Fannes, and present a number of examples in detail. In order to relate these results to the classical information capacity, we introduce a weak form of covariance of a channel. We then identify various instances of weakly covariant channels for which we can infer the additivity of the classical information capacity. Both additivity results apply to the case of an arbitrary number of different channels. Finally, we relate the obtained results to instances of bi-partite quantum states for which the entanglement cost can be calculated.

A detailed analysis of the energy transfer system between ExB turbulence and zonal flows is given. Zonal flows, driven by the ExB Reynolds stress of the turbulence, are coupled to pressure disturbances with sinusoidal poloidal structure in toroidal geometry through the geodesic curvature. These pressure 'sidebands' are nonlinearly coupled not only back to the turbulence, but also to the global Alfvén oscillation whose rest state is the Pfirsch–Schlüter current in balance with the pressure gradient. The result is a statistical equilibration between turbulence, zonal flows and sidebands, and additionally the various poloidally asymmetric parallel dynamical subsystems. Computations in three-dimensional flux surface geometry show this geodesic transfer effect to be the principal mechanism which limits the growth of zonal flows in tokamak edge turbulence in its usual parameter regime, by means of both control tests and statistical analysis. As the transition to the magnetohydrodynamic (MHD) ballooning regime is reached, the Maxwell stress takes over as the main drive, forcing the Reynolds stress to become a sink.

We give a new description of quantum Brownian motion in terms of stochastic pure states. The corresponding path integral propagator allows us to establish a direct connection to the classical Langevin equation, in the Schrödinger picture. We show that in the quantum domain, one is naturally led to consider two stochastic processes driving the Brownian dynamics, one of them representing thermal fluctuations, as in the classical case. The second process reflects growing entanglement between the Brownian particle and its environment, and is therefore a truly quantum noise process. Technically, our result rests on the representation of the full propagator of the Brownian particle and its environment in a coherent state basis. For open system dynamics that may be described by a master equation, such stochastic schemes have already been proven to be efficient Monte Carlo methods. Here, we give a new stochastic scheme that covers low temperatures and strong friction, the latter limit being the case Einstein originally investigated.

Pulsed dc magnetron reactive sputtering of dielectrics provides a deposition process without arcing. The deposition process is usually carried out with pulsing frequencies in the range 10–350 kHz and duty cycles in the range 50–90%. The operating conditions are typically optimized empirically and are critically dependent on the properties of the pulsed plasma in the immediate vicinity of the magnetron. We show how a combination of time-resolved Langmuir probe measurements and time-resolved optical emission spectroscopy can be used to characterize the pulsed magnetron plasma and gain insights that can only be obtained conclusively by correlating the results obtained by both techniques. The pulsed dc sputtering of Al and Ti targets and the reactive sputtering of their oxides were used as examples in our experiments. Our experiments were carried out at total pressures in the range 0.4–1.3 Pa in either pure Ar ('metallic' mode) or in Ar–O₂ mixtures ('oxide' or 'reactive' mode) with mixing ratios from 1:1 to 1:4.

We propose and analyse a simple method to measure simultaneously the real and imaginary parts of the effective refractive index of a turbid suspension of particles. The method is based on measurements of the angle of refraction and transmittance of a laser beam that traverses a hollow glass prism filled with a colloidal suspension. We provide a comprehensive assessment of the method. It can offer high sensitivity while still being simple to interpret. We present results of experiments using an optically turbid suspension of polystyrene particles and compare them with theoretical predictions. We also report experimental evidence showing that the refractive behaviour of the diffuse component of light coming from a suspension depends on the volume fraction of the colloidal particles.

Bell's theorem states that, to simulate the correlations created by measurement on pure entangled quantum states, shared randomness is not enough: some 'non-local' resources are required. It has been demonstrated recently that all projective measurements on the maximally entangled state of two qubits can be simulated with a single use of a 'non-local machine'. We prove that a strictly larger amount of this non-local resource is required for the simulation of pure non-maximally entangled states of two qubits |ψ(α)⟩ = cosα|00⟩ + sinα|11⟩ with .

We consider an ideal gas of Bose and Fermi atoms in a harmonic trap, with a Feshbach resonance in the interspecies atomic scattering that can lead to the formation of fermionic molecules. We map out the phase diagram for this three-component mixture in chemical and thermal equilibrium. Considering adiabatic association and dissociation of the molecules, we identify a possible cooling cycle, which in ideal circumstances can yield an exponential increase of the phase-space density.

We discuss leptonic mixing and CP violation at low and high energies, emphasizing possible connections between leptogenesis and CP violation at low energies, in the context of lepton flavour models. Furthermore, we analyse weak-basis invariants relevant for leptogenesis and for CP violation at low energies. These invariants have the advantage of providing a simple test of the CP properties of any lepton flavour model.

We study the quantum dynamics of a Bose–Einstein condensate trapped in a double-well potential with a rising interwell barrier. We analytically find the characteristic timescales of the splitting process and compare our results with numerical analyses available in the literature. In the first stage of the dynamics, the relative phase of the two condensates evolves adiabatically. At a critical time t_ad, small-amplitude phase fluctuations around the average trajectory increase exponentially fast, signalling the breakdown of adiabaticity. After this stage, it is still not possible to neglect the tunnelling exchange between the two wells. We find highly non-trivial dependences of the dephasing time on t_ad and on the ramping time of the interwell barrier.

The origin of the colossal magnetoresistance (CMR) observed in La_1−xCa_xMnO₃, at x ≈ 0.3, is still largely debated. Actually, the precursor phase of the ferromagnetic (F) metallic state, defined as the concentration ranges 0.125 ≤ x_Ca < 0.22 and 0.1 ≤ x_Sr < 0.17 in La_1−xCa_xMnO₃ and La_1−xSr_xMnO₃ systems respectively, is poorly understood. At these concentrations where the compounds are F, a very specific temperature behaviour is observed, the systems evolving from a quasi-metallic state below T_C towards an insulating state at lower temperatures. The double-exchange coupling alone is insufficient to describe the physics of these compounds and other interactions have to be considered, the origin of which is still unclear. In this paper, we mainly review and discuss neutron scattering studies performed on three compounds, La_0.83Ca_0.17MnO₃, La_0.875Sr_0.125MnO₃ and La_0.8Ca_0.2MnO₃. The magnetic excitations, as well as the dispersion of acoustic and lower optical phonons have been determined using inelastic neutron scattering. In the three systems, and over the whole temperature range, the spin-wave excitation spectrum is characterized by a splitting into several levels or several branches which are more or less dispersed. In the quasi-metallic state, particularly studied in the two Ca-doped compounds, these levels can be characterized as spin-waves confined within nanosize F spin domains. Within some model, a quantitative analysis of these excitations is proposed which determines their sizes, shapes and magnetic couplings. These couplings are found to be anisotropic, a feature characteristic of some orbital-ordering. These observations are interpreted by a charge segregation. Moreover, in the low-temperature state of La_0.83Ca_0.17MnO₃, the existence of an underlying periodicity for the magnetic characteristics is revealed, suggesting a charge-ordered state similar to the case of La_0.875Sr_0.125MnO₃. Finally, a coupling of magnons with acoustic and lower optical phonon branches is strongly suggested, particularly in La_0.875Sr_0.125MnO₃, where a coincidence between the magnon and acoustic phonon energies is observed in a large q-range along the [1 0 0] direction.

We investigate entanglement properties in dimerized and frustrated spin-one models by applying the concept of negativity. On the basis of the relation between negativity and correlators, negativities are numerically calculated. We study the effects of dimerization and frustration on entanglement. We also consider the case of finite temperature, and determine the threshold temperature before which the thermal state is doomed to be entangled.

Magnetic domain walls (MDWs) can move when driven by an applied magnetic field. This motion is important for numerous devices, including magnetic recording read/write heads, transformers and magnetic sensors. A magnetic film, with a sawtooth profile, localizes MDWs in discrete positions at the narrowest parts of the film. We propose a controllable way to move these domain walls between these discrete locations by applying magnetic field pulses. In our proposal, each applied magnetic pulse can produce an increment or step-motion for an MDW. This could be used as a shift register. A similarly patterned magnetic film attached to a large magnetic element at one end of the film operates as an XOR logic gate. The asymmetric sawtooth profile can be used as a ratchet resulting in either oscillating or running MDW motion, when driven by an ac magnetic field. Near a threshold drive (bistable point) separating these two dynamical regimes (oscillating and running MDW), a weak signal encoded in very weak oscillations of the external magnetic field drastically changes the velocity spectrum, greatly amplifying the mixing harmonics. This effect can be used either to amplify or shift the frequency of a weak signal.

We report an experimental and theoretical investigation of the growth and structure of large carbon clusters produced in a supersonic expansion by a pulsed microplasma source. The absence of a significant thermal annealing during the cluster growth causes the formation of disordered structures, where sp² and sp hybridizations coexist for particles larger than ∼90 atoms. Among the various structures, we recognize sp² closed networks encaging sp chains. This 'nutshell' configuration can prevent the fragmentation of sp species upon deposition of the clusters, thus allowing the formation of nanostructured films containing carbynoid species, as shown by Raman spectroscopy. Atomistic simulations confirm that the observed Raman spectra are the signature of the sp/sp² hybridization typical of isolated clusters and surviving in the film and they provide information about the structure of the sp chains. Endohedral sp chains in sp² cages represent a novel way in which carbon nanostructures may be organized with potentially interesting functional properties.

We have investigated the orbital and magnetic correlations of the manganese sub-lattice in the archetypical half-doped manganite La_0.5Sr_1.5MnO₄ using the enhanced sensitivity of resonant soft x-ray diffraction at the Mn L edges. We have separately measured the energy dependence of the scattered intensity from both the and reflections indicative of the long-range correlations of orbital and magnetic order respectively. Measurements of the integrated intensity of the different features in the resonant energy spectrum show the different origins of the orbital correlations and their variation with temperature. The results also give evidence for a strong coupling between the orbital and spin degrees of freedom.

Particle-in-cell (kinetic) simulations of shear Alfvén wave (AW) interaction with one-dimensional, across the uniform-magnetic field, density inhomogeneity (phase mixing) in collisionless plasma were performed for the first time. As a result, a new electron acceleration mechanism is discovered. Progressive distortion of the AW front, due to the differences in local Alfvén speed, generates electrostatic fields nearly parallel to the magnetic field, which accelerate electrons via Landau damping. Surprisingly, the amplitude decay law in the inhomogeneous regions, in the kinetic regime, is the same as in the MHD approximation described by Heyvaerts and Priest (1983 Astron. Astrophys.117 220).

We have carried out extensive path integral Monte Carlo simulations of two-dimensional para-hydrogen (p-H₂) embedded in a crystalline matrix of alkali atoms. Our results show that, at low temperatures (⪅5 K), the thermodynamically stable phase of p-H₂ is a solid, commensurate with the underlying lattice. A nonzero superfluid signal for p-H₂ is observed in simulated systems of very small size (e.g. 13 molecules); however, results for systems of larger sizes are altogether consistent with absence of superfluidity in the thermodynamic limit.

A relationship giving the entropy production as the difference between a time-reversed entropy per unit time and the standard one is applied to stochastic processes of diffusion of Brownian particles between two reservoirs at different concentrations. The entropy production in the nonequilibrium steady state is interpreted in terms of a time asymmetry in the dynamical randomness between the forward and backward paths of the diffusion process.

In the present paper, we analyse a mechanism of the onset of mixing and its interconnection with exponential instability of trajectories. We study statistical characteristics of chaotic oscillations that correspond to attractors of the spiral type and of the Lorenz type. It has been established that a random process of the 'harmonic noise' type can serve as a mathematical model of spiral chaos and a random telegraph signal as a model of the Lorenz attractor. It has been revealed that the instantaneous phase dynamics plays an important role in the case of spiral chaos and determines regularities of autocorrelation decay and power spectrum formation. The effect of external Gaussian noise sources on characteristics of chaotic oscillations is analysed in detail.

Gravitation has interesting consequences for electromagnetic wave propagation in vacuum. The propagation of plane waves with phase velocity directed opposite to the time-averaged Poynting vector is investigated for a generally curved spacetime. Conditions for such negative-phase-velocity (NPV) propagation are established in terms of the spacetime metric components for general and special cases. Implications of the negative energy density of NPV propagation are discussed.

In the layered quantum spin systems TiOCl and TiOBr, the magnetic susceptibility shows a very weak temperature-dependence at high temperatures and transition-induced phenomena at low temperatures. There is a clear connection between the observed transition temperatures and the distortion of the octahedra and the layer separation. Band structure calculations point to a relation between the local coordinations and the dimensionality of the magnetic properties. While from magnetic Raman scattering only a small decrease of the magnetic exchange by −5 to 10% is derived comparing TiOCl with TiOBr, the temperature dependence of the magnetic susceptibility favours a much bigger change.

We demonstrate that a translation-invariant chain of interacting quantum systems can be used for high efficiency transfer of quantum entanglement and the generation of multiparticle entanglement over large distances and between arbitrary sites without the requirement of precise spatial or temporal control. The scheme is largely insensitive to disorder and random coupling strengths in the chain. We discuss harmonic oscillator systems both in the case of arbitrary Gaussian states and in situations when at most one excitation is in the system. The latter case, which we prove to be equivalent to an xy-spin chain, may be used to generate genuine multiparticle entanglement. Such a 'quantum data bus' may prove useful in future solid state architectures for quantum information processing.

We employ external optical white-noise fields as the actual driving fields of two atoms inside an optical cavity and investigate how controllable entanglement between two atoms arises in such a situation. Two different action configurations of noise are considered. If two atoms are simultaneously driven by two independent white-noise fields with the same intensity, the entanglement between them is suppressed and eventually completely destroyed by the noise. However, if only one atom is exposed in the white-noise fields, the steady state of the two atoms exhibits entanglement. A stochastic-resonance-like behaviour of steady state entanglement is also revealed. Finally, we examine the Bell violation between atoms and show that the steady state does not violate the Bell inequality even though it is inseparable. The stochastic-resonance-like behaviour cannot be observed in the Bell violation of two atoms during the evolution. The stronger the noise intensity, the more rapidly the Bell violation disappears.

We study numerically and analytically the behaviour of vortex matter in artificial flow channels confined by pinned vortices in the channel edges (CEs). The critical current density J_s for channel flow is governed by the interaction with the static vortices in the CEs. Motivated by early experiments which showed oscillations of J_s on changing (in)commensurability between the channel width w and the natural vortex row spacing b₀, we study structural changes associated with (in)commensurability and their effect on J_s and the dynamics. The behaviour depends crucially on the presence of disorder in the arrays in the CEs. For ordered CEs, maxima in J_s occur at commensurability w = nb₀ (n is an integer), while for w ≠ nb₀ defects along the CEs cause a vanishing J_s. For weak disorder, the sharp peaks in J_s are reduced in height and broadened via nucleation and pinning of defects. The corresponding structures in the channels (for zero or weak disorder) are quasi-1D n row configurations, which can be adequately described by a (disordered) sine-Gordon model. For larger disorder, matching between the longitudinal vortex spacings inside and outside the channel becomes irrelevant and, for w ≃ nb₀, the shear current J_s levels at ∼30% of the value J_s⁰ for the ideal commensurate lattice. Around 'half filling' (w/b₀ ≃ n ± 1/2), the disorder leads to new phenomena, namely stabilization and pinning of misaligned dislocations and coexistence of n and n ± 1 rows in the channel. At sufficient disorder, these quasi-2D structures cause a maximum in J_s around mismatch, while J_s smoothly decreases towards matching due to annealing of the misaligned regions. Near threshold, motion inside the channel is always plastic. We study the evolution of static and dynamic structures on changing w/b₀, the relation between the J_s modulations and transverse fluctuations in the channels and find dynamic ordering of the arrays at a velocity with a matching dependence similar to J_s. We finally compare our numerical findings at strong disorder with recent mode-locking experiments, and find good qualitative agreement.

It is well known that a charged particle moving with constant velocity in vacuum does not radiate. In a medium, the situation can be different. If the so-called Cherenkov condition is satisfied, i.e. the particle velocity exceeds the phase speed in the medium, the particle will radiate. We show that a charge moving with a constant velocity in a gas of photons emits Cherenkov radiation, even in the γ-ray regime, due to nonlinear quantum electrodynamical effects. Our result is evaluated with respect to the radiation background in the early universe, and it is argued that the effect can be significant.

The existence of manifestly nonlinear electrostatic modes in pair plasmas is shown analytically by means of the quasi-potential method applied to the Vlasov–Poisson system. These modes owe their existence to the trapping of particles in the potential trough(s) and are typically characterized by a notch in the particle distribution functions at resonant velocity, forming vortices in phase space. Both entities, wave structure Φ(x) and phase velocity v₀, are uniquely characterized by two parameters, the periodicity parameter k₀ and the spectral parameter B. Whereas k₀ = 0 describes double layers, with a phase velocity in the thermal range, k₀ ≠ 0 represents a periodic wave train which can propagate with two rather distinct phase velocities. One is related to the fast plasma wave, the other one to the slow acoustic mode.

We used photoemission electron microscopy (PEEM) to investigate the lateral distribution of the photoemission yield from a defined system of silver clusters supported by a highly oriented pyrolytic graphite (HOPG) substrate. For threshold photoemission using conventional photoemission (PE) and two-photon photoemission (2PPE) we find that distinct, well-separated emitters are responsible for the measured integral photoemission yield. Complementary characterization of the surface using STM shows that the emitter density as probed by PEEM is reduced by about three orders of magnitude in comparison to the actual cluster density. Wavelength and light polarization scans in combination with two-photon-PEEM clearly show that the origin of the 2PPE signal is related to small silver particles. Furthermore, the PEEM differentiates between inhomogeneous and homogeneous broadening effects in the 2PPE signal. This observation allows one to assign the origin of the local photoemission signal to either a distinct single silver particle or a number of coherently coupled silver particles. We conclude that the 2PPE-yield is highly selective with respect to specific properties of the supported silver particles. Our results show that in future experiments, PEEM as a highly local field probe, may be a key tool in the identification of these properties.

This is an informal paper that contains a list of 'things we know' and 'things we do not know' in manganites and other compounds. It is adapted from the Conclusions chapter of a recent book by the author, Nanoscale Phase Separation and Colossal Magnetoresistance. The Physics of Manganites and Related Compounds (Berlin: Springer-Verlag; 2002), but it also contains a summary of some of the most important recent results in the field. It is argued that the current main theoretical and experimental frameworks to rationalize the results of recent manganite investigations are based on the discovery of tendencies towards nanoscale inhomogeneous states, both in experiments and in simulations of models. The colossal magnetoresistance effect appears to be closely linked to these mixed-phase tendencies, although considerably more work is needed to fully confirm these ideas. The paper also includes information on cuprates, diluted magnetic semiconductors, relaxor ferroelectrics, cobaltites and organic and heavy fermion superconductors. These materials potentially share some common phenomenology with the manganites, such as a temperature scale T* above the ordering temperature where anomalous behaviour starts. Many of these materials also present low-temperature phase competition. The possibility of colossal-like effects in compounds that do not involve ferromagnets is briefly discussed. In particular, colossal effects in cuprates are explained. Overall, it is concluded that inhomogeneous 'clustered' states should be considered as a new paradigm in condensed matter physics, since their presence appears to be far more common than previously anticipated.

In this review, we concentrate on the orbital physics in ruthenates. Two issues will be addressed. One is related to the spin/orbital ordering in Ca_2−xSr_xRuO₄. Based on detailed first-principles calculations, we will discuss the crucial role of orbital degree of freedom and its relationship to structure distortions and magnetic properties. The orbital-dependent magnetic phase control in Ca_2−xSr_xRuO₄ will be illustrated. The second issue is concerned with the electronic state and quantum transport in the presence of the spin order, namely the anomalous Hall effect (AHE) in ferromagnetic Ca_1−xSr_xRuO₃. By detailed first-principles calculations and comparison with experimental result, we provide firm evidence for the intrinsic mechanism of AHE due to the existence of Berry phase in the presence of spin–orbital coupling, which is entirely topological.

We study the connection between charged lepton electric dipole moments, d_l(l = e,μ,τ), and see-saw neutrino-mass generation in a simple two-Higgs doublet extension of the Standard Model plus three right-handed neutrinos (RHN) N_a, a = 1, 2, 3. For RHN with hierarchical masses and at least one with mass in the 10 TeV range, we obtain the upper bounds of |d_e| < 9 × 10⁻³⁰ e-cm and |d_μ| < 2 × 10⁻²⁶ e-cm. Our scenario favours the normal mass hierarchy for the light neutrinos. We also calculated the cross section for e⁻e⁻ → W⁻W⁻ in a high luminosity collider with constraints from neutrinoless double beta decay of nuclei included. Among the rare muon-decay experiments we find that μ → eγ is most sensitive and the upper limit is <8 × 10⁻¹³.

The apparent difficulty in recovering classical nonlinear dynamics and chaos from standard quantum mechanics has been the subject of a great deal of interest over the last 20 years. For open quantum systems—those coupled to a dissipative environment and/or a measurement device—it has been demonstrated that chaotic-like behaviour can be recovered in the appropriate classical limit. In this paper, we investigate the entanglement generated between two nonlinear oscillators, coupled to each other and to their environment. Entanglement—the inability to factorize coupled quantum systems into their constituent parts—is one of the defining features of quantum mechanics. Indeed, it underpins many of the recent developments in quantum technologies. Here, we show that the entanglement characteristics of two 'classical' states (chaotic and periodic solutions) differ significantly in the classical limit. In particular, we show that significant levels of entanglement are preserved only in the chaotic-like solutions.

We have operated for the first time a liquid-argon TPC immersed in a magnetic field up to 0.55 tesla. We show that the imaging properties of the detector are not affected by the presence of the magnetic field. The magnetic bending of the ionizing particle allows one to discriminate their charge and estimate their momentum. These figures have up to now not been accessible in the non-magnetized liquid-argon TPC.

The states of linear momentum that satisfy the equality in the Heisenberg uncertainty principle for position and momentum, that is the intelligent states, are also the states that minimize the uncertainty product for position and momentum. The corresponding uncertainty relation for angular momentum and angular position, however, is more complicated and the intelligent states need not be the constrained minimum uncertainty product states. In this paper, we investigate the differences between the intelligent and the constrained minimum uncertainty product states for the angular case by means of instructive approximations, a numerical iterative search and the exact solution. We find that these differences can be quite significant for particular values of angular position uncertainty and indeed may be amenable to experimental measurement with the present technology.

There is a renewed interest in constraining the sum of the masses of the three neutrino flavours by using cosmological measurements. Solar, atmospheric, reactor and accelerator neutrino experiments have confirmed neutrino oscillations, implying that neutrinos have non-zero mass, but without pinning down their absolute masses. While it has been established that the effect of light neutrinos on the evolution of cosmic structure is small, the upper limits derived from a large-scale structure could help significantly to constrain the absolute scale of the neutrino masses. It is also important to know the sum of neutrino masses as it is degenerate with the values of other cosmological parameters, e.g. the amplitude of fluctuations and the primordial spectral index. A summary of the cosmological neutrino mass limits is given. Current results from cosmology set an upper limit on the sum of the neutrino masses at ∼1 eV, somewhat dependent on the datasets used in the analyses and assumed priors on cosmological parameters. It is important to emphasize that the total neutrino mass ('hot dark matter') is derived by assuming that the other components in the universe are baryons, cold dark matter and dark energy. We assessed the impact of neutrino masses on the matter power spectrum, the cosmic microwave background, peculiar velocities and gravitational lensing. We also discuss possible methods to improve the mass upper limits by an order of magnitude.

The structure and dynamics of Ni_N, Ag_N and Au_N (N = 6–30) clusters have been studied extensively by Monte Carlo and molecular dynamics methods based on the Sutton–Chen many-body potential. An exhaustive search for low-energy minima on the potential energy surface was carried out using the eigenvector-following technique. The exponential increase in the number of isomers with atomic size is demonstrated and compared. The binding energies and point groups of global minimum and first two isomers of Ni_N, Ag_N and Au_N (N = 6–14) clusters are listed. The melting properties and temperatures of the clusters are reported.