Table of contents

In the past ten years or so 'Spintronics' has evolved into a central part of solid state physics that addresses the intimate relation of electron charge and spin transport. To date, it is a huge field covering topics that range from applied micromagnetics to quantum computing. This invited focus issue of New Journal of Physics has become possible only by focusing on structures with 'reduced dimensions', which can be realized in semiconductor, molecular and carbon-based systems.

Injection and detection of spin accumulation in non-magnetic materials using ferromagnetic contacts is at the root of spintronics. Ferromagnetic semiconductors attract much interest as bulk materials and as contacts. Recent years have seen a surge of interest in spin generation by spin orbit coupling in otherwise non-magnetic materials, leading to the observation of, e.g., the Aharonov–Casher and spin Hall effects. Molecular spintronics, including carbon nanotubes and graphene, is the newest branch of our field.

This focus issue attempts to provide an overview of the ongoing basic research in the field. The issue contains invited papers from leading theoreticians and experimentalists studying spintronics in reduced dimensions in different materials. The collected papers give an up-to-date perspective of a multi-facetted field, which in our opinion is still far from reaching its peak. We hope the reader will enjoy perusing these papers as much as we enjoyed collecting them.

The articles below represent the first accepted contributions and further additions will appear in the near future.

Focus on Spintronics in Reduced Dimensions Contents

Single spin universal Boolean logic gate H Agarwal, S Pramanik and S Bandyopadhyay

Zeeman ratchets: pure spin current generation in mesoscopic conductors with non-uniform magnetic fieldsMatthias Scheid, Dario Bercioux and Klaus Richter

Extracting current-induced spins: spin boundary conditions at narrow Hall contactsİ Adagideli, M Scheid, M Wimmer, G E W Bauer and K Richter

Phase transition between the quantum spin Hall and insulator phases in 3D: emergence of a topological gapless phaseShuichi Murakami

Magnetic focusing of charge carriers from spin-split bands: semiclassics of a Zitterbewegung effect U Zülicke, J Bolte and R Winkler

Detailed transport investigation of the magnetic anisotropy of (Ga,Mn)As K Pappert, C Gould, M Sawicki, J Wenisch, K Brunner, G Schmidt and L W Molenkamp

Electronic states of magnetic quantum dots Ramin M Abolfath, Pawel Hawrylak and Igor Zutic

Mesoscopic fluctuations of spin currents Yuli V Nazarov

TAMR effect in (Ga,Mn)As-based tunnel structuresM Ciorga, M Schlapps, A Einwanger, S Geißler, J Sadowski, W Wegscheider and D Weiss

Anomalous Hall effect in spin-polarized two-dimensional electron gases with Rashba spin–orbit interactionTakashi Kato, Yasuhito Ishikawa, Hiroyoshi Itoh and Jun-ichiro Inoue

Spin photocurrents in (110)-grown quantum well structuresH Diehl, V A Shalygin, V V Bel'kov, Ch Hoffmann, S N Danilov, T Herrle, S A Tarasenko, D Schuh, Ch Gerl, W Wegscheider, W Prettl and S D Ganichev

Spin transport across carbon nanotube quantum dots Sonja Koller, Leonhard Mayrhofer and Milena Grifoni

Local Hanle-effect studies of spin drift and diffusion in n:GaAs epilayers and spin-transport devices M Furis, D L Smith, S Kos, E S Garlid, K S M Reddy, C J Palmstrøm, P A Crowell and S A Crooker

Spin dynamics in rolled-up two-dimensional electron gases Maxim Trushin and John Schliemann

Spin injection in quantum wells with spatially dependent rashba interaction Arne Brataas, A G Mal'shukov and Yaroslav Tserkovnyak

Magneto-transport through single-molecule magnets: Kondo-peaks, zero-bias dips, molecular symmetry and Berry's phase Maarten R Wegewijs, Christian Romeike, Herbert Schoeller and Walter Hofstetter

Spin-Hall conductivity of a spin-polarized two-dimensional electron gas with Rashba spin–orbit interaction and magnetic impurities C P Moca and D C Marinescu

Dimensionally constrained D'yakonov–Perel' spin relaxation in n-InGaAs channels: transition from 2D to 1D A W Holleitner, V Sih, R C Myers, A C Gossard and D D Awschalom

Time reversal Aharonov–Casher effect using Rashba spin–orbit interaction Junsaku Nitta and Tobias Bergsten

Gerrit E W Bauer, Delft University of Technology, The Netherlands Laurens W Molenkamp, Universität Wuerzburg, Germany

Phase transitions between the quantum spin Hall (QSH) and the insulator phases in three dimensions (3D) are studied. We find that in inversion-asymmetric systems there appears a gapless phase between the QSH and insulator phases in 3D which is in contrast with the 2D case. Existence of this gapless phase stems from a topological nature of gapless points (diabolical points) in 3D, but not in 2D.

We present a theoretical study of the interplay between cyclotron motion and spin splitting of charge carriers in solids. While many of our results apply more generally, we focus especially on the Rashba model describing electrons in the conduction band of asymmetric semiconductor heterostructures. Appropriate semiclassical limits are distinguished that describe various situations of experimental interest. Our analytical formulae, which take full account of Zeeman splitting, are used to analyse recent magnetic-focusing data. Surprisingly, it turns out that the Rashba effect can dominate the splitting of cyclotron orbits even when the Rashba and Zeeman spin-splitting energies are of the same order. We also find that the origin of spin-dependent cyclotron motion can be traced back to Zitterbewegung (ZB)-like oscillatory dynamics of charge carriers from spin-split bands. The relation between the two phenomena is discussed, and we estimate the effect of ZB-related corrections to the charge carriers' canonical position.

This paper discusses transport methods for the investigation of the (Ga,Mn)As magnetic anisotropy. Typical magnetoresistance behaviour for different anisotropy types is discussed, focusing on an in depth discussion of the anisotropy fingerprint technique and extending it to layers with primarily uniaxial magnetic anisotropy. We find that in all (Ga,Mn)As films studied, three anisotropy components are always present. The primary biaxial along ([100] and [010]) along with both uniaxial components along the and [010] crystal directions which are often reported separately. Various fingerprints of typical (Ga,Mn)As transport samples at 4 K are included to illustrate the variation of the relative strength of these anisotropy terms. We further investigate the temperature dependence of the magnetic anisotropy and the domain wall nucleation energy with the help of the fingerprint method.

We study quantum states of electrons in magnetically doped quantum dots as a function of exchange coupling between electron and impurity spins, the strength of Coulomb interaction, confining potential and the number of electrons. The magnetic phase diagram of quantum dots, doped with a large number of magnetic Mn impurities, can be described by the energy-gap in the spectrum of electrons and the mean field electron–Mn exchange coupling. A competition between these two parameters leads to a transition between spin-unpolarized and spin-polarized states, in the absence of applied magnetic field. Tuning the energy-gap by electrostatic control of nonparabolicity of the confining potential can enable control of magnetization even at the fixed number of electrons. We illustrate our findings by directly comparing Mn-doped quantum dots with parabolic and Gaussian confining potential.

Spin currents may be generated by applying bias voltages V to the nanostructures even in the absence of spin-active ferromagnetic interfaces. Most theoretical proposals concentrate on a concrete spin–orbit interaction and on the disorder-averaged effect. It remains underappreciated that any spin–orbit interaction produces random spin currents with a typical amplitude not affected by disorder. This work addresses such mesoscopic fluctuations of spin currents for a generic model of a nanostructure where several quantum connectors meet in a single node. The analysis is performed in the framework of recently developed quantum circuit theory of G_Q corrections and reveals four distinct mechanisms of spin current fluctuations. The results are elaborated for simple models of tunnel and ballistic connectors.

We discuss the results of our experiments on tunnel devices based on (Ga,Mn)As structures. Those include p⁺-(Ga, Mn)As/n⁺-GaAs Esaki diodes and laterally defined narrow nanoconstrictions in (Ga,Mn)As epilayers. We found in those structures strong anisotropic magnetoresistance behaviour with features that could be attributed to the novel tunnelling anisotropic magnetoresistance effect. We argue however, that in case of nanoconstricted (Ga,Mn)As wires, some other physics has to be additionally employed to fully explain the observed effects.

We present details of the analysis of the effect of disorder on the intrinsic anomalous Hall conductivity (AHC) in a spin-polarized two-dimensional electron gas with a Rashba-type spin–orbit interaction. We show that the AHC derived by the Kubo formula vanishes when the Fermi energy is larger than the exchange energy unless the lifetime is spin-dependent. Results obtained by numerical integration of the general expressions of the AHC suggest that the AHC could depend on the lifetime, in contrast to previous results.

The circular photogalvanic effect and the circular photon drag effect are observed and investigated in detail in (110)-grown quantum well structures. The experimental data are well described by phenomenological theory and microscopic models of both effects are developed being in agreement with experimental data.

We investigate linear and nonlinear transport in interacting single-wall carbon nanotubes (SWCNTs) that are weakly attached to ferromagnetic leads. For the reduced density matrix of a SWCNT quantum dot, equations of motion which account for an arbitrarily vectored magnetization of the contacts are derived. We focus on the case of large diameter nanotubes where exchange effects emerging from short-ranged processes can be excluded and the four-electron periodicity at low bias can be observed. This yields in principle four distinct resonant tunnelling regimes, but due to symmetries in the involved groundstates, each two possess a mirror-symmetry. With a non-collinear configuration, we recover at the resonances the analytical results known for the angular dependence of the conductance of a single level quantum dot or a metallic island. The two other cases are treated numerically and show on the first glance similar, yet not analytically describable dependences. In the nonlinear regime, negative differential conductance features occur for non-collinear lead magnetizations.

In electron-doped GaAs, we use scanning Kerr-rotation microscopy to locally probe and spatially resolve the depolarization of electron spin distributions by transverse magnetic fields. The shape of these local Hanle-effect curves provides a measure of the spin lifetime as well as spin transport parameters including drift velocity, mobility and diffusion length. Asymmetries in the local Hanle data can be used to reveal and map out the effective magnetic fields due to spin–orbit coupling. Finally, using both spin imaging and local Hanle effect studies, we investigate the drift and diffusion of electrically-injected spins in lateral Fe/GaAs spin-detection devices, both within and outside the current path.

A curved two-dimensional electron gas with spin–orbit interactions due to the radial confinement asymmetry is considered. At a certain relation between the spin–orbit coupling strength and curvature radius the tangential component of the electron spin becomes a conserved quantity for any spin-independent scattering potential that leads to a number of interesting effects such as persistent spin helix and strong anisotropy of spin relaxation times. The effect proposed can be utilized in non-ballistic spin-field-effect transistors.

We consider Rashba spin–orbit effects on spin transport driven by an electric field in semiconductor quantum wells. We derive spin diffusion equations that are valid when the mean free path and the Rashba spin–orbit interaction vary on length scales larger than the mean free path in the weak spin–orbit coupling limit. From these general diffusion equations, we derive boundary conditions between regions of different spin–orbit couplings. We show that spin injection is feasible when the electric field is perpendicular to the boundary between two regions. When the electric field is parallel to the boundary, spin injection only occurs when the mean free path changes within the boundary, in agreement with the recent work by Tserkovnyak et al (Preprint cond-mat/0610190).

We theoretically analyse coherent electron transport through a single-molecule magnet (SMM) in the regime where charge fluctuations are suppressed. Using the numerical renormalization group (NRG) technique, we calculate the low-temperature conductance as a function of the SMMs magnetic anisotropy parameters and the strength and orientation of an external magnetic field. We show how the microscopic magnetic symmetry of the molecule affects the transport via a Kondo effect with non-trivial dependence on a longitudinal field. In addition, we show how Berry's phase and the Kondo effect, both associated with reversal of the SMMs spin, appear when both the magnetic field amplitude and direction are varied. It is shown that both effects involve the magnetic excitations of the SMM in an essential way.

The Kubo formula is used to calculate the spin-Hall conductivity σ_sH in a spin-polarized two-dimensional electron system with Rashba-type spin–orbit interaction. As in the case of the unpolarized electron system, σ_sH is entirely determined by states at the Fermi level, a property that persists in the presence of magnetic impurities. In the clean limit, the spin-Hall conductivity decreases monotonically with the Zeeman splitting, a result of the ordering effect on the electron spins produced by the magnetic field. In the presence of magnetic impurities, the spin-dependent scattering determines a finite renormalization of the static part of the fully dressed vertex correction of the velocity operator that leads to an enhancement of σ_sH, an opposite behaviour to that registered in the presence of spin-independent disorder. The variation of σ_sH with the strength of the Rashba coupling and the Zeeman splitting is studied.

We investigate both the spin dynamics and the magnetotransport properties of two-dimensional (2D) n-InGaAs channels as a function of the channel width. We find that the electron spin scattering in the channels is limited by a dimensionally constrained D'yakonov–Perel' mechanism, while the magnetotransport reveals purely 2D behaviour. For submicron channels the spin relaxation times increase for decreasing widths, while the magnetotransport data exhibit no band bending effects for the investigated samples. Temperature and photon energy dependent measurements rule out dissipative effects and further corroborate the experimental observation of a dimensionally constrained spin relaxation.

We propose a spin interferometer using Rashba spin–orbit interaction. A spin interference effect is demonstrated in small arrays of mesoscopic InGaAs rings. This spin interference is the time reversal Aharonov–Casher (AC) effect. The AC interference oscillations are controlled over several periods. This result shows evidence for electrical manipulation of the spin precession angle in an InGaAs two-dimensional electron gas channel. We control the precession rate in a precise and predictable way with an electrostatic gate.

Low-temperature scanning tunneling spectroscopy (STS) allows us to probe electronic properties of clusters at surfaces with unprecedented accuracy. Recent experimental determination of the differential conductance of supported clusters yield considerable deviations with respect to the expected density of states and suggest that many cluster states are invisible to STS measurements. By means of fully self-consistent quantum transport calculations, using realistic tunneling tips, we show that, depending on the tip shape, only a small fraction of the electronic states contribute to the STS spectra, thus explaining the experimental findings. We demonstrate that the unambiguous characterization of the states on the supported clusters can be achieved with energy-resolved images, obtained from a theoretical analysis which mimics the experimental imaging procedure.

We report a systematic density functional theory investigation of adsorption of small Au_n (n =1–6) clusters on ideal and defected MgO(100) single crystal surfaces and Mo(100) supported thin MgO(100) films. As a model defect, we consider a neutral surface oxygen vacancy (F_s). Optimal adsorption geometries and energies, cluster formation energies and cluster charges are discussed and compared in detail over four different substrates. For a given cluster size, the adsorption energy among these substrates increases in the order MgO, F_s/MgO, MgO/Mo and F_s/MgO/Mo. While cluster growth by association of atoms from gas phase is exothermic on all the substrates, cluster growth by diffusion and aggregation of pre-adsorbed Au atoms is an endothermic process for Au₁→Au₂, Au₃→Au₄ and Au₅→Au₆ on MgO/Mo and Au₂→Au₃ and Au₅→Au₆ on F_s/MgO/Mo. The adsorbed clusters are close to neutral on MgO, but adopt a significant anionic charge on other supports with the increasing order: MgO/Mo, F_s/MgO and F_s/MgO/Mo. The adsorption strength thus correlates with the amount of negative charge transferred from the substrate to gold.

The structure on the atomic and mesoscopic scale of Pb adsorbed on Si(557) has been investigated by high-resolution low energy electron diffraction (SPA-LEED). Depending on Pb coverage in the range between 1.2 and 1.6 monolayers (ML), formation of various facets [(112), (335), (223), and a meta-stable (557) orientation] is induced by the Pb layers. The facet orientation in general does not coincide with the macroscopic orientation of the (557) surface. After an initial annealing step to 600 K, starting with 1.2 ML of Pb, this new vicinality can be tuned gradually and reversibly even at temperatures below 180 K by further adsorption, but also by desorption of Pb. Superstructures of the Pb layers on the terraces were identified on the most stable (223) facets. Here parts of the devil's staircase and the stripe-incommensurate (SIC) phases known from Si(111) surfaces (Yakes et al 2004 Phys. Rev. B 69 224103) develop. A new mechanism for facet formation with different orientations, based on avoidance of step decoration by adsorbed Pb, is proposed.

We have studied the optical properties of silicon quantum dots (QDs) embedded in a silicon oxide matrix using photoluminescence (PL) and time-resolved PL. A broad luminescence band is observed in the red region, in which the time evolution exhibits a stretched exponential decay. With increasing excitation intensity a significant saturation effect is observed. Direct electron–hole recombination is the dominant effect in the red band. A relatively narrow peak appears around 1.5 eV, which is attributed to the interface states overlapping with transition from the ground state of the silicon QDs. The saturation factor increases slowly with detection photon energy between 1.5 and 1.8 eV, which is attributed to the emission from zero-phonon electron–hole recombination. At higher photon energies the significantly increased saturation factor suggests a different emission mechanism, most likely the defect states from silicon, silicon oxide or silicon rich oxide.

The present research in particle physics has been progressing very quickly in recent decades thanks to the effort of a large and motivated community of experimentalists and theorists. According to an oversimplified scheme, the experimental effort goes along two main lines which we could broadly identify as the 'high-intensity' (or 'high-luminosity') and the 'high-energy' roads. The former includes high-precision and relatively low-energy experiments, aiming at pinning down tiny effects related to the exchange of virtual particles of new physics and giving rise to departures from the predictions of the Standard Model of particles and interactions (SM). We can mention, as not exhaustive examples, neutrino experiments and measurements of CP violation and flavor changing neutral currents (FCNC) in the quark sector and analogous lepton flavor violation (LFV) in the lepton sector. The other line of research has the scope of extending the energy frontier of the collisions, entering the 'terra incognita' of particle physics thanks to the availability of more and more powerful particle accelerators and of complex and performing experimental apparatus, able to stand extremely high collision rates and doses. The operation in recent years of large accelerators such as the SPS Collider and the LEP at CERN, the Tevatron at FERMILAB, and HERA at DESY has allowed particle physicists to gather valuable data, leading to a profound understanding of the Standard Model and a consequent major achievement in our endeavor to understand fundamental interactions and elementary particles at the shortest distances. Namely, we now know that the Standard Model of particle physics correctly describes such fundamental physics up to energies of O(100 GeV).

As deep and outstanding as this achievement may be, we still have good reasons to claim that the Standard Model represents only a layer in our knowledge of fundamental interactions, i.e. new physics has to show up at energies larger than the 100 GeV level. There is both observational and theoretical support for such an important claim. On the observational side, the non-vanishing neutrino masses, the presence of a large amount of non-baryonic Dark Matter and the need to have an efficient dynamical mechanism to give rise to the cosmic matter–antimatter asymmetry (baryogenesis) call for extensions of the Standard Model with new particles and interactions. Theoretically, we blame the SM for not offering an answer to questions that we usually consider as fundamental: (i) the SM fails to give a rationale for the puzzling spectrum of fermion masses and mixings, (ii) it does not achieve a true unification of fundamental interactions since it still has three gauge coupling constants to account for the electroweak and strong interactions and (iii) in the SM the spontaneous breaking of the electroweak symmetry is achieved through the introduction of a fundamental scalar, the Higgs boson, whose mass is not protected by any symmetry against huge radiative corrections leading to a destabilization of the energy scale where the electroweak breaking has to occur (gauge hierarchy problem). This third deficiency of the SM actually provides the main motivation for our firm belief that new physics has to show up at a scale related to the electroweak symmetry breaking, i.e. in the TeV range. Indeed, no matter how one chooses to provide an ultraviolet completion of the SM to allow for the above-mentioned stabilization (dynamical electroweak symmetry breaking à la Technicolor, low-energy supersymmetry, large extra dimensions, 'little Higgs solution', etc), one unavoidably ends up with the presence of new physics signatures at the TeV scale. In some cases, the new physics at the electroweak scale may entail very interesting candidates for Dark Matter or may provide a nice unification of the electroweak and strong gauge couplings at some larger energy scales.

In this spirit, there is general consensus that the present and the next generations of high-energy, high-intensity (luminosity) machines will bring new fundamental discoveries, since the new physics outlined above is expected to be 'just beyond' the energy scale explored so far. We talk therefore of 'physics at the TeV scale', the energy domain that will be soon explored by the proton LHC machine at CERN, and later on by the electron colliders ILC and CLIC. The latter are expected to be the first machines to be conceived, designed, funded and operated as a genuinely worldwide effort. Finally, special attention has to be devoted to the detectors employed with these accelerators. As an example, the requirements on particle detection and on the measurement of their kinematical quantities at the LHC have pushed the various detector techniques to their limits, calling for new solutions and bringing forward a remarkable development of the field.

This invited focus issue of New Journal of Physics aims to provide a survey of the field of 'physics at the TeV scale' courtesy of selected papers from leading experimentalists and theorists directly involved in key aspects of the research. On the eve of the LHC start-up, we hope that this collection will prove to be a useful resource in the hands of a diversified scientific community which is tackling the difficult task of finding the first traces of a new physics that particle physicists have been (desperately) seeking for more than three decades.

Focus on Particle Physics at the TeV Scale Contents

Is SUSY natural? Keith R Dienes, Michael Lennek, David Sénéchal and Vaibhav Wasnik

Energy measurement at the TeV scale Richard Wigmans

Innovations in ILC detector design using a particle flow algorithm approach Stephen R Magill

Tracking at LHC F Ragusa and L Rolandi

The Large Hadron Collider Lyndon Evans

Triggering at high luminosity colliders Hans Peter Beck

TeV physics and the Planck scale Vernon Barger, Paul Langacker and Gabe Shaughnessy

Physics during the first two years of the LHC Fabiola Gianotti

Antonio Ereditato, University of Bern, Switzerland Takaaki Kajita, University of Tokyo, Japan Antonio Masiero, Università degli Studi di Padova, Italy

Precise tracking is an indispensable tool for the study of many phenomena at new energy frontier accessible with the CERN Large Hadron Collider (LHC). The tracking detectors of ATLAS and CMS have been designed to cope with the harsh experimental conditions of the LHC interaction region. In this paper, we discuss and compare the tracking performance of these two detectors.

The Large Hadron Collider (LHC), now close to completion at CERN will provide proton–proton collisions with unprecedented luminosity and energy. It will allow the Standard Model of physics to be explored in an energy range where new phenomena can be studied. This includes the validity of the Higgs mechanism, supersymmetry and CP violation. The machine presents a number of novel features discussed in detail below.

This paper discusses the techniques used to select online promising events at high energy and high luminosity colliders. After a brief introduction, explaining some general aspects of triggering, the more specific implementation options for well established machines like the Tevatron and Large Hadron Collider (LHC) are presented. An outlook on what difficulties need to be met is given when designing trigger systems at the Super Large Hadron Collider, or at the International Linear Collider.

Supersymmetry is one of the best motivated possibilities for new physics at the TeV scale. However, both concrete string constructions and phenomenological considerations suggest the possibility that the physics at the TeV scale could be more complicated than the minimal supersymmetric standard model (MSSM), e.g. due to extended gauge symmetries, new vector-like supermultiplets with non-standard SU(2)× U(1) assignments, and extended Higgs sectors. We briefly comment on some of these possibilities, and discuss in more detail the class of extensions of the MSSM involving an additional standard model singlet field. The latter provides a solution to the μ problem, and allows significant modifications of the MSSM in the Higgs and neutralino sectors, with important consequences for collider physics, cold dark matter, and electroweak baryogenesis.

The CERN Large Hadron Collider (LHC) presents the most extraordinary challenges that particle physics has ever faced. By colliding high-intensity proton beams at a centre-of-mass energy of 14 TeV, it will explore in great detail the previously unaccessible territory of the TeV scale. We discuss the LHC physics goals and potential during the first two years of operation, and outline the fundamental questions that it might be able to address by the end of 2009.

We have studied the thermal oxidation of the intermetallic alloy CoGa in situ, in real time on the atomic scale, during the growth of an ultrathin, epitaxial Ga oxide layer. On the basis of an extended set of surface x-ray diffraction data, density functional theory calculations and core level spectroscopy data, we find that the oxide film consists of an oxygen ion double layer, which contains the basic building block of bulk β-Ga₂O₃. The oxide formation takes place via the nucleation of two-dimensional, anisotropic oxide islands which laterally grow and coalesce. A dramatic increase of the oxide island size is observed for low O₂ pressures in the 10⁻⁸ mbar regime, which we interpret as the onset of a step flow like growth mode. This allows us to conclude that thermal oxidation can be considered as a hetero-epitaxial growth process, that follows similar atomistic growth principles to molecular beam epitaxy. As a consequence, the structural perfection of the oxide layer can be tailored by the appropriate choice of oxygen pressure and temperature.

Stable multi-armed spiral patterns have been observed in a gas discharge by using water electrodes. It is found that the multi-armed spirals in our experiments generally result from the interaction between a single-armed spiral pattern and dislocations. The number of spiral arms can be increased or decreased depending on the topological charge of the dislocation when it glides into the spiral core. The complex spatiotemporal dynamics of the multi-armed spiral tips has also been investigated. The spiral tips rotate about a common circle for a two-armed spiral pattern. The core dynamics of a three-armed spiral pattern involves intermittent pairwise collision of tips at or near their tips, and it is more complex for a four-armed spiral pattern.

We studied a spin transfer torque (STT) effect in Ag₂Co granular films, induced by a high current density 10^{8 - 9} A cm^{− 2} injected via a point contact. The system consists of single domain Co nanoparticles randomly embedded in Ag matrix, with a mean distance corresponding to the typical quantity of layer thicknesses used for the nonmagnetic space in multilayer nanopillars. A large giant magnetoresistance (GMR) effect of 55% measured at 4.2 K is an indication for high spin scattering anisotropy which is required for STT observations. Supposedly, a certain amount of large-sized Co particles saturated in external magnetic field H acts here as the spin polarizer for the injected current I and small-sized particles unsaturated at 4.2 K and H_max (90 kOe) even act as the detector for switching. A novel STT effect was observed thereby as I rises across a threshold value I_c, showing a sharp decrease in R (ΔR/R = 130% with two steps), which arises accordingly from further alignment of the small-sized Co granules. The behavior is polar and hysteretic, similar to properties measured for multilayer nanopillars. The two-step behavior could be an effect related to a lognormal particle size distribution. Depending on the spin polarization, I_c is found to be field disproportional, indicating a larger STT efficiency at a higher H.

Recently, composed vortex fields generated by means of a spatial light modulator working in a dynamical regime have been examined as prospective information carriers. In this paper, an advanced optical set-up utilizing vortices for transfer of information is proposed and experimentally verified. Its operation is based on a sophisticated design of phase-only masks enabling information encoding and decoding. In the proposed vortex communication channel, the photolithographically prepared masks are used for generation of a composed vortex field carrying information. Dynamics of information encoding and decoding is achieved by switching of an array of light sources illuminating the phase masks.

A series of dedicated experiments with the Plasma Kristal Experiment (PKE)-Nefedov (Nefedov et al 2003 New J. Phys. 5 33) set-up were performed on board the International Space Station to measure the dispersion relation (DR) for the longitudinal dust-acoustic (DA) waves in quasi-isotropic three-dimensional (3D) complex plasmas. The waves were excited by applying ac electric modulation of variable frequency to the radio frequency (rf) electrodes. The amplitude of excitation was varied with frequency to ensure a 'sufficiently linear' regime of the dust density perturbations. The DR was obtained by measuring the induced density perturbations, revealing fairly good agreement with a simple multispecies theory of DA waves.

The authors report the transmission properties of omega shaped metallic inclusions on a dielectric medium that exhibits bianisotropic properties. The resonance frequencies of single omega resonators are investigated experimentally and numerically. The resonance frequency of an Ω structure depends on its orientation with respect to the incident electric field. Increasing the tail length of the Ω resonator causes a decrease in resonance frequency. Band gaps due to the magnetoelectric resonances are observed for various types of periodic omega arrays. A transmission band is observed when a periodic Ω media is combined with a negative permittivity media of periodic thin wires. The transmission band appears below the band gap of periodic omega media, in turn indicating right-handed behavior. A dual transmission band is obtained by composing two different types of metamaterials that are arranged periodically.

The present contribution focuses on the phenomenology and mechanisms of stress generation in Ge thin films during keV ion bombardment. Experimentally, amorphous Ge (a-Ge) thin films were grown from vapor, and subsequently bombarded with Ar⁺ ions with energies of up to 3 keV. Stress generation is monitored by a laser beam deflection method. In order to identify the underlying nanoscale physics, molecular dynamics simulations were performed, in which crystalline and (a-Ge) films of different densities were irradiated. Experiments and simulations both show generation of compressive stresses, which saturate at ≈200 MPa and can be attributed to generation of voids with sizes of approximately 1 nm several nanometres below the surface.

It is shown that a new type of metamaterial, a 3D-array of toroidal solenoids, displays a significant toroidal response that can be readily measured. This is in sharp contrast to materials that exist in nature, where the toroidal component is weak and hardly measurable. The existence of an optimal configuration, maximizing the interaction with an external electromagnetic field, is demonstrated. In addition, it is found that a characteristic feature of the magnetic toroidal response is its strong dependence on the background dielectric permittivity of the host material, which suggests possible applications. Negative refraction and backward waves exist in a composite toroidal metamaterial, consisting of an array of wires and an array of toroidal solenoids.

It has been shown that two-dimensional arrays of rigid or fluidlike cylinders in a fluid or a gas define, in the limit of large wavelengths, a class of acoustic metamaterials whose effective parameters (sound velocity and density) can be tailored up to a certain limit. This work goes a step further by considering arrays of solid cylinders in which the elastic properties of cylinders are taken into account. We have also treated mixtures of two different elastic cylinders. It is shown that both effects broaden the range of acoustic parameters available for designing metamaterials. For example, it is predicted that metamaterials with perfect matching of impedance with air are now possible by using aerogel and rigid cylinders equally distributed in a square lattice. As a potential application of the proposed metamaterial, we present a gradient index lens for airborne sound (i.e. a sonic Wood lens) whose functionality is demonstrated by multiple scattering simulations.

We study the multi-spin entanglement for the 1D anisotropic XY model concentrating on the simplest case of three-spin entanglement. As compared to the pairwise entanglement, three-party quantum correlations have a longer range and they are more robust on increasing the temperature. We find regions of the phase diagram of the system where bound entanglement (BE) occurs, both at zero and finite temperature. BE in the ground state can be obtained by tuning the magnetic field. Thermal BE emerges naturally due to the effect of temperature on the free ground state entanglement.

As fluids approach their gas–liquid critical points, the physical properties such as the specific heat and compressibility diverge due to the formation of large molecular clusters. Incident light cannot penetrate near-critical fluids because of the large clusters, a phenomenon known as critical opalescence. In this paper, we irradiate near-critical carbon dioxide (ncCO₂), the critical temperature and pressure of which are 31.0°C and 7.38 MPa, with a laser beam of 213, 266, 355 and 532 nm wavelength and show that CO₂ is dissociated and particles are produced when the system is set so close to the critical point that critical opalescence occurs in the case of 213 and 266 nm wavelength, whereas no particles are produced when the temperature is made to deviate from the critical value. We also apply a dc electric field to ncCO₂ during irradiation with a laser beam of 213 and 266 nm wavelength and find that particles are formed on both anode and cathode. As the intensity of the electric field increases, films are formed on the electrodes. Electron diffraction patterns and energy-dispersive x-ray, Auger electron, x-ray photoelectron and Raman spectroscopic analyses show that the particles and films are composed of amorphous carbon.

Ultraviolet light induces ferroelectric domains in SrTiO₃ at low temperatures. The ferroelectric domains are related to anharmonic soft modes of the crystal. We show the sextic anharmonicity is the lowest order for the soft mode to give rise to ferroelectricity in SrTiO₃ after photoexcitation. The occurrence of a quartic anharmonicity and/or harmonicity for the soft mode potential will suppress the domain structure in the crystal.

Recent advances in quantum key distribution (QKD) have given rise to systems that operate at transmission periods significantly shorter than the dead times of their component single-photon detectors. As systems continue to increase in transmission rate, security concerns associated with detector dead times can limit the production rate of sifted bits. We present a model of high-speed QKD in this limit that identifies an optimum transmission rate for a system with given link loss and detector response characteristics.

We generalize three symmetric cloners to the asymmetric cases and present three explicit cloning transformations as well as their corresponding fidelity distributions in d dimensions. The three asymmetric cloners, including the optimal asymmetric phase-covariant cloning (APCC) and the suboptimal asymmetric economical phase-covariant cloning (AEPCC) working without ancilla and the optimal asymmetric real state cloning (ARSC), together with the optimal asymmetric universal quantum cloning (AUQC) construct a generic cloning, where the quantum information of initial systems of different pure input states in d dimensions with their information not completely known can be optimally distributed to different final systems. By comparison of the fidelity distributions of the four asymmetric cloners, some interesting results can be obtained.

We have carried out the bulk-sensitive and high-resolution soft x-ray photoelectron spectroscopy on Lu substituted intermediate-valence compound Yb_1−xLu_xAl₃ (x = 0.4) at temperatures from 200 to 20 K. The temperature dependences of the bulk Yb 4f photoelectron spectra revealed in our preceding works on high purity YbAl₃ have not been observed in this Lu substituted system. The temperature dependences of the bulk Yb 4f peak positions and the Yb valence in this system can be well reproduced by the single impurity Anderson model (SIAM), whereas the spectral behaviors in YbAl₃ were not at all reproduced by the SIAM. These results confirm the importance of the Kondo lattice effects for YbAl₃, for which the coherent lattice periodicity plays essential roles.

We study the dynamics of a symmetric two-level system strongly coupled to a broadened harmonic mode. Upon mapping the problem on to a spin–boson model with peaked spectral density, we show how analytic solutions can be obtained, at arbitrary detuning and finite temperatures, in the case of large Q-factors of the oscillator. One, two or more dominating oscillation frequencies of the two-level particle can be observed as a consequence of the entanglement with the oscillator. Our approximated analytical solution agrees well with numerical predictions obtained within the non-interacting blip approximation.

The radiative biexciton–exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization-entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4 K to 68% at 30 K. This is especially encouraging for future implementations in practical devices.

A variety of optical quantum information networks based on the multipartite entanglement of amplitude and phase quadratures of an electromagnetic field have been proposed and experimentally realized in recent years. The multipartite entanglement of optical continuous variables provides flexible and reliable quantum resources for developing unconditional quantum information networks. In this paper, we review the generation schemes of the multipartite entangled states of optical continuous quantum variables and some applications in the quantum communication networks with emphasis on the experimental implementations.

It is shown that the collision integral describing the nonlocal character of collisions leads to the same mean-field fluctuations in the one-particle distribution as proposed by Boltzmann–Langevin pictures. It is argued that this appropriate collision integral contains the fluctuation–dissipation theorems in equilibrium itself and therefore there is no need to assume additionally stochasticity. This leads to tremendous simplifications in numerical simulation schemes.

We have investigated the remanent coupling between a soft-magnetic and a hard-magnetic film separated by a nonmagnetic spacer layer. It turned out that the remanent coupling angle of the soft-magnetic layer relative to the hard-magnetic layer can be adjusted by temporarily applied external fields. The underlying mechanism is the competition between the exchange field and the soft layer coercive field. This could be applied in modern magnetic recording techniques like MRAMs to realize multiple or even continuous states per memory cell, thus constituting a re-emergence of analogue recording technology with the potential of much higher information density.

This work reviews several concentric measurements of the topology of complex networks and then applies feature selection concepts and methods in order to quantify the relative importance of each measurement with respect to the discrimination between four representative theoretical network models, namely Erdös–Rényi, Barabási–Albert, Watts–Strogatz, as well as a geographical type of network. Progressive randomizations of the geographical model have also been considered. The obtained results confirmed that the four models can be well-separated by using a combination of measurements. In addition, the relative contribution of each considered feature for the overall discrimination of the models was quantified in terms of the respective weights in the canonical projection into two-dimensions, with the traditional clustering coefficient, concentric clustering coefficient and neighborhood clustering coefficient being particularly effective. Interestingly, the average shortest path length and concentric node degrees contributed little for the separation of the four network models.

We show that the amount of coherent quantum information that can be reliably transmitted down a dephasing channel with memory is maximized by separable input states. In particular, we model the channel as a Markov chain or a multimode environment of oscillators. While in the first model, the maximization is achieved for the maximally mixed input state, in the latter it is convenient to exploit the presence of a decoherence-protected subspace generated by memory effects. We explicitly compute the quantum channel capacity for the first model while numerical simulations suggest a lower bound for the latter. In both cases memory effects enhance the coherent information. We present results valid for arbitrary input size.