Table of contents

If we look around, everything we see is surfaces. What we cannot see, however, are the atomistic and electronic processes that occur at surfaces (and interfaces), playing a crucial role in the properties, function, and performance of advanced materials and in nanoscale technologies.

Basic research in surface and interface science is highly interdisciplinary, covering the fields of physics, chemistry, bio-physics, geo-, atmospheric and environmental sciences, material science, chemical engineering, and more. The various phenomena are interesting by themselves, and they are most important in nearly all modern technologies, as for example electronic, magnetic, and optical devices, sensors, catalysts, lubricants, hard and thermal-barrier coatings, protection against corrosion and crack formation under harsh environments. In fact, detailed understanding of the elementary processes at surfaces is necessary to support and to advance the high technology that very much founds the prosperity and lifestyle of our society. The strength of surface science as a discipline has been recognized by the award of the 2007 Nobel Prize in Chemistry to Prof. Gerhard Ertl for his studies of chemical processes on solid surfaces.

Current state-of-the-art experimental studies of elementary processes at surfaces, of surface properties and functions employ a variety of sophisticated tools. Some are capable of revealing the location and motion of individual atoms. Others measure excitations (electronic, magnetic, vibronic), for example employing special light sources such as synchrotrons, high magnetic fields, or free electron lasers. The surprising variety of intriguing physical phenomena at surfaces, interfaces, and nanostructures also poses a persistent challenge for the development of theoretical descriptions, methods, and even basic physical concepts.

This Focus Issue in New Journal of Physics provides a synoptic view on pertinent developments in the field.

Focus on Advances in Surface and Interface Science Contents

Thermal contact delocalization in atomic scale friction: a multitude of friction regimes Sergey Yu Krylov and Joost W M Frenken

Ultrafast optical excitations of metallic nanostructures: from light confinement to a novel electron source Claus Ropers, Thomas Elsaesser, Giulio Cerullo, Margherita Zavelani-Rossi and Christoph Lienau

Complex magnetism of the Fe monolayer on Ir(111) Kirsten von Bergmann, Stefan Heinze, Matthias Bode, Gustav Bihlmayer, Stefan Blügel and Roland Wiesendanger

Adsorption-induced chirality in highly symmetric hydrocarbon molecules: lattice matching to substrates of lower symmetry Neville V Richardson

Dynamics of electron transfer at polar molecule–metal interfaces: the role of thermally activated tunnelling J Stähler, M Meyer, X Y Zhu, U Bovensiepen and M Wolf

Simulating adsorption of complex molecules using the linearity between interaction energies and tunnelling currents: the case of hexabenzocoronene on a Ag/Pt dislocation network K Palotás and W A Hofer

Adsorbate induced self-ordering of germanium nanoislands on Si(113) Thomas Schmidt, Torben Clausen, Jan Ingo Flege, Subhashis Gangopadhyay, Andrea Locatelli, Tevfik Onur Mentes, Fang Zhun Guo, Stefan Heun and Jens Falta

ARPES and STS investigation of Shockley states in thin metallic films and periodic nanostructures D Malterre, B Kierren, Y Fagot-Revurat, S Pons, A Tejeda, C Didiot, H Cercellier and A Bendounan

Ultrafast energy flow in model biological membranes Marc Smits, Avishek Ghosh, Jens Bredenbeck, Susumu Yamamoto, Michiel Müller and Mischa Bonn

Epitaxy of GaN on silicon—impact of symmetry and surface reconstruction A Dadgar, F Schulze, M Wienecke, A Gadanecz, J Bläsing, P Veit, T Hempel, A Diez, J Christen and A Krost

Effect of quantum confinement of surface electrons on adatom–adatom interactions V S Stepanyuk, N N Negulyaev, L Niebergall and P Bruno

Temporal step fluctuations on a conductor surface: electromigration force, surface resistivity and low-frequency noise E D Williams, O Bondarchuk, C G Tao, W Yan, W G Cullen, P J Rous and T Bole

Surface resonances on transition metals as low-dimensional model systems M Minca, S Penner, E Dona, A Menzel, E Bertel, V Brouet and J Redinger

Symmetry breaking in few layer graphene films Aaron Bostwick, Taisuke Ohta, Jessica L McChesney, Konstantin V Emtsev, Thomas Seyller, Karsten Horn and Eli Rotenberg

Matthias Scheffler, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany Wolf-Dieter Schneider, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

This paper summarizes and extends results from a series of recent investigations of atomic scale friction, in which an ultra-low effective mass and a corresponding thermal delocalization of the contact play a dominant role. A rich variety of physically different regimes of friction concerned with the contact delocalization are analyzed in a systematic way and visualized by advanced numerical calculations. The results shed an essentially new light on what is actually measured in friction force microscopy and suggest the necessity to reinterpret many seemingly standard experiments. Even more importantly, our results can possibly be extended to the asperities that establish the contact between two sliding bodies thus predicting a much more pronounced role of thermally driven dynamics in macroscopic sliding than has ever been imagined. The paper is supplied with a detailed introduction to the subject, aimed at a general physical audience.

Combining ultrafast coherent spectroscopy with nano-optical microscopy techniques offers a wealth of new possibilities for exploring the structure and function of nanostructures. In this paper, we describe newly developed nano-optical methods based on short-pulse laser sources with durations in the 10 fs regime. These techniques are used to unravel some of the intricate dynamics of elementary excitations in metallic nanostructures. Specifically, we explore light localization and storage in plasmonic crystals, demonstrate field enhancement and second harmonic generation from metallic nanotips and describe a novel nanometre-sized source of electron pulses. The rapid progress in this area offers exciting new prospects for probing and controlling electron dynamics in metallic nanostructures with femtosecond temporal and nanometre spatial resolution.

The electronic and magnetic properties of Fe on Ir(111) have been investigated experimentally by spin-polarized scanning tunneling microscopy (SP-STM) and theoretically by first-principles calculations based on density functional theory. While the growth of an Fe monolayer is in-plane commensurate, deposition of a double-layer shows a rearrangement of atoms due to strain relief accompanied by local variations of the electronic structure. Both stackings of the monolayer, i.e. face centered cubic (fcc) and hexagonal closed packed (hcp), are observed experimentally. The magnetic structure of both types is imaged with SP-STM. From these experiments, we propose a nanoscale magnetic mosaic structure for the fcc-stacking with 15 atoms in the unit cell. For hcp-stacking, the tunneling spectra are similar to the fcc case, however, the magnetic contrast in the SP-STM images is not as obvious. In our first-principles calculations, a collinear antiferromagnetic (AFM) state with a 15 atom in-plane unit cell (AFM 7 : 8 state) is found to be more favorable than the ferromagnetic state for both fcc- and hcp-stacking. Calculated SP-STM images and spectra are also in good agreement with the experimental data for the fcc case. We performed spin spiral calculations which are mapped to a classical Heisenberg model to obtain the exchange-interaction constants. From these calculations, it is found that the AFM 7 : 8 state is energetically more favorable than all solutions of the classical Heisenberg model. While the obtained magnetic exchange constants are rather similar for the fcc and hcp stacking, a comparison with the experiments indicates that competing interactions could be responsible for the differences observed in the magnetically sensitive measurements.

For molecules of high symmetry and lateral interactions dominated by van der Waals' interactions, such as some planar aromatic hydrocarbons, there is a preference for hexagonal close packing on adsorption. Optimization of packing by minimization of the interadsorbate spacing may favour correlated rotations of the molecules, which reduces the symmetry and leads to chiral properties in the two-dimensional overlayer. Independently, mapping of the preferred hexagonal packing on to lower symmetry substrates, which provide pseudo-hexagonal lattices, can lead to alternative mirror image lattices. The interaction of these independent chiral phenomena gives rise to diastereoisomerism in the adsorbed array. Coronene and its substituted, larger derivative hexa-tert-butyl hexabenzocoronene adsorbed on copper surfaces provide examples of these phenomena. A new structure is proposed for coronene on Cu{100} while new STM and LEED data are presented for this molecule adsorbed on Cu{110}. Finally, the adsorption of hexa-tert-butyl hexabenzocoronene on Cu{110} is re-examined and the implications of the competition between two, closely related, pseudo-hexagonal lattices are considered.

Heterogeneous electron transfer (ET) across interfaces is frequently discussed on the basis of Marcus theory taking into account the rearrangements of the solvent along a nuclear coordinate q. The ET process itself occurs via tunnelling through a barrier normal to the interface. The key point is not whether tunnelling occurs, but whether thermally activated solvent fluctuations initiate the tunnelling. Here, we discuss the role of thermally activated tunnelling in heterogeneous ET versus direct ET due to the strong electronic coupling to a metal substrate. As a model system, we investigate the ultrafast dynamics of ET at amorphous ice–metal interfaces (4–6 bilayers D₂O/Cu(111) and Ru(001), respectively) by time-resolved two-photon photoelectron spectroscopy. We find that the ET rate is independentof temperature within the first 500 fs after excitation, which demonstrates that for this system interfacial ET occurs in the strong-coupling limit and that thermally assisted tunnelling plays a negligible role.

We present a method for determining the adsorption position of complex molecules on surfaces from first-principles calculations. We use electron transport theory through a vacuum barrier (theory of scanning tunnelling microscopy), and the relationship between the tunnelling current and the interaction energy between surface and tip. This method is especially useful for obtaining reasonable adsorption positions of relatively large molecules (>50 atoms) on reconstructed surfaces, e.g. dislocation networks. The main advantage is that this approach is computationally efficient and does not require mapping of all adsorption sites by separate simulations. The reliability is illustrated by simulating the adsorption of hexabenzocoronene on a model Ag/Pt(111) surface.

The impact of Ga preadsorption on the spatial correlation of nanoscale three-dimensional (3D) Ge-islands has been investigated by low-energy electron microscopy and low-energy electron diffraction. Submonolayer Ga adsorption leads to the formation of a 2D chemical nanopattern, since the Ga-terminated (2×2) domains exclusively decorate the step edges of the Si(113) substrate. Subsequent Ge growth on such a partially Ga-covered surface results in Ge 3D islands with an increased density as compared to Ge growth on clean Si(113). However, no pronounced alignment of the Ge islands is observed. Completely different results are obtained for Ga saturation coverage, which results in the formation of (112) and (115) facets regularly arranged with a periodicity of about 40 nm. Upon Ge deposition, Ge islands are formed at a high density of about 1.3×10¹⁰ cm⁻². These islands are well ordered as they align at the substrate facets. Moreover, the facet array induces a reversal of the Ge islands' shape anisotropy as compared to growth on planar Si(113) substrates.

Due to their extreme surface sensitivity, the Shockley states of (111) noble metal surfaces can be used to study the modifications of atomic and electronic properties of epitaxial ultra thin films and self-organized nanostructures. In metallic interfaces, the different parameters of the Shockley surface state bands (energy, effective mass and eventually spin–orbit splitting) have been shown to be strongly thickness dependent. It was also possible by scanning tunneling spectroscopy to evidence a spectroscopic signature of buried interfaces. Moreover, superperiodic surface structures like the reconstruction on Au(111) vicinal surfaces or self-organized nanodots, lead to spectacular spectroscopic effects. In the vicinal Au(23 23 21) surface, the opening of tiny energy gaps associated with the reconstruction potential of such surfaces has been evidenced. Peculiar growth on these Au vicinal surfaces allows us to obtain high quality self-assembled metallic nanostructures which exhibit homogeneous electronic properties on a large spatial scale resulting from a coherent scattering of the Shockley states.

We report on the energy flow dynamics in model membranes, investigated by surface-specific time-resolved (femtosecond) sum frequency generation spectroscopy. This recently developed technique allows us to probe energy dynamics selectively at the water/lipid interface. We report vibrational relaxation dynamics of C–H stretch modes in the lipid alkyl chains, and reveal that incoherent energy transfer occurs from the excited CH₂ groups to the terminal CH₃ groups. We also find evidence for strong anharmonic coupling between different CH₂ and CH₃ modes. Relaxation and the energy transfer processes within the lipid alkyl chain occur on (sub-)picosecond timescales. Studies of the dynamics on different lipid phases (gel or liquid crystalline phase) reveal a marked independence of the dynamics on the precise molecular conformation of the lipids. In addition, we report the energy transfer dynamics between membrane-bound water and lipids, and find that the transfer of heat between water and lipids occurs remarkably fast: heat is transferred across the monolayer, from the polar head group region of the lipid to the end of the alkyl chain, within 1 ps. These results demonstrate the potential of using ultrafast surface-specific spectroscopies to elucidate biomolecular dynamics at membrane surfaces.

GaN-on-silicon is a low-cost alternative to growth on sapphire or SiC. Today epitaxial growth is usually performed on Si(111), which has a threefold symmetry. The growth of single crystalline GaN on Si(001), the material of the complementary metal oxide semiconductor (CMOS) industry, is more difficult due to the fourfold symmetry of this Si surface leading to two differently aligned domains. We show that breaking the symmetry to achieve single crystalline growth can be performed, e.g. by off-oriented substrates to achieve single crystalline device quality GaN layers. Furthermore, an exotic Si orientation for GaN growth is Si(110), which we show is even better suited as compared to Si(111) for the growth of high quality GaN-on-silicon with a nearly threefold reduction in the full width at half maximum (FWHM) of the -scan. It is found that a twofold surface symmetry is in principal suitable for the growth of single crystalline GaN on Si.

The quantum confinement of surface-state electrons in atomic-scale nanostructures is studied by means of the Korringa–Kohn–Rostoker (KKR) Green's function method. We demonstrate that the surface-state mediated interaction between adatoms can be significantly modified by the quantum confinement of surface electrons. We show that quantum corrals and quantum mirrors constructed on metal surfaces can be used to tailor the exchange interaction between magnetic adatoms at large distances. We discuss the self-organization of adatoms on metal surfaces caused by quantum confinement.

Scattering of charge carriers from surface structures will become an increasing factor in the resistivity as the structure decreases in size to the nanoscale. The effects of scattering at the most basic surface defect, a kink in a step edge, are here analyzed using the continuum step model. Using a Langevin analysis, it has been shown that the electromigration force on the atoms at the step edge causes changes in the temporal evolution of the step-edge. For an electromigration force acting perpendicular to the average step edge and mass-transport dominated by step-edge diffusion, significant deviations from the usual t^1/4 scaling of the displacement correlation function occur dependent on a critical time τ and the direction of the force relative to the step edge (i.e. uphill or downhill). Experimental observations of step fluctuations on Ag(111) show the predicted changes among step fluctuations without current, and with current in the up- and down-hill directions for a current density of order 10⁵ A cm⁻². The results yield the magnitude of the electromigration force acting on kinked sites at the step-edge. This in turn yields the contribution of the fluctuating steps to the surface resistivity, which exceeds 1% of the bulk resistivity as wire diameters decrease below 10s of nanometres. The temporal fluctuations of kink density can thus also be related to resistivity noise. Relating the known fluctuation spectrum of the step displacements to fluctuations in their lengths, the corresponding resistivity noise is predicted to show spectral signatures of ∼f^−1/2 for step fluctuations governed by random attachment/detachment, and ∼f^−3/4 for step fluctuations governed by step-edge diffusion.

Finding and investigating low-dimensional model systems is essential to improve the understanding of metals with strong electron correlation. Here, we show that suitably chosen transition metal surfaces can provide such model systems. Comparing the band structure from density functional theory (DFT) with angular resolved photoemission (ARPES) for Pt(110), we give evidence for a low-dimensional surface resonance. Details of the band topology and fingerprints of low-dimensional behaviour are presented.

Recently, it was demonstrated that the quasiparticle dynamics, the layer-dependent charge and potential, and the c-axis screening coefficient could be extracted from measurements of the spectral function of few layer graphene films grown epitaxially on SiC using angle-resolved photoemission spectroscopy (ARPES). In this paper we review these findings, and present detailed methodology for extracting such parameters from ARPES. We also present detailed arguments against the possibility of an energy gap at the Dirac crossing E_D.

The basic operating element of standard quantum computation is the qubit, an isolated two-level system that can be accurately controlled, initialized and measured. However, the majority of proposed physical architectures for quantum computation are built from systems that contain much more complicated Hilbert space structures. Hence, defining a qubit requires the identification of an appropriate controllable two-dimensional sub-system. This prompts the obvious question of how well a qubit, thus defined, is confined to this subspace, and whether we can experimentally quantify the potential leakage into states outside the qubit subspace. We demonstrate how subspace leakage can be characterized using minimal theoretical assumptions by examining the Fourier spectrum of the oscillation experiment.

Patterns and collective phenomena such as firing synchronization are studied in networks of nonhomogeneous oscillatory neurons and mixtures of oscillatory and excitable neurons, with dynamics of each neuron described by a two-dimensional (2D) Rulkov map neuron. It is shown that as the coupling strength is increased, typical patterns emerge spatially, which propagate through the networks in the form of beautiful target waves or parallel ones depending on the size of networks. Furthermore, we investigate the transitions of firing synchronization characterized by the rate of firing when the coupling strength is increased. It is found that there exists an intermediate coupling strength; firing synchronization is minimal simultaneously irrespective of the size of networks. For further increasing the coupling strength, synchronization is enhanced. Since noise is inevitable in real neurons, we also investigate the effects of white noise on firing synchronization for different networks. For the networks of oscillatory neurons, it is shown that firing synchronization decreases when the noise level increases. For the missed networks, firing synchronization is robust under the noise conditions considered in this paper. Results presented in this paper should prove to be valuable for understanding the properties of collective dynamics in real neuronal networks.

We consider the possibility to extract spins that are generated by an electric current in a two-dimensional electron gas with Rashba–Dresselhaus spin–orbit interaction (R2DEG) in the Hall geometry. To this end, we discuss boundary conditions for the spin accumulations between a spin–orbit (SO) coupled region and a contact without SO coupling, i.e. a normal two-dimensional electron gas (2DEG). We demonstrate that in contrast to contacts that extend along the whole sample, a spin accumulation can diffuse into the normal region through finite contacts and be detected by e.g. ferromagnets. For an impedance-matched narrow contact the spin accumulation in the 2DEG is equal to the current induced spin accumulation in the bulk of R2DEG up to a geometry-dependent numerical factor.

A novel bionic network nanostructure of zinc oxide (ZnO), which is similar to the microstructure of a butterfly wing, was first fabricated by a vapor-phase transport method using zinc powder as a source. These bionic nanostructures are composed of three ordered multi-aperture gratings. Similar to the optical effect of butterfly wings, the diffraction patterns of the bionic network of ZnO were observed. The mechanism of the optical function was discussed based on the physical model of multi-aperture diffraction.

Based on a combination of cluster dynamical mean field theory (DMFT) and density functional calculations, we calculated the angle-integrated spectral density in the layered s=1/2 quantum magnet TiOCl. The agreement with recent photoemission and oxygen K-edge x-ray absorption spectroscopy experiments is found to be good. The improvement achieved with this calculation with respect to previous single-site DMFT calculations is an indication of the correlated nature and low-dimensionality of TiOCl.

Hybrid systems of thin films of oxide ferromagnets and high-temperature superconductors have been investigated by scanning Hall probe microscopy (SHPM) to analyze the local magnetic flux density distribution at low temperatures. In addition to the intrinsic properties of the films themselves, such structures exhibit novel phenomena due to complex interactions arising at the interface between them. The latter can be divided into processes originating from either electronic or magnetic coupling, respectively. As a direct consequence, the distribution of vortices in the superconductor is strongly influenced by the magnetic background arising from the ferromagnet. The local magnetic information obtained from SHPM images provides clear evidence for the presence of a magnetic dipolar interaction between the magnetic domains of the ferromagnetic component and the vortex ensemble in the superconductor.

In this paper, we consider instabilities of localized solutions in planar neural field firing rate models of Wilson–Cowan or Amari type. Importantly we show that angular perturbations can destabilize spatially localized solutions. For a scalar model with Heaviside firing rate function, we calculate symmetric one-bump and ring solutions explicitly and use an Evans function approach to predict the point of instability and the shapes of the dominant growing modes. Our predictions are shown to be in excellent agreement with direct numerical simulations. Moreover, beyond the instability our simulations demonstrate the emergence of multi-bump and labyrinthine patterns.

With the addition of spike-frequency adaptation, numerical simulations of the resulting vector model show that it is possible for structures without rotational symmetry, and in particular multi-bumps, to undergo an instability to a rotating wave. We use a general argument, valid for smooth firing rate functions, to establish the conditions necessary to generate such a rotational instability. Numerical continuation of the rotating wave is used to quantify the emergent angular velocity as a bifurcation parameter is varied. Wave stability is found via the numerical evaluation of an associated eigenvalue problem.

Cosmic acceleration is explained quantitatively, purely in general relativity with matter obeying the strong energy condition, as an apparent effect due to quasilocal gravitational energy differences that arise in the decoupling of bound systems from the global expansion of the universe. 'Dark energy' is recognized as a misidentification of those aspects of gravitational energy which by virtue of the equivalence principle cannot be localized. Matter is modelled as an inhomogeneous distribution of clusters of galaxies in bubble walls surrounding voids, as we observe. Gravitational energy differences between observers in bound systems, such as galaxies, and volume-averaged comoving locations in freely expanding space can be so large that the time dilation between the two significantly affects the parameters of any effective homogeneous isotropic model one fits to the universe. A new approach to cosmological averaging is presented, which implicitly solves the Sandage–de Vaucouleurs paradox. Comoving test particles in freely expanding space, which observe an isotropic cosmic microwave background (CMB), possess a quasilocal 'rest' energy E=⟨γ(τ,x)⟩mc² on the spatial hypersurfaces of homogeneity. Here : the lower bound refers to fiducial reference observers at 'finite infinity', which is defined technically in relation to the demarcation scale between bound systems and expanding space. Within voids γ>1, representing the quasilocal gravitational energy of expansion and spatial curvature variations. Since all our cosmological measurements apart from the CMB involve photons exchanged between objects in bound systems, and since clocks in bound systems are largely unaffected, this is entirely consistent with observation. When combined with a non-linear scheme for cosmological evolution with back-reaction via the Buchert equations, a new observationally viable model of the universe is obtained, without 'dark energy'. A quantitative scheme is presented for the recalibration of average cosmological parameters. It uses boundary conditions at the time of last scattering consistent with primordial inflation. The expansion age is increased, allowing more time for structure formation. The baryon density fraction obtained from primordial nucleosynthesis bounds can be significantly larger, yet consistent with primordial lithium abundance measurements. The angular scale of the first Doppler peak in the CMB anisotropy spectrum fits the new model despite an average negative spatial curvature at late epochs, resolving the anomaly associated with ellipticity in the CMB anisotropies. Non-baryonic dark matter to baryonic matter ratios of about 3:1 are typically favoured by observational tests. A number of other testable consequences are discussed, with the potential to profoundly change the whole of theoretical and observational cosmology.

We investigate the impulsively excited acoustic dynamics of nanoscale Au triangles of different sizes and thicknesses on silicon and glass substrates. We employ high-speed asynchronous optical sampling in order to study the damping of the acoustic vibrations with high sensitivity in the time domain. From the observed damping dynamics we deduce the reflection coefficient of acoustic energy from the gold–substrate interface. The observed damping times of coherent acoustic vibrations are found to be significantly longer than expected from the acoustic impedance mismatch for an ideal gold–substrate interface, hence pointing towards a reduced coupling strength. The strength of the coupling can be determined quantitatively. For Au triangles with large lateral size-to-thickness ratio, i.e. a small aspect ratio, the acoustic dynamics is dominated by a thickness oscillation similar to that of a closed film. For triangles with large aspect ratio the coherent acoustic excitation consists of a superposition of different three-dimensional modes which exhibit different damping times.

We investigate the phase structure of an asymmetric fermion superfluid with inter- and intra-species pairings. The introduction of the intra-species pairing mechanism in the canonical ensemble changes significantly the phase diagram and brings in a new state with coexisting inter- and intra-species pairings. Different from the case with only inter-species pairing, all the fermion excitations are fully gapped in the region with intra-species pairing.

The evolutional optical behaviours (turn-on dynamics) of a four-level double-control tripod-configuration (electromagnetically induced transparency) system are considered based on the transient solution to the equation of motion of the probability amplitudes of the atomic levels. As the most remarkable property (quantum interference between the two control transitions) will arise in the present tripod-configuration system, the transient evolution of the permittivity in cases of both destructive and constructive quantum interferences is presented. It can be shown that the four-level double-control vapour can become a destructive-interference medium, (exhibiting a two-level resonant absorption) and a constructive-interference medium, (exhibiting transparency to the probe field), respectively, under certain conditions (related to the ratio of the two control field intensities). The present double-control scenario can be applicable to designs of some new photonic and quantum optical devices such as logic and functional devices (logic gate circuits) and the key component of the technology of quantum coherent information storage.

We investigate the influence of a deterministic but non-synchronous update on random Boolean networks, with a focus on critical networks. Knowing that 'relevant components' determine the number and length of attractors, we focus on such relevant components and calculate how the length and number of attractors on these components are modified by delays at one or more nodes. The main findings are that attractors decrease in number when there are more delays and that periods may become very long when delay times are not integer multiples of the basic update step.

Many-body effects in solids are related to the correlation among electrons. This mutual interaction between the electrons can be probed by electron pair emission spectroscopy. We have investigated the electron pair emission from a LiF(100) surface upon excitation with low kinetic energy electrons. Our angular distributions clearly show that the emission direction of one electron is surrounded by a reduced intensity of the other electron. This depletion zone of electronic intensity is a manifestation of the exchange and correlation hole. We show that we are able to observe the full extension and shape of the depletion zone. It has an angular extension of ≈1.2 rad and is independent of the electron energy. Additionally, we discovered that the angle between the trajectories of the electrons has a profound effect on the two-dimensional energy distribution.

We report the experimental realization of squeezed quantum states of light, tailored for new applications in quantum communication and metrology. Squeezed states in a broad Fourier frequency band down to 1 Hz have been observed for the first time. Nonclassical properties of light in such a low frequency band are required for high efficiency quantum information storage in electromagnetically induced transparency (EIT) media. The states observed also cover the frequency band of ultra-high precision laser interferometers for gravitational wave detection and can be used to reach the regime of quantum non-demolition interferometry. Furthermore, they cover the frequencies of motion of heavy macroscopic objects and might therefore support attempts to observe entanglement in our macroscopic world.

Recent experiments have shown that truncated Gauss sums allow us to find the factors of an integer N. This method relies on the fact that for a factor the absolute value of the Gauss sum is unity. However, for every integer N there exist integers which are not factors, but where the Gauss sum reaches a value which is arbitrarily close to unity. In order to distinguish such ghost factors from real factors we need to amplify this difference. We show that a proper choice of the truncation parameter of the Gauss sum suppresses the ghost factors below a threshold value. We derive the scaling law of the truncation parameter on the number to be factored. Moreover, we show that this scaling law is also necessary for the success of our factorization scheme, even if we relax the threshold or allow limited error tolerance.

A general procedure is presented which permits the form of an extended spin Hamiltonian to be established for a given magnetic solid and the magnitude of its terms to be evaluated from spin polarized, Hartree–Fock or density functional calculations carried out for periodic models. The computational strategy makes use of a general mapping between the energy of pertinent broken-symmetry solutions and the diagonal terms of the spin Hamiltonian in a local representation. From this mapping it is possible to determine not only the amplitude of the well-known two-body magnetic coupling constants between near-neighbor sites, but also the amplitudes of four-body cyclic exchange terms. A scrutiny of the on-site spin densities provides additional information and control of the many broken-symmetry solutions which can be found. The procedure is applied to the La₂CuO₄, Sr₂CuO₂F₂, Sr₂CuO₂Cl₂ and Ca₂CuO₂Cl₂ square lattices and the SrCu₂O₃ ladder compound. It is shown that a proper description of the magnetic structure of these compounds requires that two- and four-body terms are explicitly included in the spin Hamiltonian. The implications for the interpretation of recent experiments are discussed.

We report on the observation of two-photon electron emission from silver nanoparticles suspended in nitrogen flow resulting from irradiating them with continuous wave and pulsed laser light with photon energies below the threshold of the single-photon photoelectric effect. The photoelectron yield is quadratic in the light intensity, and the two-photon electron emission threshold is evident. The efficiency of the two-photon photoelectric effect is determined for nanoparticles of various sizes. These experiments offer the net information on nonlinear quantum properties of an isolated single nanoparticle which is crucial for developing theoretical models.

This work aims at a more fundamental understanding of the rheological behaviour of nanofluids and the interpretation of the discrepancy in the recent literature. Both experiments and theoretical analyses are carried out with the experimental work on ethylene glycol (EG)-based nanofluids containing 0.5–8.0 wt% spherical TiO₂ nanoparticles at 20–60 °C and the theoretical analyses on the high shear viscosity, shear thinning behaviour and temperature dependence. The experimental results show that the EG-based nanofluids are Newtonian under the conditions of this work with the shear viscosity as a strong function of temperature and particle concentration. The relative viscosity of the nanofluids is, however, independent of temperature. The theoretical analyses show that the high shear viscosity of nanofluids can be predicted by the Krieger–Dougherty equation if the effective nanoparticle concentration is used. For spherical nanoparticles, an aggregate size of approximately 3 times the primary nanoparticle size gives the best prediction of experimental data of both this work and those from the literature. The shear thinning behaviour of nanofluids depends on the effective particle concentration, the range of shear rate and viscosity of the base liquid. Such non-Newtonian behaviour can be characterized by a characteristic shear rate, which decreases with increasing volume fraction, increasing base liquid viscosity, or increasing aggregate size. These findings explain the reported controversy of the rheological behaviour of nanofluids in the literature. At temperatures not very far from the ambient temperature, the relative high shear viscosity is independent of temperature due to negligible Brownian diffusion in comparison to convection in high shear flows, in agreement with the experimental results. However, the characteristic shear rate can have strong temperature dependence, thus affecting the shear thinning behaviour. The theoretical analyses also lead to a classification of nanofluids into dilute, semi-dilute, semi-concentrated and concentrated nanofluids depending on particle concentration and particle structuring.

Competition between the vortex lattice and a lattice of asymmetric artificial defects is shown to play a crucial role in ratchet experiments in superconducting films. We present a novel and collective mechanism for current reversal based on a reconfiguration of the vortex lattice. In contrast to previous models of vortex current reversal, the mechanism is based on the global response of the vortex lattice to external forces.

We show storage of the circular polarization of an optical field, transferring it to the spin-state of an individual electron confined in a single semiconductor quantum dot. The state is subsequently read out through the electronically-triggered emission of a single photon. The emitted photon shares the same polarization as the initial pulse but has a different energy, making the transfer of quantum information between different physical systems possible. With an applied magnetic field of 2 T, spin memory is preserved for at least 1000 times more than the exciton's radiative lifetime.

The sensitivity of magnetoresistive devices, i.e. their ability to be switched by very small fields, depends on the softness of their soft electrode. In this paper, we show the possibility to tune the coercive field of the CoFe₂ alloy, commonly used as a soft electrode, from intrinsic values down to zero, by varying the pulsed laser ablation conditions of CoFe₂/CoFe₂O₄ bilayers. This tuning possibility relies on the existence of a frustration of the spins of the CoFe₂ layer originating from both the ferrimagnetic nature of the CoFe₂O₄ layer and the oxide/metal interface roughness.

We present a simple and cost-effective optical protocol to realize contrast-enhancement imaging (such as dark-field, optical-staining and oblique illumination microscopy) of transparent samples on a conventional widefield microscope using commercial multimedia projectors. The projector functions as both light source and mask generator implemented by creating slideshows of the filters projected along the illumination planes of the microscope. The projected optical masks spatially modulate the distribution of the incident light to selectively enhance structures within the sample according to spatial frequency thereby increasing the image contrast of translucent biological specimens. Any amplitude filter can be customized and dynamically controlled so that switching from one imaging modality to another involves a simple slide transition and can be executed at a keystroke with no physical filters and no moving optical parts. The method yields an image contrast of 89–96% comparable with standard enhancement techniques. The polarization properties of the projector are then utilized to discriminate birefringent and non-birefringent sites on the sample using single-shot, simultaneous polarization and optical-staining microscopy. In addition to dynamic pattern generation and polarization, the projector also provides high illumination power and spectral excitation selectivity through its red-green-blue (RGB) channels. We exploit this last property to explore the feasibility of using video projectors to selectively excite stained samples and perform fluorescence imaging in tandem with reflectance and polarization reflectance microscopy.

Dual hybridization of highly fluorescent conjugated polymers and highly luminescent nanocrystals (NCs) is developed and demonstrated in multiple combinations for controlled white light generation with high color rendering index (CRI) (> 80) for the first time. The generated white light is tuned using layer-by-layer assembly of CdSe/ZnS core-shell NCs closely packed on polyfluorene, hybridized on near-UV emitting nitride-based light emitting diodes (LEDs). The design, synthesis, growth, fabrication and characterization of these hybrid inorganic–organic white LEDs are presented. The following experimental realizations are reported: (i) layer-by-layer hybridization of yellow NCs (λ_PL=580 nm) and blue polyfluorene (λ_PL=439 nm) with tristimulus coordinates of (x, y)=(0.31, 0.27), correlated color temperature of T_c=6962 K and CRI of R_a=53.4; (ii) layer-by-layer assembly of yellow and green NCs (λ_PL=580 and 540 nm) and blue polyfluorene (λ_PL=439 nm) with (x, y)=(0.23, 0.30), T_c=14395 K and R_a=65.7; and (iii) layer-by-layer deposition of yellow, green and red NCs (λ_PL=580, 540 and 620 nm) and blue polyfluorene (λ_PL=439 nm) with (x, y)=(0.38, 0.39), T_c=4052 K and R_a= 83.0. The CRI is demonstrated to be well controlled and significantly improved by increasing multi-chromaticity of the NC and polymer emitters.

In a recent article Anders et al 2006 Phys. Rev. Lett. 97 107206, we have presented a class of states which is suitable as a variational set to find ground states in spin systems of arbitrary spatial dimension and with long-range entanglement. Here, we continue the exposition of our technique, extend from spin 1/2 to higher spins and use the boson Hubbard model as a non-trivial example to demonstrate our scheme.

Polymers are remarkable molecules that have relaxation times that can span 15 orders of magnitude. The very longest of the relaxation times for high molecular weight polymers are sufficiently long to overlap with fluid mechanical times scales; under those circumstances, polymers can influence the flow. A well-known example that is still not fully understood is polymer drag reduction. It has been known since Toms (1949 Proc. 1st Int. Congress on Rheology 2 135–41) that parts per million (mass basis) concentrations of polymers can reduce the drag on a solid surface by as much as 80%. Understanding the mechanism of drag reduction requires an understanding of the dynamics of the dissolved polymer chain in response to local fluctuations in the turbulent flow field. We investigate this by using Brownian dynamics simulations of bead-spring models of polymers immersed in a turbulent solvent that is separately computed using direct numerical simulations. We observe that polymer chains with parameters that are effective for drag reduction generally remain stretched for long periods of time and only occasionally relax. The relatively restricted configuration space they sample makes it reasonable to represent their behavior with simpler dumbbell models. We also study the spatial structure of the polymer stresses using a Lagrangian strategy. The results explain the need for relatively high spatial resolution for numerical simulations of polymer flows.

Metamaterials are constructed such that, for a narrow range of frequencies, the momentum density depends on the local displacement gradient and the stress depends on the local acceleration. In these models the momentum density generally depends not only on the strain, but also on the local rotation, and the stress is generally not symmetric. A variant is constructed for which, at a fixed frequency, the momentum density is independent of the local rotation (but still depends on the strain) and the stress is symmetric (but still depends on the acceleration). Generalizations of these metamaterials may be useful in the design of elastic cloaking devices.

We report the experimental demonstration of a novel method to slow atoms and molecules with permanent magnetic moments using pulsed magnetic fields. In our experiments, we observe the slowing of a supersonic beam of metastable neon from 461.0 ± 7.7 to 403 ± 16 m s⁻¹ in 18 stages, where the slowed peak is clearly separated from the initial distribution. This method has broad applications as it may easily be generalized, using seeding and entrainment into supersonic beams, to all paramagnetic atoms and molecules.

Based on macroscopic thermodynamic analysis, we have established a generalized linear theory for non-linear diffusion in external fields and non-ideal systems, which was classically described by the Fokker–Planck equation (or the Smoluchowski equation) and the non-linear Fickian equation, respectively. The new theory includes three basic equations expressed in 'apparent variables' as defined in this paper: (i) a generalized linear flux equation for non-linear diffusion; (ii) an apparent mass conservation equation and (iii) a generalized linear non-steady state equation for non-linear diffusion. Our analysis shows that (i) all of the existing linear and non-linear equations are the special cases of the new non-steady state general diffusion equation. It was also demonstrated that the general equation of the non-steady state is equivalent to the Fokker–Planck equation; (ii) coupling diffusion with multiple driving forces can be unified to a single force: the apparent concentration gradient; (iii) the exact relationship between diffusion coefficient and concentration in the non-linear Fickian equation under non-ideal conditions could be established and (iv) the potential energy is conservative in a diffusion process. An application of the generalized linear equation showed that the solution is simple. For the first time, an analytic solution of the Smoluchowski equation with a time-dependent potential in the algebraic form was obtained.