Table of contents

The ion temperature gradient driven (ITG) mode in reversed shear tokamaks is analysed using a gyrokinetic toroidal particle code. It is found that the ITG mode in the reversed shear configuration shows a coupled mode structure between the slab and toroidal ITG modes. Especially in the q_min-region, a slablike feature due to the reversed shear slab ITG mode (Idomura Y, Tokuda S and Wakatani M 1999 Phys. Plasmas6 4658) becomes strong. This coupled eigenmode structure is changed from a slab mode to a toroidal mode depending on η_i = L_n/L_ti and L_ti. Results show that in reversed shear tokamaks the ITG mode is determined from a competition between the slab and toroidal ITG modes.

The ground-state spin moments ⟨S_z⟩, orbital moments ⟨L_z⟩ and magnetic anisotropy energy (MAE) of Co_N one-dimensional (1D) clusters (N ⩽ 12) deposited on the Pd(110) surface are determined in the framework of a self-consistent, real-space tight-binding method. Remarkably large total magnetic moments per Co atom, M_z = (2⟨S_z⟩ + ⟨L_z⟩)/N = 2.8-2.9 µ_B, are obtained, which can be understood as the result of three physically distinct effects. The first and leading contribution is given by the local spin moments ⟨S_iz⟩ at the Co atoms i = 1, N (2⟨S_iz⟩_Co ≃ 1.6 µ_B). Second, significant spin moments are induced at the Pd atoms i > N close to the Co-Pd interface, which amount to about 25% of M_z (2⟨S_iz⟩_Pd = 0.2-0.3 µ_B). Finally, enhanced orbital magnetic moments ⟨L_iz⟩ are responsible for approximately 20% of M_z. In the case of the Co atoms, ⟨L_iz⟩_Co = 0.28-0.33 µ_B is almost a factor of three larger than the Co bulk orbital moment, while in Pd atoms ⟨L_iz⟩_Pd = 0.05 µ_B represents about 15% of the total local moment µ_iz = 2⟨S_iz⟩ + ⟨L_iz⟩. These results and the associated MAEs are analysed from a local perspective. The role of the cluster-surface interactions is discussed by comparison with the corresponding results for free-standing wires. Particularly in the case of monatomic 1D Co chains we observe that the lowest-energy magnetization direction (easy axis) changes from in line to off plane upon deposition on Pd(110). Wire-substrate hybridizations are therefore crucial for the magneto-anisotropic behaviour of 1D magnetic nanostructures on metallic substrates.

Recently a new dark-matter candidate has been proposed as a consequence of universal compact extra dimensions. It was found that to account for cosmological observations, the masses of the first Kaluza-Klein (KK) modes (and thus the approximate size of the extra dimension) should be in the range 600-1200 GeV when the lightest Kaluza-Klein particle (LKP) corresponds to the hypercharge boson and in the range 1-1.8 TeV when it corresponds to a neutrino. In this paper, we compute the elastic scattering cross sections between KK dark matter and nuclei both when the LKP is a KK mode of a weak gauge boson, and when it is a neutrino. We include nuclear form factor effects which are important to take into account due to the large LKP masses favoured by estimates of the relic density. We present both differential and integrated rates for present and proposed germanium, NaI and xenon detectors. Observable rates at current detectors are typically less than one event per year, but the next generation of detectors can probe a significant fraction of the relevant parameter space.

The size-dependent magnetic properties of small iron clusters deposited on ultrathin Ni/Cu(100) films have been studied with circularly polarized synchrotron radiation. For x-ray magnetic circular dichroism studies, the magnetic moments of size-selected clusters were aligned perpendicular to the sample surface. Exchange coupling of the clusters to the ultrathin Ni/Cu(100) film determines the orientation of their magnetic moments. All clusters are coupled ferromagnetically to the underlayer.

With the use of sum rules, orbital and spin magnetic moments as well as their ratios have been extracted from x-ray magnetic circular dichroism spectra. The ratio of orbital to spin magnetic moments varies considerably as a function of cluster size, reflecting the dependence of magnetic properties on cluster size and geometry. These variations can be explained in terms of a strongly size-dependent orbital moment. Both orbital and spin magnetic moments are significantly enhanced in small clusters as compared to bulk iron, although this effect is more pronounced for the orbital moment.

Magnetic properties of deposited clusters are governed by the interplay of cluster-specific properties on the one hand and cluster-substrate interactions on the other hand. Size dependent variations of magnetic moments are modified upon contact with the substrate.

The effects of collective neutrino-plasma interactions on the linear wave spectrum supported by a magnetized electron-positron plasma in the presence of a neutrino-antineutrino medium are investigated. When a pair-symmetric background neutrino-plasma medium is perturbed by space-charge waves (electrostatic waves associated with electron-positron charge separation), our analysis shows that the neutrino and antineutrino fluids also separate and the pair symmetry of the background medium is broken. The cosmological implications of this pair-symmetry breaking mechanism are briefly discussed.

The NA52 experiment at CERN has investigated lead-lead collisions at 158 A GeV/c and searched for long-lived strange quark matter droplets, so-called strangelets, with a unique signature of a high mass-to-charge ratio. This ratio was measured in a focusing spectrometer equipped with a time-of-flight system. A total of 3×10¹¹ Pb + Pb interactions at positive and 10¹³ at negative spectrometer polarities have been recorded. No strangelet has been observed, which sets experimental upper limits (90% CL) for the strangelet production at 3×10^-9 per interaction for positively charged and at 2×10^-10 per interaction for negatively charged strangelets.

The electron relaxation in Ag nanoparticles supported on graphite is investigated by time-resolved multiphoton photoemission spectroscopy. The photoemission spectra map the transient electron energy distribution in the nanoparticles and reveal the internal thermalization and cooling of the electron gas. The excess energy stored in the electron gas is calculated using the free-electron model. In contrast to the behaviour of isolated nanoparticles the energy loss rate from the electron gas increases with the pump fluence. This indicates that the electron gas equilibration in Ag nanoparticles on graphite is modified by excited electron transport.

Using a first-principles method, we study the effect of pressure on the band gap energy of wurtzite Al_xGa_{1 - x}N, In_xGa_{1 - x}N, and In_xAl_{1 - x}N. Starting with the binaries, GaN, InN and AlN, the direct band gap is found to increase linearly with pressure but becomes indirect for AlN at 13.88 GPa. The direct band gap pressure coefficients are 31.8 meV GPa^-1 for GaN, 18.8 meV GPa^-1 for InN and 40.5 meV GPa^-1 for AlN, which are in good agreement with other calculations. For the ternary alloys, the fundamental band gaps energy are direct and increase rapidly with pressure. The pressure coefficients vary in the range of 31.9-34.5 meV GPa^-1 for Al_xGa_{1 - x}N, 19.8-24.8 meV GPa^-1 for In_xGa_{1 - x}N and 16.7-20.7 meV GPa^-1 for In_xAl_{1 - x}N; they depend on alloy composition with a strong deviation from linearity. The band gap bowing of InGaN increases linearly with pressure, but those of AlGaN and InAlN strongly decrease when the AlN band gap becomes indirect.

We investigate the optical properties of spherical gold and silver clusters with diameters of 20 nm and larger. The light scattering spectra of individual clusters are measured using dark-field microscopy, thus avoiding inhomogeneous broadening effects. The dipolar plasmon resonances of the clusters are found to have nearly Lorentzian line shapes. With increasing size we observe polaritonic red-shifts of the plasmon line and increased radiation damping for both gold and silver clusters. Apart from some cluster-to-cluster variations of the plasmon lines, agreement with Mie theory is reasonably good for the gold clusters. However, it is less satisfactory for the silver clusters, possibly due to cluster faceting or chemical effects.

An Eötvös experiment to test the weak equivalence principle (WEP) for zero-point vacuum energy is proposed using a satellite. Following the suggestion of Ross for a terrestrial experiment of this type, the acceleration of a spherical test mass of aluminium would be compared with that of a similar test mass made from another material. The estimated ratio of the zero-point vacuum energy density inside the aluminium sphere to the rest mass energy density is ~1.6×10^-14, which would allow a 1% resolution of a potential WEP violation observed in a satellite mission test that had a baseline sensitivity to WEP violations of ~10^-16. An observed violation of the WEP for vacuum energy density would constitute a significant clue as to the origin of the cosmological constant and the source of dark energy, and test a recently proposed resolution of the cosmological constant problem, based on a model of nonlocal quantum gravity and quantum field theory.

We propose to model three-dimensional (3D) knots as effective vertex networks with vanishing topological exponents to study their equilibrium behaviour. This model is self-consistent and predicts weak localization in knots with up to five essential crossings. The resulting localization exponent for the 3D trefoil is in numerical agreement with a recent simulation study. In more complex knots, however, delocalization is expected. It is shown that this approach corresponds to the decomposition of the knot into C coupled loops of variable size, where C is the number of essential crossings.

It is well known that almost all eigenstates of a classically ergodic system are individually ergodic on coarse-grained scales. This has important implications for the quantization ambiguity in ergodic systems: the difference between alternative quantizations is suppressed compared with the O(ℏ²) ambiguity in the integrable or regular case. For two-dimensional ergodic systems in the high-energy regime, individual eigenstates are independent of the choice of quantization procedure, in contrast with the regular case, where even the ordering of eigenlevels is ambiguous. Surprisingly, semiclassical methods are shown to be much more precise in any dimension for chaotic than for integrable systems.

The surface morphology after deposition of Sb₄ clusters onto MoS₂(0001) at 90 K has been studied in detail with scanning tunnelling microscopy in ultrahigh vacuum. It has been found that during the initial stage of growth two-dimensional nanoparticles are formed. With increasing coverage these nanoparticles grow layer by layer and each layer has the height of one monolayer of undissociated Sb₄ clusters. The interface width (surface roughness) has been quantitatively determined as a function of total coverage. Ideal three-dimensional layer-by-layer growth has been identified for the single particles according to a Poisson distribution of exposed areas of the layers of the particles. Consequently, antimony grows on MoS₂(0001) without interlayer diffusion which is suppressed by an effective step edge (Schwoebel-Ehrlich) barrier.

In this paper, we perform a systematic study of particle production and neutrino yields for different incident proton energies E_p and baselines L, with the aim of optimizing the parameters of a neutrino beam for the investigation of θ₁₃-driven neutrino oscillations in the Δm² range allowed by Superkamiokande results. We study the neutrino energy spectra in the `relevant' region of the first maximum of the oscillation at a given baseline L. We find that to each baseline L corresponds an `optimal' proton energy E_p which minimizes the required integrated proton intensity needed to observe a fixed number of oscillated events. In addition, we find that the neutrino event rate in the relevant region scales approximately linearly with the proton energy. Hence, baselines L and proton energies E_p can be adjusted and the performance for neutrino oscillation searches will remain approximately unchanged provided that the product of the proton energy times the number of protons on target remains constant. We apply these ideas to the specific cases of 2.2, 4.4, 20, 50 and 400 GeV protons. We simulate focusing systems that are designed to best capture the secondary pions of the `optimal' energy. We compute the expected sensitivities to sin ²2θ₁₃ for the various configurations by assuming the existence of new-generation accelerators able to deliver integrated proton intensities on target times the proton energy of the order of (5×10²³) GeV×pot/year.

Cigar-shaped maghemite (γ-Fe₂O₃) nanocrystals dispersed in aqueous solution are subjected to a magnetic field during the deposition (process) on graphite. The nanocrystals can thus be oriented along their long axis to form ribbons at a mesoscopic scale whereas without a field the nanocrystals remain randomly oriented on the substrate. The magnetic properties markedly depend on the organization of the nanocrystals within the mesostructures.

Nano-electromechanical systems (NEMS) are ideal for sensor applications and ultra-sensitive force detection, since their mechanical degree of freedom at the nanometre scale can be combined with semiconductor nano-electronics. We present a system of coupled nanomechanical beam resonators in silicon which is mechanically fully Q-tunable ~700-6000. This kind of resonator can also be employed as a mechanical charge shuttle via an insulated metallic island at the tip of an oscillating cantilever. Application of our NEMS as an electromechanical single-electron transistor (emSET) is introduced and experimental results are discussed. Three animation clips demonstrate the manufacturing process of the NEMS, the Q-tuning experiment and the concept of the emSET.

We have studied the magnetic behaviour of ultra-thin films produced by depositing pre-formed gas phase Fe and Co nanoclusters, containing typically a few hundred atoms, in ultra-high vacuum (UHV) conditions. Two types of sample were prepared, that is, clusters embedded at very low volume fractions (⩽2%) within Ag matrices to obtain the isolated particle properties, and pure cluster-assembled films with no matrix that were transferred without a capping layer into the magnetometer in UHV. The dilute assemblies both display ideal superparamagnetism, with an H/T scaling of the magnetization curves, above 50 K for Fe clusters and 150 K for Co clusters. Fitting the magnetization data above these temperatures to Langevin functions enabled an accurate determination of the size distribution and gave a median size of 3 nm for the Fe and 2.8 nm for the Co clusters. At 2 K the magnetic isotherms are characteristic of assemblies of blocked particles with a uniaxial anisotropy axis and anisotropy constants of 2.6×10⁵ and 7.7×10⁵ J m^-3 for Fe and Co particles respectively. The magnetic behaviour of the pure cluster films was analysed using a random anisotropy model including parameters determined from the isolated cluster films. The approach to saturation of the Fe and Co cluster films indicates that the ground state is a correlated super-spin glass over the temperature range 10-300 K in both cases.

The growth and saturation of magnetic field in conducting turbulent media with large magnetic Prandtl numbers are investigated. This regime is very common in low-density hot astrophysical plasmas. During the early (kinematic) stage, weak magnetic fluctuations grow exponentially and concentrate at the resistive scale, which lies far below the hydrodynamic viscous scale. The evolution becomes nonlinear when the magnetic energy is comparable to the kinetic energy of the viscous-scale eddies. A physical picture of the ensuing nonlinear evolution of the MHD dynamo is proposed. Phenomenological considerations are supplemented with a simple Fokker-Planck model of the nonlinear evolution of the magnetic-energy spectrum. It is found that, while the shift of the bulk of the magnetic energy from the subviscous scales to the velocity scales may be possible, it occurs very slowly - at the resistive, rather than dynamical, timescale (for galaxies, this means that the generation of large-scale magnetic fields cannot be explained by this mechanism). The role of Alfvénic motions and the implications for the fully developed isotropic MHD turbulence are discussed.

Phase space is the framework best suited for quantizing superintegrable systems, naturally preserving the symmetry algebras of the respective Hamiltonian invariants. The power and simplicity of the method is fully illustrated through new applications to nonlinear σ-models, specifically for de Sitter N-spheres and chiral models, where the symmetric quantum Hamiltonians amount to compact and elegant expressions. Additional power and elegance is provided by the use of Nambu brackets to incorporate the extra invariants of superintegrable models. Some new classical results are given for these brackets, and their quantization is successfully compared to that of Moyal, validating Nambu's original proposal.

We examine the possibility of secure key exchange between a ground station and a low earth orbit satellite using the technique of quantum cryptography. The study suggests there are no technical obstacles to building a system that could exchange keys at kilobaud rates between a metre diameter telescope on the ground and a satellite with a 10 cm diameter lightweight telescope.

A semi-continuum model is introduced to study the dynamics of the formation of granular heaplets in tapped granular layers. By taking into account the energy dissipation of collisions and screening effects due to avalanches, this model is able to reproduce qualitatively the pattern of these heaplets. Our simulations show that the granular heaplets are characterized by an effective surface tension which depends on the magnitude of the tapping intensity. Also, we observe that there is a coarsening effect in that the average size of the heaplets, V, grows as the number of taps k increases. The growth law at intermediate times can be fitted by a scaling function V~k^z but the range of validity of the power law is limited by size effects. The growth exponent z appears to diverge as the tapping intensity is increased.

The problem of the two-photon coherent generation of entanglement photon pairs in quantum optics has been intensively studied over the passing years. It is important to note that the two-quanta cooperative effects also play a main role in other fields of physics. One example is superconductivity, where the Cooper pairs are created due to the simultaneous two-phonon exchange between electrons. It occurs when the one-phonon exchange integral between the band electrons is smaller than that of the two-phonon exchange. This is possible in many-band superconducting materials, in which the two-phonon exchange integral arises through the virtual bands of the material. Some estimates of the two-phonon superconductivity have already been given.

A more realistic model which takes into account the specificities of the many-band aspects of superconductor materials will be proposed. In two-phonon processes, a more complicated temperature dependence on the order parameter is expected. A rigorous study of this anomalous temperature dependence on the order parameter of superconductors is presented. One expects that the two-phonon exchange effects can amplify the superconductivity in a way similar to the way the thermal field amplifies the two-photon super-radiance in a microcavity.

In this experiment we have measured the temperature dependence of field emission characteristics for boron-doped diamond and diamond-like films at various temperatures and low fields. The threshold voltage increases and emission current density decreases as the temperature decreases. These field emission characteristics can be successfully explained by the semiconductor thermionic emission theory. This measurement apparently reveals that the electron affinity is not changed while bending down of the surface band is enhanced as temperature decreases, resulting in reduction of the emission current.

Nanoscience is certain to play a crucial role within many areas of technological development over the coming years. It is therefore necessary, at the beginning of the new century, to adapt to this extremely important subject. Due to the highly interdisciplinary nature of the field, comprising of features from different parts of physics (solid state, magnetism, quantum mechanics, optics, thermodynamics, surface physics, etc), chemistry, electronic engineering, and, so far to a lower extent, also biology, it is necessary to create a discussion platform which encompasses some of its facets.

A vast range of experimental and theoretical work has shown that isolated metal clusters possess many interesting features which differ distinctly from what is known, from both surface/solid state physics and atomic and molecular physics. For example, metal (as well as semiconductor or carbon) clusters exhibit electronic level structures which often change completely when the number of atoms in the cluster, N, changes by just one unit. Correspondingly the work functions are very sensitive to N, as are the chemical reactivities. Similar strong N-dependencies have been found in the optical absorption and the magnetism of clusters.

To realise possible technical applications of these new properties, a prerequisite is to bring the cluster into an environment, such as an encapsulating matrix or a surface. Due to the interaction with the contact medium, the properties of the clusters might change or even disappear. Additionally, with increasing coverage or volume filling, electronic and/or magnetic coupling will lead to a change of the functionality and enable a fine-tuning of the behaviour of the new devices. The physics of the cluster-on-surface system is therefore of fundamental importance.

Investigative efforts must focus on both the surface and the cluster science. In the past the two communities have more or less been separated although, despite this, there are similarities in some of the theoretical and experimental methods employed. The aim of this Focus Issue is therefore to fuel the discussion between these two fields in order to answer questions related to the interaction of clusters with surfaces. Accordingly, topics including atom diffusion, aggregation, self-organisation, and electronic and optical properties, as well as questions of possible applications (e.g. catalysis) are addressed.

The rapid development within the fields of cluster and nanoparticle physics clearly indicates the vital role that will be played by these systems in the future. At this point, however, it is clear that the area of nanostructure physics is expanding in a number of different directions, each requiring specific attention. There is a community working on large particles, i.e. on systems with several thousands of atoms where the exact number of atoms plays a minor role (e.g., in catalysis or magnetism). Other groups concentrate on clusters and nanostructures that are in the size range where each atom counts. This means that the physical and chemical properties might change dramatically as a function of the size or even the geometrical structure of a system.

While it is necessary to further investigate the new physics of nanoparticles, particular effort must also be placed on studying the functionality of clusters in appropriate environments. For example, nanomagnetism is a highly demanding fundamental problem as well as being important for future storage devices - in order to increase the writing density of recording media the magnetic domains have to be reduced in size. Ultimately the question must be answered as to which minimum size a technically usable magnetic domain can be reduced. Despite not yet having a complete picture of nanomagnetism, it is possible to observe (e.g., enhanced magnetic moments in small clusters), and to record hysteresis curves of single nanoparticles. The example of nanomagnetism clearly shows the strong interrelation between electronic and structural properties and functionality.

Focus on Clusters at Surfaces Contents

Al₂O₃-films on Ni₃Al(111): a template for nanostructured cluster growthC Becker, A Rosenhahn, A Wiltner, K von Bergmann, J Schneider, P Pervan, M Milun, M Kralj and K Wandelt

Quantum-dot systems prepared by 2D organization of nanoclusters preformed in the gas phase on functionalized substratesA Perez, L Bardotti, B Prevel, P Jensen, M Treilleux, P Mélinon, J Gierak, G Faini and D Mailly

Synthesis and magnetic properties of nanoscale bimetallic Co₁Rh₁ particlesD Zitoun, C Amiens, B Chaudret, M Respaud, M-C Fromen, P Lecante and M-J Casanove

Stabilization of Si-based cage clusters and nanotubes by encapsulation of transition metal atomsAntonis N Andriotis, Giannis Mpourmpakis, George E Froudakis and Madhu Menon

Magnetic behaviour of thin films produced by depositing pre-formed Fe and Co nanoclustersC Binns and M J Maher

Organization and magnetic properties of cigar-shaped ferrite nanocrystalsA T Ngo and M P Pileni

Subsequent layer growth of supported nanoparticles by deposition of Sb₄ clusters onto MoS₂(0001)B Stegemann, B Kaiser and K Rademann

Plasmon resonances in large noble-metal clustersC Sönnichsen, T Franzl, T Wilk, G von Plessen and J Feldmann

Transient electron energy distribution in supported Ag nanoparticlesM Merschdorf, C Kennerknecht, K Willig and W Pfeiffer

Spin and orbital magnetic moments of deposited small iron clusters studied by x-ray magnetic circular dichroism spectroscopyJ T Lau, A Föhlisch, M Martins, R Nietubyc, M Reif and W Wurth

Spin moments, orbital moments and magnetic anisotropy of finite-length Co wires deposited on Pd(110)R Félix-Medina, J Dorantes-Dávila and G M Pastor

Thermal emission of electrons from highly excited Na₁₆⁺ to Na₂₅₀⁺M Maier, M Astruc Hoffmann and B von Issendorff

Surface-induced reactions and dissociations of small acetone, acetonitrile and ethanol cluster ions: competitive chemical reactions, dissociation mechanisms and determination of dissociation energyC Mair, J Fedor , M Lezius, P Scheier, M Probst, Z Herman and T D Märk

Nanostructuration with visible-light-emitting silicon nanocrystalsF Huisken, D Amans, G Ledoux, H Hofmeister, F Cichos and J Martin

Nonequilibrium electron energy-loss kinetics in metal clustersC Guillon, P Langot, N Del Fatti and F Vallée

A study of charge quantization on ligand-stabilized Au₅ cluster monolayersH Zhang, D Mautes and U Hartmann

Large noble metal clusters: electron confinement and band structure effectsH Hövel and I Barke

Temperature dependence of the magnetization in Fe islands on W(110): evidence for spin-wave quantizationV Senz, R Röhlsberger, J Bansmann, O Leupold and K-H Meiwes-Broer

Karl-Heinz Meiwes-Broer, Universität Rostock, Germany Wilfried Wurth, Universität Hamburg, Germany Hans-Peter Steinrück, Universität Erlangen-Nürnberg, Germany

The encapsulation of metal atoms within Si-based cage clusters leads to stable metal-encapsulated Si cage clusters (Si-cc). The present work investigates the effect of encapsulation of transition metal atoms (TMAs). We show that the filling factor of the d-band of the TMA is the dominant factor determining the structural configuration of the Si-cc. This results in a contrasting bonding and magnetic behaviour of the endohedral Ni and V atoms. The size of the encapsulated atom is found to play a minor role. Both Ni and V were found to stabilize Si-cc. More significantly, we show that both Ni and V, in the form of one-dimensional chains, can stabilize Si nanotubes encapsulating the Ni or the V chain. Our results also show that these metal-encapsulated Si nanotubes have small conduction gaps and become metallic at infinite length.

The influence of size reduction on the magnetism of CoRh has been studied on a system of spherical bimetallic nanoparticles embedded in a polymer matrix. By varying the concentration and the nature of the polymer, we achieved the chemical synthesis of different sizes from 1.7 to 4.1 nm from organometallic precursors. Pulsed fields up to 30 T were used in order to approach the magnetic saturation M_S. Particles with a mean diameter of 1.7 nm display a value of 2.38 µ_B per CoRh unit, strongly enhanced compared to values calculated or measured on a bulk alloy. For all samples, the magnetic moment per atom and the effective anisotropy constant are found to decrease with size but are still enhanced compared to bulk values. These results were interpreted as first evidence of the cooperative role of both alloying and size reduction in the enhancement of the magnetization and the anisotropy in this system associating a three-dimensional ferromagnetic metal with a 4d metal.

The low-energy cluster beam deposition (LECBD) technique is used to deposit gold nanoclusters preformed in the gas phase on functionalized graphite substrates (highly oriented pyrolitic graphite (HOPG)), to prepare 2D-organized arrays of cluster assembled dots. Functionalized HOPG substrates are obtained using the focused ion beam (FIB) nanoengraving technique to pattern 2D-organized arrays of defects (nanoholes, nanobumps) which act as traps for the diffusing clusters. Depending on the deposition conditions (nature, size and fluence of the deposited clusters) and the functionalized substrates (nature and size of the FIB-induced defects, geometry of the 2D array of defects and temperature during deposition) high-quality quantum-dot arrays can be obtained with well controlled and reproducible morphologies. Kinetic Monte Carlo simulations of the cluster deposition experiments on functionalized substrates allow us to obtain quite good fits of the experimental images performed by tapping mode atomic force microscopy (TMAFM), leading to systematic investigations of the best conditions to realize high-quality quantum dots systems. This combined top-down-bottom-up approach (LECBD-FIB) seems a promising method for preparing high-integration-density devices (~Tbit cm^-2) well suited for future applications to data storage, nanoelectronics, nano-optics, nanomagnetic systems.

In scanning tunnelling microscope images of thin Al₂O₃-films grown on Ni₃Al(111) at 1000 K two super-lattices with periodicities of 2.6 and 4.5 nm, respectively, can be identified. These well-ordered nanostructures can be used as nucleation centres for metal particle growth. It can be shown that both nanostructures act as a template for the fabrication of ordered assemblies of metal clusters by mere physical vapour deposition. The degree of ordering of these nanostructures is largely dependent on the metal deposited. Here we report on the growth of Cu, Ag, Au, Mn, and V clusters on the Al₂O₃-films. The best results as far as ordering of the clusters is concerned was reached for V deposition at 550 K, which resulted in a nearly perfect hexagonal array of clusters with a spacing of 2.6 nm.

A previously calculated universal pattern (Berry M V 2002 New J. Phys.4 66) describes colours near an isolated phase singularity (diffraction zero), generated with white light and visible when the dark light of the singularity is scaled to isoluminance. Here the pattern is illustrated in several different situations: near the zeros of random and regular superpositions of plane waves, and near the zeros inside and outside the diffraction pattern decorating the geometrical cusp catastrophe. The universal colours emerge in miniature, close to the zeros, when an initially achromatic diffraction pattern is perturbed by switching on an asymptotic `chromaticity parameter', that can be chosen in several different ways.

This paper discusses relationships between topological entanglement and quantum entanglement. Specifically, we propose that it is more fundamental to view topological entanglements such as braids as entanglement operators and to associate with them unitary operators that are capable of creating quantum entanglement.

Discrete bright breathers are well known phenomena. They are localized excitations that consist of a few excited oscillators in a lattice and the rest of them having very small amplitude or none. In this paper we are interested in the opposite kind of localization, or discrete dark breathers, where most of the oscillators are excited and one or a few units of them have very small amplitude. We investigate, using band analysis, Klein-Gordon lattices at frequencies not close to the linear ones. Dark breathers at low coupling are shown to be stable for Klein-Gordon chains with soft on-site potentials and repulsive dispersive interaction, and with hard on-site potentials and attractive dispersive interactions. At higher coupling dark breathers lose their stability via subharmonic, harmonic or oscillatory bifurcations, depending on the model. However, most of these bifurcations are harmless in the sense that they preserve dark localization. None of these bifurcations disappear when the system is infinite. Dark breathers in dissipative systems are found to be stable for both kinds of dispersive interaction.

We analyse the properties of a spontaneously broken D = 4, N = 4 supergravity without cosmological constant, obtained by gauging translational isometries of its classical scalar manifold.

This theory offers a suitable low energy description of the super-Higgs phases of certain type-IIB orientifold compactifications with 3-form fluxes turned on.

We study its N = 3, 2, 1, 0 phases and their classical moduli spaces and we show that this theory is an example of no-scale extended supergravity.

A plasma called a resonant transfer (rt) plasma formed with a low field (1 V cm^-1), at low temperatures (e.g. ≈10³ K), from atomic hydrogen generated at a tungsten filament and strontium which was vapourized by heating the metal. Strong vacuum ultraviolet emission was observed that increased with the addition of argon, but not when sodium, magnesium or barium replaced strontium or with hydrogen, argon or strontium alone. Characteristic strontium and argon emission was observed which supported a resonant-energy-transfer mechanism. Significant Balmer α line broadening corresponding to an average hydrogen atom temperature of 14, 24, and 23-45 eV was observed for strontium and argon-strontium rt plasmas and discharges of strontium-hydrogen, helium-hydrogen, argon-hydrogen, strontium-helium-hydrogen and strontium-argon-hydrogen, respectively, compared to ≈3 eV for pure hydrogen, krypton-hydrogen, xenon-hydrogen and magnesium-hydrogen. To achieve that same optically measured light output power, hydrogen-sodium, hydrogen-magnesium and hydrogen-barium mixtures required 4000, 7000 and 6500 times the power of the hydrogen-strontium mixture, respectively, and the addition of argon increased these ratios by a factor of about two. A glow discharge plasma formed for hydrogen-strontium mixtures at an extremely low voltage of about 2 V compared to 250 V for hydrogen alone and sodium-hydrogen mixtures, and 140-150 V for hydrogen-magnesium and hydrogen-barium mixtures.

We report measurements on the guiding of cold ⁸⁷Rb atoms from a magneto-optical trap by a continuous light beam over a vertical distance of 6.5 mm. For moderate laser power (<85 mW) we are able to capture around 40% of the cold atoms. We systematically study the guided fraction as a function of laser power and detuning, and give an analytical expression that agrees well with our results. Although the guide is red detuned, the optical scattering rate at this detuning (≈70 GHz) is acceptably low. For lower detuning (<30 GHz) a larger fraction is guided but radiation pressure starts to push the atoms upward, effectively lowering the acceleration due to gravity.

We analyse J/ψ-production in nucleon-nucleon collisions near threshold in the framework of a general model-independent formalism, which can be applied to any reaction N + N→N + N + V⁰, where V⁰ = ω, ϕ or J/ψ. Such reactions show large isotopic effects: a large difference for pp and pn collisions, which is due to the different spin structure of the corresponding matrix elements. The analysis of the spin structure and of the polarization observables is based on symmetry properties of the strong interaction. Using existing experimental data on the different decays of the J/ψ meson, we suggest a model for N + N→N + N + J/ψ, based on t channel η + π exchanges. We predict polarization phenomena for the n + p→n + p + J/ψ reaction and the ratio of cross sections for np and pp collisions. For the processes η(π) + N→N + J/ψ we apply two different approaches: vector meson exchange and local four-particle interaction. In both cases we find larger J/ψ production in np collisions than in pp collisions.

The role of large-scale fluctuation structures in electrostatic drift-wave-type plasma turbulence is highlighted. In particular, well-defined laboratory experiments allow one to study the dynamics of drift wave mode structures as well as `eddies' in drift wave turbulence. In the present paper we discuss the mutual relationships between observations made in linear magnetic geometry, purely toroidal geometry and magnetic confinement. The simplest structure, a saturated, nonlinear drift mode, is the starting point for a Ruelle-Takens-Newhouse transition route to chaos and weakly developed turbulence. Both spectral and phase space analysis are applied to characterize in detail the transition scenario, which is enforced due to an increased drive by the plasma equilibrium state. In addition to direct multi-probe observation, statistical approaches are most revealing for the systematic study of the spatiotemporal dynamics in fully developed drift wave turbulence. In particular, the propagation of large-scale `eddy' structures is traced by conditional statistics methods. Finally, the control of drift wave turbulence by spatiotemporal synchronization is discussed.

For illumination with white light, the spectra near a typical isolated phase singularity (nodal point of the component wavelengths) can be described by a universal function of position, up to linear distortion and a weak dependence on the spectrum of the source. The appearance of the singularity when viewed by a human observer is predicted by transforming the spectrum to trichromatic variables and chromaticity coordinates, and then rendering the colours, scaled to constant luminosity, on a computer monitor. The pattern far from the singularity is a white that depends on the source temperature, and the centre of the pattern is flanked by intensely coloured `eyes', one orange and one blue, separated by red, and one of the eyes is surrounded by a bright white circle. Only a small range of possible colours appears near the singularity; in particular, there is no green.

The scattering and capture of right-handed neutrinos by an Abelian cosmic string in the SO(10) grand unification model are considered. The scattering cross section of neutrinos per unit length due to the interaction with the gauge and Higgs fields of the string is much larger in its scaling regime than in the friction regime because of the larger infrared cutoff of the former. The probability of capture in a zero mode of the string accompanied by the emission of a gauge or Higgs boson shows a resonant peak for the neutrino momentum of the order of its mass. Due to the decrease in the number of strings per unit of comoving volume in the scaling epoch, the cosmological consequences of the superconducting strings formed in this regime will be much smaller than those which may already be produced in the friction regime; in particular as possible sources of ultraenergetic cosmic rays.

This work deals with the electronic properties, in different crystal phases, of AlN (wurtzite and zincblende) compounds computed using an all electron ab initio linearized augmented plane wave method. Results include band structure, total and partial density of states, charge density and the ionicity factor. Most of the calculated band parameters, of direct bandgap, total- and upper-valence bandwidths and antisymmetric gap for wurtzite-AlN are close to those of c-AlN to within 1%. The charge distributions have similar features, meaning that AlN has the same ionicity factor in both structures.

The present status of the experimental searches for supersymmetry at LEP and Tevatron is reviewed. Prospects at future machines, i.e. the Large Hadron Collider and lepton colliders, are also discussed. The phenomenology of several scenarios, the experimental strategies and the analysis methods are described, and the sensitivities and reaches of the various machines are compared.

The rapid delocalization of chaotic Hamiltonian dynamics can be described by a local measure on phase space, the microscopic heterogeneity, both for classical and quantum systems. The properties of this new measure are discussed and studied numerically for a one-dimensional nonlinearly driven oscillator. An intricate and highly structured phase space dependence is observed. The correspondence between the classical and quantum evolution of the delocalization is investigated.

Advances in ultrafast optics in recent years have revived a keen interest in laser-induced dielectric breakdown study. While it is widely accepted that femtosecond laser pulses with peak powers reaching gigawatts can propagate over tens of metres under laboratory conditions, the dynamics underlying this highly nonlinear phenomenon is yet not fully understood. Although initial research on laser-triggered lightning was started with infrared lasers, it was found that they are not suitable to initiate lightning. Recent published literature and experimental work favour the use of ultraviolet (UV) laser pulses as the appropriate means for laser-induced lightning discharge. An analytical solution based on Maxwell's equations has been developed for UV filamentation in air, arising from a dynamic oscillating balance between self-focusing, diffraction and plasma defocusing. This model suggests that UV (220-420 nm) 200 ps laser pulses with a peak power of around 50 MW (or 12.5 mJ input energy) and a beam size of 100 µm are the optimal tool to trigger outdoor lightning. The laser beam size remains relatively small (less than 0.3 mm) after a propagation distance of 200 m up into the normally cloudy and damp atmospheric conditions.

Equilibrated three-dimensional Pb crystallites, supported on Ru(001) and of about 1 µm diameter, were imaged by scanning tunnelling microscopy at 298-393 K. The top section of the crystallites exhibited large (111) facets and, depending on temperature, smaller (112) facets. The vicinal shapes close to (111) were analysed in detail to determine the critical shape exponent and the step-step interaction energy as well as the interaction constant of the potential. Analyzing the complete shape in sections of 1° or 3° azimuthal increments and averaging over all sections of one crystallite, we found a shape exponent of 1.490. The exponent is very close to the theoretically predicted universal value of 3/2 and as such clear evidence for the 1/x² step interaction potential. Several crystallites had dislocations threading the (111) facet. For those crystallites the step interaction energy was determined as 16 meV Å^-2 at about 350 K, equivalent to a dipole interaction energy of 8.1 meV Å^-2 at 0 K. The interaction constant for the dipole-dipole part of step interaction was found to be 115 meV Å.

Calculation by Douglas and Shenker of the tension ratios for vortices of different N-alities in the softly broken = 2 supersymmetric SU(N) Yang-Mills theory is carried to the second order in the adjoint multiplet mass m. Corrections to the ratios violating the well known sine formula are found, showing that it is not a universal quantity.

We have recently introduced a realistic, covariant, interpretation for the reduction process in relativistic quantum mechanics. The basic problem for a covariant description is the dependence of the states on the frame within which collapse takes place. A suitable use of the causal structure of the devices involved in the measurement process allowed us to introduce a covariant notion for the collapse of quantum states. However, a fully consistent description in the relativistic domain requires the extension of the interpretation to quantum fields. The extension is far from straightforward. Besides the obvious difficulty of dealing with the infinite degrees of freedom of the field theory, one has to analyse the restrictions imposed by causality concerning the allowed operations in a measurement process. In this paper we address these issues. We shall show that, in the case of partial causally connected measurements, our description allows us to include a wider class of causal operations than the one resulting from the standard way of computing conditional probabilities. This alternative description could be experimentally tested. A verification of this proposal would give stronger support to the realistic interpretations of the states in quantum mechanics.

Using a form of modified dispersion relations derived in the context of quantum geometry, we investigate limits set by current observations on potential corrections to Lorentz invariance. We use a phenomological model in which there are separate parameters for photons, leptons and hadrons. Constraints on these parameters are derived using thresholds for the processes of photon stability, photon absorption, vacuum Cerenkov radiation, pion stability and the GZK cutoff. Although the allowed region in parameter space is tightly constrained, non-vanishing corrections to Lorentz symmetry due to quantum geometry are consistent with current astrophysical observations.

We have measured depolarized light scattering in liquid benzene over the whole accessible temperature range and over four decades in frequency. Between 40 and 180 GHz we find a susceptibility peak due to structural relaxation. This peak shows stretching and time-temperature scaling as known from alpha relaxation in glass-forming materials. A simple mode-coupling model provides consistent fits of the entire data set. These qualitative and quantitative results show that structural relaxation in ordinary liquids and alpha relaxation in glass-forming materials are one and the same physical process. Thus, a deeper understanding of equilibrium liquids is reached by applying concepts that were originally developed in the context of glass-transition research.

It is shown that a limited number of supersymmetric potentials obtained from factorization methods, which are reflectionless, lead to the description of KdV Hamiltonians and their related KdV conservation laws are derived. Also, single-soliton solutions of the KdV equations corresponding to these potentials are presented.

We study the new structures appearing due to noncommutative effects in the inclusive decay b→sγ*, in the standard model. We present the corresponding coefficients which carry the space-space and space-time noncommutativity.

Direct comparisons between numerical simulations and the measured plasma fluctuations and transport are presented by performing nonlinear two-fluid simulations with the BOUT code (Xu X Q and Cohen R H 1998 Contrib. Plasma Phys.38 158). BOUT models boundary-plasma turbulence in a realistic divertor geometry using modified Braginskii equations for plasma vorticity, density (n_i), electron and ion temperature (T_e, T_i) and parallel momenta. The BOUT code solves for the plasma fluid equations in a 3D toroidal segment, including the region somewhat inside the separatrix and extending into the scrape-off layer; the private flux region is also included. In this paper, the physics of resistive X-point turbulence and its relation to flow shear generation is discussed. We present comparisons between the boundary plasma turbulence observed in the BOUT code and experiments on DIII-D (Luxon J L et al 1986 Int. Conf. on Plasma Physics and Controlled Nuclear Fusion (Vienna: IAEA) p 159), the National Spherical Torus Experiment (Peng Y-K M 2000 Phys. Plasmas7 1681), and C-Mod (Hutchinson I H et al 1994 Phys. Plasmas1 1511). In an L-mode discharge in the DIII-D tokamak, both BOUT simulations and beam emission spectroscopy show a similar flow pattern and blob size across the last closed flux surface. In an L-mode discharge, both BOUT simulations and gas puff imaging show similar filament structures along the field line and similar frequency spectrum at the outboard midplane. In simulations of the quasi-coherent mode in the EDA regime of C-Mod, the particle flux measured from BOUT simulation is consistent with Langmuir probe measurements on C-Mod at the midplane near the separatrix. The qualitative comparisons thus indicate that BOUT contains much of the relevant physics for boundary plasma turbulence in the experimentally relevant X-point divertor geometry of present-day tokamaks and spherical tori.

Drift wave turbulence, in general a balance between E×B drift turbulence in planes perpendicular to, and dissipative wave dynamics parallel to, a background magnetic field, is a hallmark example of nonlinearity in plasma physics. The turbulence generally has the same basic character in a sheared magnetic field lying in closed surfaces whether linear instabilities are present or not. Only when the linear forcing terms are dominant does this situation not prevail; it is not analogous to neutral fluid turbulence where pure linear forcing is balanced by pure nonlinear mixing and decorrelation. Detailed computations show that two types of nonlinearity are simultaneously present: advection of fluid vorticity by the E×B flows, which tends to have a scattering character, and E×B advection of pressure disturbances, which has the familiar diffusive mixing character. The vorticity nonlinearity excites the turbulence, acting against the mostly linear parallel dynamics which constrains it, while the pressure nonlinearity provides dissipation via transfer to ever smaller scales. The practical result is that the saturated level of the turbulence and the resulting averaged thermal energy transport are controlled principally by these nonlinear mechanisms even when moderate linear instabilities are present. The model is mostly applicable to tokamak edge turbulence, for which the linear forcing effects are sufficiently moderate that the nonlinear physics is allowed to operate.

A view of recent experimental results and progress in the characterization of plasma turbulence in magnetically confined devices is given. An empirical similarity in the scaling properties of the probability distribution function of turbulent transport has been observed in the plasma edge region. This result supports the view that turbulent transport displays universality in fusion plasmas and emphasizes the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the traditional way of characterizing transport based on the computation of effective transport coefficients. Comparative studies in different magnetically confined plasmas show that fluctuations and sheared poloidal flows organize themselves to be close to marginal stability. This property should be considered as a critical test for improved confinement transition models. Magnetic configuration scan experiments in stellarator devices have shown a complex interplay between transport and sheared radial electric fields in the proximity of rational surfaces. The development of new fluctuation analysis techniques based on the investigation of velocity fluctuations opens a new way to investigate turbulent transport and dynamical electric fields in the plasma core region.

The dipole strength function and the photoabsorption cross section for some He even-even isotopes close to the neutron drip-line region are systematically studied within a selfconsistent random phase approximation method in the continuum. In the systematic analysis several parametrizations of the Skyrme interaction with density and momentum dependent terms are used as the p-h residual interaction. Proton and neutron partial contributions to the total dipole strength and cross section are determined and the ratio R = σ(γ, p)/σ(γ, p) is calculated and compared to the measured data. On the same footing, calculations of the mean square radius and interaction cross section are carried out for the secondary beams of ⁴He, ⁶He and ⁸He produced through the fragmentation of ¹¹Be primary beam with targets ⁹Be, ¹²C and ²⁷Al at 800 MeV/nucleon. It is determined that some of the Skyrme force parametrizations give better results than others when compared to the experimental data. Finally, a determination of nuclear skins for the He isotopes is given.

We investigate a specific set of two-loop self-energy corrections involving squared decay rates and point out that their interpretation is highly problematic. The corrections cannot be interpreted as radiative energy shifts in the usual sense. Some of the problematic corrections find a natural interpretation as radiative nonresonant corrections to the natural line shape. They cannot uniquely be associated with one and only one atomic level. While the problematic corrections are rather tiny when expressed in units of frequency (a few hertz for hydrogenic P levels) and do not affect the reliability of quantum electrodynamics at the current level of experimental accuracy, they may be of importance for future experiments. The problems are connected with the limitations of the so-called asymptotic-state approximation, which means that atomic in- and out-states in the S-matrix are assumed to have an infinite lifetime.

We first consider teleportation of entangled states shared between Claire and Alice to Bob1 and Bob2 when Alice and the two Bobs share a single copy of a GHZ state and where all the four parties are at distant locations. We extend this result to a more general state than GHZ-state and show that still a class of pure entangled states can be teleported, where the entanglement of this class ranges from 0 to e (⩽1), depending on the entanglement (defined in the text) of the channel state. We then generalize this situation to the case of teleportation of entangled states shared between Claire1, Claire2, ..., Claire(N - 1) and Alice to Bob1, Bob2, ..., BobN when Alice and the N Bobs share a single copy of a GHZ-class state and where again all the 2N parties are at distant locations.

In our modern era of telecommunications and the Internet, information has become a valuable commodity. Sometimes it must therefore be protected against theft - in this case, loss of secret information to an eavesdropper. Most of today's transactions are protected using encryption unproven to be secure against a computational attack by a classical computer and, in fact, the standardly used encryption algorithms are provably vulnerable to the mind-boggling parallelism of a quantum computer, should one ever be physically realized. Enter quantum cryptography. Underlying nearly all forms of encryption is the necessity for a truly secret key, a random string of zeros and ones; the basic notion of quantum cryptography is to employ single photon transmissions (or the closest attainable approximation to these) to distribute the random key material, while removing the threat of an undetected eavesdropper. Now, nearly twenty years since the seminal quantum cryptography paper by Bennett and Brassard (Bennett C H and Brassard G 1984 Proc. IEEE Int. Conf. on Computers, Systems, and Signal Processing (Bangalore) (New York: IEEE) pp 175-9), we take a look at several state-of-the-art implementations, and glimpse how future quantum cryptosystems might look.

We start with papers from three of the world's leading experimental quantum cryptography efforts: Stucki et al and Bethune and Risk describe working systems for quantum key distribution (QKD) over telecommunications fibres (at 1550 nanometres and 1300 nanometres, respectively). The former's achievement of quantum key exchange over 67 kilometres of optical fibre is a world record, as is the experimental demonstration by Hughes et al of daylight free-space QKD over a 10 km atmospheric range.

Next, Lütkenhaus and Jahma explore the possible vulnerabilities of such systems (which employ attenuated laser pulses instead of actual single photon states) to conceivable future eavesdropping technologies. Enzer et al have implemented a totally new protocol, using polarization-entangled photons, which in some circumstances can tolerate higher error rates than the traditional one of Bennett and Brassard; moreover, the use of entanglement provides a means of `automatic source verification'.

Finally, looking to the future, Elliott gives a provocative view of how these technologies may be merged into network operation, and Shapiro describes a method to combine a novel source of entangled photons with a means to transfer the photons' quantum state to trapped-atom quantum memories. If realized, these systems could presage the world's first quantum network.

Focus on Quantum Cryptography Contents

Quantum key distribution over 67 km with a plug&play systemD Stucki, N Gisin, O Guinnard, G Ribordy and H Zbinden

Autocompensating quantum cryptographyDonald S Bethune and William P Risk

Practical free-space quantum key distribution over 10 km in daylight and at nightRichard J Hughes, Jane E Nordholt, Derek Derkacs and Charles G Peterson

Quantum key distribution with realistic states: photon-number statistics in the photon-number splitting attackNorbert Lütkenhaus and Mika Jahma

Entangled-photon six-state quantum cryptographyDaphna G Enzer, Phillip G Hadley, Richard J Hughes, Charles G Peterson and Paul G Kwiat

Building the quantum networkChip Elliott

Architectures for long-distance quantum teleportationJeffrey H Shapiro

Ground to satellite secure key exchange using quantum cryptographyJ G Rarity, P R Tapster, P M Gorman and P Knight

Method for decoupling error correction from privacy amplificationHoi-Kwong Lo

Paul G Kwiat University of Illinois at Urbana-Champaign, USA

A system architecture for achieving long-distance, high-fidelity teleportation and long-duration quantum storage is proposed. It uses polarization-entangled photons and trapped-atom quantum memories and is compatible with transmission over standard telecommunication fibre. An extension of this architecture permits long-distance transmission and storage of Greenberger-Horne-Zeilinger states.

We show how quantum key distribution (QKD) techniques can be employed within realistic, highly secure communications systems, using the internet architecture for a specific example. We also discuss how certain drawbacks in existing QKD point-to-point links can be mitigated by building QKD networks, where such networks can be composed of trusted relays or untrusted photonic switches.

We have implemented the `six-state' quantum cryptography protocol using polarization-entangled photon pairs, in which the polarization of each photon of a pair is measured in one of three randomly chosen bases. For a given amount of eavesdropping, this protocol results in a larger error rate than in four- or two-state protocols, but reduces the number of key-producing events. We have experimentally investigated several incoherent eavesdropping strategies, and verified the predicted enhancement in error rate. However, we demonstrate that for low error rates, the efficiency for secret key generation is higher when using the four-state protocol.

Quantum key distribution can be performed with practical signal sources such as weak coherent pulses. One example of such a scheme is the Bennett-Brassard protocol that can be implemented via polarization of the signals, or equivalent signals. It turns out that the most powerful tool at the disposition of an eavesdropper is the photon-number splitting attack. We show that this attack can be extended in the relevant parameter regime so as to preserve the Poissonian photon number distribution of the combination of the signal source and the lossy channel.

We have demonstrated quantum key distribution (QKD) (Bennett C H and Brassard G 1984 Proc. IEEE Int. Conf. on Computers, Systems, and Signal Processing (Bangalore, India) p 175) over a 10 km, 1-airmass atmospheric range during daylight and at night. Secret random bit sequences of the quality required for the cryptographic keys used to initialize secure communications devices were transferred at practical rates with realistic security. By identifying the physical parameters that determine the system's secrecy efficiency, we infer that free-space QKD will be practical over much longer ranges under these and other atmospheric and instrumental conditions.

Quantum cryptographic key distribution (QKD) uses extremely faint light pulses to carry quantum information between two parties (Alice and Bob), allowing them to generate a shared, secret cryptographic key. Autocompensating QKD systems automatically and passively compensate for uncontrolled time-dependent variations of the optical fibre properties by coding the information as a differential phase between orthogonally polarized components of a light pulse sent on a round trip through the fibre, reflected at mid-course using a Faraday mirror. We have built a prototype system based on standard telecom technology that achieves a privacy-amplified bit generation rate of ~1000 bits s^-1 over a 10 km optical fibre link. Quantum cryptography is an example of an application that, by using quantum states of individual particles to represent information, accomplishes a practical task that is impossible using classical means.

We present a fibre-optical quantum key distribution system. It works at 1550 nm and is based on the plug&play set-up. We tested the stability under field conditions using aerial and terrestrial cables and performed a key exchange over 67 km between Geneva and Lausanne.

This paper addresses the application of the operator-valued function and small-parameter methods to the analysis of the wave propagation and interaction in a family of slotted waveguides. For narrow slots, the eigenwaves and field distributions in the cross-sectional domains are calculated as segments of asymptotic series in powers of the characteristic small parameter of the problems. For larger slots, the method of infinite-matrix (summation) equations is applied. The results show that the interaction of waves occurs in slotted structures and that semi-analytical methods are an efficient tool to analyse this phenomenon.

This work is devoted to the study of the reactivity of CH₄ in a nitrogen afterglow. We show that CH₄ reacts efficiently in the nitrogen afterglow producing the simple radicals CH₃, CH₂ or recombined species containing C-N bonds in the form of C₂N₂, CN or HCN. A reaction rate constant ranging between 1×10^-18 and 6×10^-18 m³ s^-1 is found for the global reaction producing mainly CH₂ and CH₃ radicals. Investigations are also performed in order to determine which `active' nitrogen species are involved in the dissociation mechanism of CH₄ in the nitrogen afterglow. To do this we study elementary reactive processes between CH₄ and specific nitrogen species such as N₂(A ³Σ_u⁺,v) and N atoms. N₂(A ³Σ_u⁺,v) is selectively produced by energy transfer from Ar(³P₂) metastable species to N₂, before reacting with CH₄. We show that this reaction is efficient and leads mainly to the formation of CH₃ and C₂H₆. According to the literature, the higher the vibrational level is the faster is the rate of the vibrational relaxation of N₂(A ³Σ_u⁺,v). We also study the reaction of CH₄ with N atoms in the nitrogen afterglow. We measure a reaction rate constant ranging between 2×10^-18 and 6×10^-18 m³ s^-1. This value is larger than the value reported elsewhere in the literature and close to the value given for the reaction of CH_x<4 with N atoms. So we propose the following reaction mechanism for the dissociation of CH₄ in a nitrogen afterglow. First CH₄ reacts with vibrationally excited metastable species N₂(A ³Σ_u⁺,v) coming from the nitrogen discharge that produces CH_x<4 simple radicals. Then these radicals react with N atoms producing hydrogen cyanide, cyanogen and hydrocarbons.

In the scrape-off layer and outermost edge of the Wendelstein 7-AS (W7-AS) stellarator, fluctuations of, mostly, ion saturation current I_sat and floating potential Φ_fl have been measured with high poloidal, radial (both 2-3 mm) and temporal (0.5 µs) resolution. Parallel to the magnetic field, measurements with two probe tips on the same magnetic field line were performed.

The basic spatio-temporal structure of the fluctuations is presented in terms of correlation functions and of wavenumber-frequency (kf) spectra. They are characterized by the parameters of a fit function to the correlation function, which describe the spatio-temporal structure. The behaviour of these fluctuation parameters under variations of the discharge parameters is investigated.

A high correlation of the fluctuations parallel to the magnetic field is documented, and we analyse, which frequency components contribute to this high correlation.

In our radially and poloidally resolving measurements, we find that the fluctuation structures are inclined in the poloidal-radial plane at our position of measurement. The consequences of such inclined structures for the interpretation of measurements with one-dimensional arrays are discussed, with an emphasis on the risk of an erroneous interpretation of purely radially resolving measurements. There is strong evidence that this inclination is due to the local magnetic shear in W7-AS in conjunction with the high correlation of the fluctuations parallel to the magnetic field. A further contribution to the inclination may originate from the radial shear of the poloidal velocity of the fluctuations in the plasma edge. The different consequences of magnetic shear and velocity shear for an inclination of the fluctuations in the poloidal-radial plane are discussed.

Some exact solutions of Einstein's field equations represent a rotating universe. One example is Gödel's cosmological model. Bianchi solutions generalize the Gödel metric and include the expansion of the universe. We propose a measurement of the cosmic rotation using a light or matter wave interferometer based on the Sagnac effect. Entanglement between the quanta employed in this quantum gyroscope enhances the accuracy, thereby coming closer to the more-than-challenging requirements of such experiments.

The functional expansion of the non-uniform first-order direct correlation function (DCF) around the bulk density was truncated at the first order, then the functional counterpart of the Lagrangian theorem of differential calculus was employed to make the truncation formally exact. The actual procedure is as follows. According to the Lagrangian theorem and the definition of the DCF, the original expansion coefficient, i.e. the uniform second-order DCF, was replaced by the non-uniform second-order DCF whose argument is the appropriate mixture of the density distribution and the bulk density with an adjustable parameter determined by a hard-wall sum rule. With reference to an earlier paper (Khein A and Ashcroft N W 1999 Phys. Rev. E 59 1803), the non-uniform second-order DCF was then approximated by its uniform counterpart with a weighted density as its density argument. The truncated expansion was incorporated into the density functional theory formalism to predict the non-uniform hard-sphere fluid density distribution - in very good agreement with simulation data for three confining geometries: a single hard wall, a spherical cavity and a bulk hard-sphere particle whose resulting external potential leads to a radial distribution function of the bulk hard-sphere fluid whose prediction by the present theory was also in good agreement with the corresponding simulation data.

Electromagnetic gyrokinetic simulations of electron-temperature-gradient-driven modes on electron gyroradius scales are performed in the geometry of an advanced stellarator fusion experiment, Wendelstein 7-AS. Based on linear simulations, a critical electron-temperature-gradient formula is established which happens to agree quite well with a previously derived formula for tokamaks in the appropriate limit. Nonlinear simulations are used to study the turbulence and transport characteristics which are dominated by the presence of high-amplitude radially elongated vortices or `streamers'. The role of Debye shielding effects is also examined.

A toroidal low-temperature plasma is used for comparative turbulence studies. Measurements are carried out with Langmuir probe arrays in the entire plasma cross section. The data are closely compared with drift-Alfvén turbulence simulations. Although the parameters in a low-temperature plasma are very different from those in fusion plasmas, the dimensionless parameters governing the drift-wave physics are comparable. Hence this study can also give insight into high-temperature plasma turbulence. In order to identify relevant characteristic signatures of different turbulence-driving mechanisms, simulations were carried out for a wide range of plasma parameters. The simulation results are compared with first measurements.

The coordinated and efficient distribution of limited resources by individual decisions is a fundamental, unsolved problem. When individuals compete for road capacities, time, space, money, goods, etc, they normally make decisions based on aggregate rather than complete information, such as TV news or stock market indices. In related experiments, we have observed a volatile decision dynamics and far-from-optimal payoff distributions. We have also identified methods of information presentation that can considerably improve the overall performance of the system. In order to determine optimal strategies of decision guidance by means of user-specific recommendations, a stochastic behavioural description is developed. These strategies manage to increase the adaptibility to changing conditions and to reduce the deviation from the time-dependent user equilibrium, thereby enhancing the average and individual payoffs. Hence, our guidance strategies can increase the performance of all users by reducing overreaction and stabilizing the decision dynamics. These results are highly significant for predicting decision behaviour, for reaching optimal behavioural distributions by decision support systems and for information service providers. One of the promising fields of application is traffic optimization.

We review constraints on the minimal supersymmetric extension of the standard model (MSSM) coming from direct searches at accelerators such as LEP, indirect measurements such as b→sγ decay and the anomalous magnetic moment of the muon. The recently corrected sign of pole light-by-light scattering contributions to the latter is taken into account. We combine these constraints with those due to the cosmological density of stable supersymmetric relic particles. The possible indications on the supersymmetric mass scale provided by fine-tuning arguments are reviewed critically. We discuss briefly the prospects for future accelerator searches for supersymmetry.

We consider domain walls obtained by embedding the (1+1)-dimensional ϕ⁴-kink in higher dimensions. We show that a suitably adapted dimensional regularization method avoids the intricacies found in other regularization schemes in both supersymmetric and non-supersymmetric theories. This method allows us to calculate the one-loop quantum mass of kinks and surface tensions of kink domain walls in a very simple manner, yielding a compact d-dimensional formula which reproduces many of the previous results in the literature. Among the new results is the non-vanishing one-loop correction to the surface tension of a (2+1)-dimensional N = 1 supersymmetric kink domain wall with chiral domain-wall fermions.

Turbulence is a ubiquitous phenomenon, playing a role in many aspects of our daily life, and constituting an essential element in many fields of physics research. At the same time, its theoretical analysis remains one of the great challenges of physics. The advent of supercomputers has given us the new instrument of numerical modelling, and promises qualitative progress in our theoretical understanding of turbulence and ultimately also in our capability to predict quantitatively its consequences.

The presence of a magnetic field in a plasma adds a completely new element. In a one-fluid model it extends the set of ideal invariants of the Navier-Stokes equations from two (in the three-dimensional (3D) case: energy and kinetic helicity) to three (energy, cross-helicity and magnetic helicity). As in the 2D (but not the general 3D) hydrodynamic case, one of the invariants - the magnetic helicity - exhibits an inverse cascade, suggesting that the system drives towards self-organization and the formation of large-scale structures. More complexity is added to the system by the fact that for many phenomena a simple one-fluid description of the plasma is not adequate and has to be substituted by a more complex two-fluid model with kinetic corrections, or even a fully kinetic model.

Explaining the behaviour of magnetically confined laboratory plasmas has been a driving force in theoretical and experimental turbulence studies. The papers in this focus issue are concerned with configurations in which large-scale macroscopic instabilities are absent. In this situation, namely, in toroidal configurations with nested, closed magnetic surfaces, turbulence driven by smaller scale linear (or sometimes non-linear) instabilities determines the transport of energy across field lines. The turbulent structures are highly anisotropic, with scales of the order of the torus circumference along and (typically) of several ion Larmor radii perpendicular to the field lines. The large correlation length of turbulence along field lines requires that, in general, at least some aspects of the geometry of the plasma, and in particular its toroidal nature, have to be taken into account in these calculations. The driving mechanisms and the characteristics of the turbulence vary with the parameter regime of the plasma between the core of the toroidal plasma column and the edge, and also depend on whether electrons or ions are the plasma component that has been primarily heated. Plasma turbulence therefore involves, in principle, scales ranging from the electron gyro-radius to the plasma cross section. In numerical simulations it is therefore necessary to find a compromise between the complexity of the plasma model, the spatial resolution of the calculations and the geometrical extent of the simulated plasma region. Clearly, the advent of more powerful computers will allow these limits to be pushed back further.

The most striking result of magnetic confinement research during the last two decades has been the observation of so-called `transport barriers', across which the turbulent transport is strongly reduced, and which contribute, in spite of their narrow width, dominantly to the thermal insulation of the core plasma. Such barriers were first observed near the plasma edge (the `H-mode' barrier), but were later also produced in the plasma interior (`ITBs': internal transport barriers). Much theoretical and experimental evidence has accumulated that they are produced by sheared flows (or, equivalently, gradients in the electric field perpendicular to the flux surfaces). Both analytic and numerical models have shown that such flows, which are of a large scale within a flux surface, can spontaneously arise from smaller scale turbulence. The identification of the conditions under which they form a stable barrier, and the self-consistent simulation of such a transition remain, however, an important research topic. Transport barriers also arise in very different plasma parameter regimes, and can affect in different ways the particle, the electron, and the ion heat transport.

The articles included in this focus issue of New Journal of Physics highlight the state of the art in this field, which is characterized on the one hand by a rapid development of our capability to construct a virtual, magnetically confined plasma on a computer, and on the other, by a growing experimental effort to test the more `microscopic' predictions of these models by fluctuation studies and advanced data interpretation methods.

Focus on Turbulence in Magnetized Plasmas Contents

Aspects of flow generation and saturation in drift-wave turbulenceVolker Naulin

The role of radial electric fields in linear and nonlinear gyrokinetic full radius simulationsS J Allfrey, A Bottino, O Sauter and L Villard

Zonal flow generation in the improved confinement mode plasma and its role in confinement bifurcationsM G Shats and W M Solomon

Comparison of turbulence measurements and simulations of the low-temperature plasma in the torsatron TJ-KC Lechte, S Niedner and U Stroth

Stellarator turbulence at electron gyroradius scalesF Jenko and A Kendl

The spatial structure of edge fluctuations in the Wendelstein 7-AS stellaratorJ Bleuel, M Endler, H Niedermeyer, M Schubert, H Thomsen and The W7-AS Team

Fluctuations, sheared radial electric fields and transport interplay in fusion plasmasC Hidalgo, M A Pedrosa and B Gonçalves

The nonlinear drift wave instability and its role in tokamak edge turbulenceBruce D Scott

Dynamical simulations of boundary plasma turbulence in divertor geometryX Q Xu, W M Nevins, R H Cohen, J R Myra and P B Snyder

Large-scale fluctuation structures in plasma turbulenceO Grulke and T Klinger

A model of nonlinear evolution and saturation of the turbulent MHD dynamoA A Schekochihin, S C Cowley, G W Hammett, J L Maron and J C McWilliams

Slablike ion temperature gradient driven mode in reversed shear tokamaksY Idomura, S Tokuda and Y Kishimoto

Turbulent electron thermal transport in tokamaksW Horton, B Hu, J Q Dong and P Zhu

Modulational instability of drift wavesK Hallatschek and P H Diamond

Sibylle Günter Max-Planck-Institut für Plasmaphysik, Garching, Germany

Unstable fluctuations develop in the initially quiescent plasma in the improved confinement mode of the H-1 heliac when the radial electric field (E_r) shear exceeds some critical value. These unstable E_r shear-driven modes are shown to generate zonal-flow-like poloidally symmetric potential structures, similar to those generated in the low confinement mode (Shats M G and Solomon W M 2002 Phys. Rev. Lett.88 045001). The structures modulate their parent waves, the background E_r shear and the fluctuation-driven radial transport. The onset of zonal flows is observed as a precursor to the plasma confinement bifurcation to an even higher confinement regime.

The pivotal role played by radial electric fields in the development of turbulence associated with anomalous transport is examined by means of global gyrokinetic simulations. It is shown that the stabilizing effect of E×B flows on ion temperature gradient (ITG) modes is quadratic in the shearing rate amplitude. For a given shearing rate it leads to an increase in the critical gradient. The electric fields (zonal flows) self-generated by ITG modes interact in a nonlinear way and it is shown that a saturated level of both the zonal flow and ITG turbulence is reached in the absence of any collisional mechanism being included in the model. The quality of the global nonlinear simulations is verified by the energy conservation which is allowed by the inclusion of nonlinear parallel dynamics. This demonstrates the absence of spurious damping of numerical origin and thus confirms the nonlinear character of zonal flow saturation mechanism.

The generation of large-scale flows by turbulence in plasmas and fluids has attracted a great deal of attention in the last years. The interplay between turbulence and sheared zonal flows has been identified as a possible candidate to explain the transition from L to H mode as well as the setup of transport barriers in general, while radial flows are made responsible for increased levels of transport. Using simplified equations for low-frequency plasma turbulence of the drift-wave type we consider some general aspects of flow generation and address the problem of saturation of drift-wave turbulence.

We analyse the relic neutralino density in supersymmetric models with an intermediate unification scale. In particular, we present concrete cosmological scenarios where the reheating temperature is as small as (1-1000 MeV). When this temperature is associated to the decay of moduli fields producing neutralinos, we show that the relic abundance increases considerably with respect to the standard thermal production. Thus the neutralino becomes a good dark matter candidate with 0.1 ≲ Ωh² ≲ 0.3, even for regions of the parameter space where large neutralino-nucleon cross sections, compatible with current dark matter experiments, are present. This is obtained for intermediate scales M_I ~ 10¹¹-10¹⁴ GeV, and moduli masses m_ϕ ~ 100-1000 GeV. On the other hand, when the above temperature is associated with the decay of an inflaton field, the relic abundance is too small.

I propose a `quantum annealing' heuristic for the problem of a combinatorial search among a frustrated set of states characterized by a cost function to be minimized. The algorithm is probabilistic, with post-selection of the measurement result. A unique parameter playing the role of an effective temperature governs the computational load and the overall quality of the optimization. Any level of accuracy can be reached with a computational load independent of the dimension N of the search set by choosing the effective temperature correspondingly low. This is much better than classical search heuristics, which typically involve computation times growing as powers of log(N).

The rare B→K*ν decay is analysed with a new up-like quark in a sequential fourth-generation model. Two possible solutions for the fourth-generation CKM (Cabibbo-Kabayashi-Maskawa) factor obtained as a function of the new -quark mass are used. The branching ratio (BR) and missing-energy spectrum of this decay in the two cases are estimated. In one case, it is shown that for  GeV a significant enhancement to the BR and the missing-energy spectrum of this decay over the SM (standard model) is recorded, while the results are almost same in the other case. If a fourth generation should exist in nature and nature chooses this case, this B-meson decay mode could be a good probe for the existence of the fourth generation, or perhaps a signal for a new physics.

Starting from a N = 1 scalar supermultiplet in 2+1 dimensions, we demonstrate explicitly the appearance of induced N = 1 vector and scalar supermultiplets of composite operators made out of the fundamental supersymmetric constituents. We discuss an extension to a N = 2 superalgebra with central extension, due to the existence of topological currents in 2+1 dimensions. We comment on the relevance of these results for an effective description of the infrared dynamics of planar high-temperature superconducting models with quasiparticle excitations near nodal points of their Fermi surface.

The evolution properties of propagating particles produced at high energies in a randomly distributed environment are studied. The finite size of the phase space of the multiparticle production region as well as the chaoticity can be derived.

Upon the addition of 5% argon to a hydrogen plasma, the Lyman α emission was observed to increase by about an order of magnitude, whereas xenon control had no effect. With a microwave input power of 40 W, the gas temperature of an argon plasma increased from 400 to over 750 °C with the addition of 3% flowing hydrogen, whereas the 400 °C temperature of a xenon plasma run under identical conditions was essentially unchanged with the addition of hydrogen. The average hydrogen-atom temperature of the argon-hydrogen plasma was measured to be 110-130 eV versus ≈3 eV for pure hydrogen or xenon-hydrogen. Mechanisms such as Stark broadening or acceleration of charged species due to high fields (e.g. over 10 kV cm^-1) cannot be invoked to explain the results with argon since the electron density was low and no high field was observationally present. The electron temperature T_e for the argon-hydrogen and xenon-hydrogen plasmas was 11 600±5% and 6500±5% K, respectively, compared to 4800±5% and 4980±5% K for argon and xenon alone, respectively. The observation of higher temperatures corresponding to three possibly independent plasma parameters for only argon with hydrogen may be explained by the release of energy from atomic hydrogen by a resonant nonradiative energy-transfer mechanism.

We consider multiplet shortening for Bogomol'nyi-Prasad-Sommerfield solitons in = 1 two-dimensional models. Examples of single-state multiplets were established previously in = 1 Landau-Ginzburg models. The shortening comes at the price of losing the fermion parity (-1)^F due to boundary effects. This implies the disappearance of the boson-fermion classification resulting in abnormal statistics. We discuss an appropriate index that counts such short multiplets.

A broad class of hybrid models which extend the Landau-Ginzburg models to include a nonflat metric on the target space are considered. Our index turns out to be related to the index of the Dirac operator on the soliton reduced moduli space (the moduli space is reduced by factoring out the translational modulus). The index vanishes in most cases, implying the absence of shortening. In particular, it vanishes when there are only two critical points on the compact target space and the reduced moduli space has nonvanishing dimension.

We also generalize the anomaly in the central charge to take into account the target space metric.

Probably the most natural energy functional to be considered for knotted strings is that given by electrostatic repulsion. In the absence of counter-charges, a charged, knotted string evolving along the energy gradient of electrostatic repulsion would progressively tighten its knotted domain into a point on a perfectly circular string. However, in the presence of charge screening self-repelling knotted strings can be stabilized. It is known that energy functionals in which repulsive forces between repelling charges grow inversely proportionally to the third or higher power of their relative distance stabilize self-repelling knots. Especially interesting is the case of the third power since the repulsive energy becomes scale invariant and does not change upon Möbius transformations (reflections in spheres) of knotted trajectories. We observe here that knots minimizing their repulsive Möbius energy show quantization of the energy and writhe (measure of chirality) within several tested families of knots.

A theory describing propagation of partially coherent pulse trains in Kerr media is proposed. It is shown that changes in the statistical properties of chaotic modulated pulses propagating under self-phase modulation (SPM) are easily obtained within the framework of cyclostationary processes. Specific applications for laser light and thermal light are considered showing that in contrast with laser light, the thermal light spectrum on propagation suffers a significant change which smears out the characteristic peaks of the SPM spectrum.

We discuss the puzzling experimental results on baryon-antibaryon production in e⁺e^- annihilation close to the threshold, in particular the fact that  ≳ . We discuss an interpretation in terms of a two-step process, via an intermediate coherent isovector state serving as an intermediary between e⁺e^- and the baryon-antibaryon system. We provide evidence that the isovector channel dominates both e⁺e^- → pions and from annihilation at rest, and show that the observed ratio of to can be understood quantitatively in this picture.

We present high statistics Monte Carlo results for the Drossel-Schwabl forest fire model in two dimensions. They extend to much larger lattices (up to 65 536 × 65 536) than previous simulations and are much closer to the critical point (up to θ≡p/f = 256 000). They are incompatible with all previous conjectures for the (extrapolated) critical behaviour, although in general they agree well with previous simulations wherever they can be directly compared. Instead, they suggest that scaling laws observed in previous simulations are spurious, and that the density ρ of trees in the critical state was grossly underestimated. While previous simulations gave ρ≈0.408, we conjecture that ρ is actually equal to the critical threshold p_c = 0.592... for site percolation in d = 2. This is still, however, far from the densities reachable with present day computers, and we estimate that we would need many orders of magnitude higher CPU times and storage capacities to reach the true critical behaviour - which might or might not be that of ordinary percolation.

We consider the dual picture of the Yang-Mills theory at large distances. The dual Higgs model is reformulated in terms of two-point Wightman functions with the equations of motion involving higher derivatives.

Effective field theories with (large) extra dimensions are studied within a physical regularization scheme provided by string theory. Explicit string calculations then allow us to consistently analyse the ultraviolet sensitivity of Kaluza-Klein theories in the presence or absence of low-energy supersymmetry.

The strange quark star is investigated within the enlarged SU(3) Nambu-Jona-Lasinio model. The stable quark star can exist until a maximal configuration with ρ_m = 3.1×10¹⁵ g cm^-3 with M_m = 1.61 M_⊙ and R_m = 8.74 km is reached. Strange quarks appear for density above ρ_c = 9.84 g cm^-3 for the quark star with radius R_c = 8.003 km and M_c = 0.77 M_⊙. A comparison of quark star properties obtained in the quark mean-field approach to a neutron star model constructed within the relativistic mean-field theory is presented.

This paper discusses the supersymmetry and shape invariance (SI) of the effective screened potential. It is shown that the effective screened potential (ESP) has SI and belongs to the first class of shape-invariant potentials, thence the energy levels of this potential are obtained. Furthermore, by using the method of point canonical transformation we find that the ESP belongs to the same subclass as Pöschl-Teller potential 1; the bound-state spectra and the eigenfunctions of this potential are obtained. The results obtained can readily be applied to the special case of the ESP, the famous Hulthén potential.

The influence of the excited states of a substitutional dopant (donor or acceptor) on the majority-carrier concentration in a wide-bandgap semiconductor is investigated theoretically and experimentally, because acceptor levels ΔE_A in SiC, GaN and diamond were reported to be deeper than 150 meV. In order to accurately determine the values of ΔE_A, the acceptor density N_A and the compensating density N_comp in Al-doped p-type 4H-SiC using the temperature dependence p(T) of the hole concentration obtained from Hall-effect measurements, a distribution function including this influence is theoretically derived. Here, an ensemble average of the ground and excited state levels of the acceptor is newly introduced into the distribution function. It is found that a curve-fitting procedure, in which we proceed to search for N_A, ΔE_A and N_comp to fit a curve to the experimental p(T) by a method of least squares, is not suitable for investigating this influence. It is experimentally demonstrated that free-carrier concentration spectroscopy (FCCS), which we have proposed, can investigate this influence in detail. By using FCCS and the distribution function proposed here, the reliable values of N_A, ΔE_A and N_comp can be obtained.

We study the violations of the Bell inequality for thermal states of qubits in a multi-qubit Heisenberg model as a function of temperature and external magnetic field. Unlike the behaviours of the entanglement, the violation cannot be obtained by increasing the temperature or the magnetic field. The threshold temperatures of the violation are found be less than that of the entanglement. We also consider a realistic cavity-QED model which is a special case of the multi-qubit Heisenberg model.

We have carried out computer simulations to identify and characterize the ground-state geometrical structures of some nickel clusters (Ni₁₃-Ni₅₅) using tight-binding molecular dynamics. A possible geometrical packing sequence was found for Ni₁₃-Ni₃₂. The result suggested that the geometrical structures of Ni₁₃-Ni₃₂ should tend to exhibit geometries based on the icositetrahedron but not the icosahedron.

We present in this paper a molecular dynamics simulation of structural and thermodynamic properties of the hypothetical IV-IV compound GeC in the zinc-blende structure. This study is performed with the use of the well-tested Tersoff potential. Various physical quantities including elastic constants, Debye temperature, thermal expansion coefficient, heat capacity, and Grüneisen parameter are predicted. The comparison with the corresponding results for SiC is also discussed.

We analyse quantum teleportation (QT) and entanglement swapping (ES) for spin systems. If the permitted operations are restricted to the Ising interaction, plus local rotations and spin measurements, high-fidelity teleportation is achievable for quantum states that are close to the maximally weighted spin state. ES is achieved, and is maximized for a combination of entangled states and Bell measurements that is different from the QT case. If more general local unitary transformations are considered, then it is possible to achieve perfect teleportation and ES.

We discuss the interactions of Goldstone particles with solitonic states. We observe that, contrary to the familiar situation in the vacuum sector, the Goldstone particles can have non-derivative interactions with the solitons. This result is applied to brane physics and in particular leads to the possibility that neutrinos in brane world scenarios are Goldstone particles for broken supersymmetry.

Based on the renormalization group (RG) method, a reduction of non-integrable multi-dimensional Hamiltonian systems has been performed. The evolution equations for the slowly varying part of the angle-averaged phase space density and for the amplitudes of the angular modes have been derived. It has been shown that these equations are precisely the RG equations. As an application of the approach developed, the modulational diffusion in a one-and-a-half-degree-of-freedom dynamical system has been studied in detail.

We demonstrate storage and manipulation of one qubit encoded into a decoherence-free subspace (DFS) of two nuclear spins using liquid state nuclear magnetic resonance techniques. The DFS is spanned by states that are unaffected by arbitrary collective phase noise. Encoding and decoding procedures reversibly map an arbitrary qubit state from a single data spin to the DFS and back. The implementation demonstrates the robustness of the DFS memory against engineered dephasing with arbitrary strength as well as a substantial increase in the amount of quantum information retained, relative to an un-encoded qubit, under both engineered and natural noise processes. In addition, a universal set of logical manipulations over the encoded qubit is also realized. Although intrinsic limitations prevent maintenance of full noise tolerance during quantum gates, we show how the use of dynamical control methods at the encoded level can ensure that computation is protected with finite distance. We demonstrate noise-tolerant control over a DFS qubit in the presence of engineered phase noise significantly stronger than observed from natural noise sources.

After the shutdown of the large electron-positron collider at CERN, the search for CP violation and flavour changing neutral current (FCNC) phenomena acquired a privileged role in our quest for new physics beyond the electroweak standard model (SM). Most extensions of the SM exhibit new sources of CP violation and FCNC. In particular, the minimal supersymmetric extension of the SM presents several new phases and flavour structures in addition to those already present in the SM. Therefore, supersymmetry may have a good chance to manifest some departure from the SM in this particularly challenging class of rare phenomena. On the other hand, it is also apparent that CP violation and FCNC generally represents a major constraint on any attempt at model building beyond the SM. In this work, we review the status of FCNC and CP violation in supersymmetric extensions of the SM and discuss the possibilities of these indirect searches in the quest for supersymmetry.

We have evaluated the total carrier mass enhancement factor f_t for MgB₂ from two independent experiments (specific heat and upper critical field). These experiments consistently show that f_t = 3.1±0.1. The unusually large f_t is incompatible with the measured reduced gap (2Δ(0)/k_BT_c = 4.1) and the total isotope-effect exponent (α = 0.28±0.04) within the conventional phonon-mediated model. We propose an unconventional phonon-mediated mechanism, which is able to quantitatively explain the values of T_c, f_t, α and the reduced energy gap in a consistent way.

We give a general analysis of OPEs of 1/2 BPS superfield operators for the D = 3, 4, 5, 6 superconformal algebras OSp(8/4, R), PSU(2, 2/4), F₄ and OSp(8*/4) which underlie maximal AdS supergravity in 4 ⩽ D + 1 ⩽ 7.

The corresponding three-point functions can be formally factorized in a way similar to the decomposition of a generic superconformal UIR into a product of supersingletons. This allows for a simple derivation of branching rules for primary superfields. The operators of protected conformal dimension which may appear in the OPE are classified and are shown to be either 1/2 or 1/4 BPS, or semishort. As an application, we discuss the `non-renormalization' of extremal n-point correlators.

We consider a five-dimensional supergravity model with SU(5) gauge symmetry and the minimal field content. Studying the arising scalar potential we find that the gauging of the U(1)_R symmetry of the five-dimensional supergravity causes instabilities. Lifting the instabilities, the vacua are of anti-de-Sitter type and SU(5) is broken along with supersymmetry. Keeping the U(1)_R ungauged the potential has flat directions along which supersymmetry is unbroken.

A recent paper by Trugenberger (C A Trugenberger 2002 New J. Phys.4 26) is commented on.

The abstract contains a misprint in the value for the star central density, ρ_s, above which strange quarks appear. A factor of 10¹⁴ was omitted from the original version. Strange quarks appear for central densities ρ_c above ρ_s = 9.85 × 10¹⁴ g cm^-3.

The abstract has also been rewritten to differentiate more clearly ρ_c, the general star central density, ρ_m, the maximal central density and ρ_s the minimal central density. It now reads as:

Abstract. The strange quark star is investigated within the enlarged SU(3) Nambu-Jona-Lasinio model. The stable star can exist until a maximal configuration with central density ρ_c = ρ_m = 3.11 ×10¹⁵ g cm^-3 with M_m = 1.61 M_⊙ and R_m = 8.74 km is reached. Strange quarks appear for densities ρ_c above ρ_s = 9.85 × 10¹⁴ g cm^-3 for the quark star with radius R_c = 8.003 km and M_c = 0.77 M_⊙. A comparison of quark star properties obtained in the quark mean-field approach to a neutron star model constructed within the relativistic mean-field theory is presented.

There are also two typographical errors in the paper:

• ρ₂ on pages 14.15 and 14.16 should have been written as ρ_m.