Table of contents

In the 1988 edition of Nature's 'News and Views', J Maddox wrote that 'one of the continuing scandals in physical sciences is that it remains in general impossible to predict the structure of even the simplest crystallographic solids from knowledge of their chemical composition' (Maddox 1988 Nature335 7). There is, however, the possibility of making some progress in this direction by combining two fundamental areas of physics: quantum mechanics and statistical physics. The starting point is an electronic structure theory density functional theory (DFT) (Hohenberg and Kohn 1964 Phys. Rev.136 864B, Kohn and Sham 1965 Phys. Rev.140 1133A) which independently establishes the range (first neighbours, second neighbours, etc), type (pairs, three body, four body, etc) and chemical character (charge transfer, atomic site effects, etc) of the interaction energies. All these can be determined from cluster expansions (CE) (Sanchez et al 1984 Physica A 128 334) which give access to both huge parameter spaces (e.g. for ground-state searches) and systems containing more than a million atoms (e.g. for microstructure studies). It will be shown that, together with Monte Carlo simulations, CE open the possibility of quantitatively studying alloy properties which possess a delicate temperature dependence, such as short-range-order effects, mixing enthalpies or dynamic processes like the ageing of microstructures. This method is extended to alloy surfaces in order to investigate geometric relaxations as well as surface segregation, i.e. the enrichment of one component in the near-surface region. To establish a complementary, experimental view of the geometrical structure and chemical composition of surfaces, experimental low energy electron diffraction spectra are analysed by the use of a multiple-scattering theory (Pendry 1974 Low Energy Electron Diffraction (London: Academic), Van Hove and Tong 1979 Surface Crystallography by LEED (Berlin: Springer)) providing a test of our DFT predicted surface properties.

This topical review discusses the influence of the surface geometry (e.g. lattice parameters and termination) and electronic structure of well-defined bimetallic surfaces on the adsorption and dissociation of benzene. The available data can be divided into two categories with combinations of non-transition metals and transition metals on the one side and combinations of two transition metals on the other. The main effect of non-transition metals in surface alloys is site blocking which can suppress chemisorption and dissociation of the molecules completely. When two transition metals are combined, the effects are less dramatic. They mainly affect the strength of the chemisorption bond and the degree of dissociation due to electronic and template effects.

A microscopic approach is developed to account for the magnitudes of the supercurrents observed experimentally in carbon nanotubes placed between superconducting contacts. We build up a model for the nanotube ropes encompassing the electron repulsion from the Coulomb interaction and the effective attraction given by phonon exchange. We show that the available experimental data are consistent with the expected decay of the supercurrents along the length of the nanotube samples. Our results stress that propagation of Cooper pairs is favoured in the thick ropes, as a consequence of the reduction in the strength of the Coulomb interaction from the electrostatic coupling between the metallic nanotubes. We also provide an explanation for the temperature dependence of the supercurrents observed in the experiments, noting the existence of a crossover from a very flat behaviour at low temperatures to a pronounced decay in the long nanotubes.

The classification of a molecule's spatial symmetry is an essential initial step in predicting its vibrational spectra. When a (chemical) substitution is made, the relationship between the spatial symmetries of the molecule and its derivatives governs the differences between their corresponding vibrational spectra. These text-book statements of molecular spectroscopy have also found recent and successful applications with much larger molecules, namely single-walled nanotubes. The purpose of this work is to review the profound interplay between the spatial symmetry and the numbers of Raman-and infrared-active vibrations in single-walled carbon and boron nitride nanotubes.

We compare our optical data collected for two families of low-dimensional correlated electron systems: the ladders and the organic Bechgaard salts. Our data display a remarkable optical anisotropy between the spectra along the three crystallographic axes, indicative of the low dimensionality in the electronic band structure. Moreover, we optically identify a dimensionality crossover, induced by Ca doping in the ladders and by chemical pressure in the Bechgaard salts. An outlook on the ongoing research on carbon and MoS₂ nanotubes, as novel one-dimensional quantum wires, is also addressed.

Spin-polarized scanning tunnelling microscopy is applied to study the growth of Fe on Cr(001) as well as the dependence of the electronic and magnetic structure on the Fe coverage. Topographic measurements show an almost perfect layer-by-layer growth except for the coverage range 1.48 ML < θ < 3 ML, where the second and third layers grow simultaneously. By high-temperature annealing, alloying between the Cr substrate and the Fe film is provoked. Scanning tunnelling spectroscopy (STS) shows that the electronic structure of the resulting c(2 × 2)-ordered Fe/Cr alloy is governed by a double peaked dI/dU spectrum with maxima at U = −0.3 and +0.15 V. A time-dependent STS investigation of Fe/Cr(001) reveals that the interface is structurally unstable against alloying. While the spectra are dominated by a single d_z²-like surface state on the day of preparation a double-peak spectrum, indicating the presence of a Fe/Cr alloy, was found one day after preparation. Spin-resolved measurements show that, at small coverage (θ ≤ 0.2 ML), the Fe islands couple antiferromagnetically to the underlying Cr(001) terraces. The magnetic contrast of the islands and the substrate starts to decrease for θ ≥ 0.22 ML and completely vanishes at θ ≥ 0.4 ML. This observation is discussed in terms of a reduction of the Cr Néel temperature due to interdiffusion of Fe into the interface-near region of the Cr substrate. For θ ≥ 3 ML a weak magnetic contrast reappears which is possibly caused by a spatial variation of the 90° coupling between the Cr substrate and the Fe overlayer.

The first investigation of magnetism in one-dimensional (1D) monatomic chains of metal atoms is reported. High-density arrays (5 × 10⁶ cm⁻¹) of parallel Co atomic wires have been grown using the vicinal Pt(997) surface as a template. Angle-resolved photoemission experiments evidence the presence of a 3d exchange-split band for the Co wires giving rise to enhanced localized spin magnetic moments. X-ray magnetic circular dichroism shows further that the orbital magnetic moment is about five times larger compared to that of bulk hcp Co as a result of the reduced atomic coordination of the 1D wires. Whereas statistical models forbid long-range ferromagnetic order in infinite 1D spin chains at any temperature greater than zero, we show that finite monatomic Co chains display both short- and long-range ferromagnetic order. The chains consist of thermally fluctuating segments of ferromagnetically coupled atoms which evolve into a ferromagnetic metastable long-range-ordered state below 15 K. Ferromagnetism in 1D is stabilized by unusually large magnetic anisotropy energy barriers (2 meV/atom) which arise from the reduced dimensionality of the wires and related large orbital magnetization.

Scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) are very efficient techniques for studying confinement effects on nanostructured surfaces. Noble metal (Cu, Ag, Au) and near-noble metal (Ni, Pd, Pt) (111) surfaces present a dispersive surface state (the Shockley state) that allows a direct investigation of these effects in real space. Both the energy spectrum and the spatial distribution of the wavefunctions can be investigated using spectroscopic imaging techniques. We present a very simple model for the confinement of surface states which is sufficient to perform a first-order analysis of our experimental data on two kinds of surface resonators (quantum boxes and linear resonators). We summarize the different acquisition modes in STM/STS that allow the study of the confinement effects via the mapping of the local density of states. These points are illustrated for the well-known case of the Cu(111) surface. The main difference between Ni-based nanostructures and similar objects on the clean Cu(111) surface is that d states are present in the energy range where the Shockley surface state is expected. We report a combined experimental and theoretical analysis of the dispersion of the surface state on the clean Ni(111) surface and on a mixed Ni–Cu(111) phase (β phase). In both cases the analysis of the electronic structure of the surface is possible only in situations of strong confinement. The presence of d states is found to affect the characteristics of the Shockley state, which opens interesting perspectives.

Eigen-modes of unconfined ferromagnetic media (spin waves) were introduced by Bloch to explain the thermodynamics of ferromagnets. In this paper we analyse the eigen-modes of laterally confined magnetic structures. The quantization of eigen-modes due to the finite size of the structures as well as their localization due to the recently discovered spin wave wells effect will be considered in detail. A general description of magnetic dynamic eigen-modes in media with a strongly nonuniform internal field, important for confined structures, will be presented. Pictorial effects of spin wave propagation, reflection and tunnelling caused by field inhomogeneity will be demonstrated and discussed.

The Halperin–Lubensky–Ma (HLM) effect of a fluctuation-induced change of the order of phase transition in thin films of type I superconductors with relatively small Ginzburg–Landau number κ is considered. Numerical data for the free energy, the order parameter jump, the latent heat and the specific heat of W, Al and In are presented to reveal the influence of film thickness and material parameters on the properties of the phase transition. We demonstrate for the first time that in contrast to the usual notion the HLM effect occurs in the most distinct way in superconducting films with high critical magnetic field H_c0 rather than in materials with small κ. The possibility of an experimental observation of the fluctuation change of the order of the superconducting phase transition in superconducting films is discussed.

Two post-treatments consisting of rapid thermal annealing (RTA) and furnace annealing (FA) were used to crystallize a-Si:H sublayers in a-Si:H/SiO₂ multilayers fabricated by alternate plasma-enhanced chemical vapour deposition of a-Si:H layers and plasma oxidation. Raman measurements show that the crystallization process of a-Si:H sublayers strongly depends on the post-treatment process. RTA is advantageous for nucleation, in which the size of the nuclei increases with higher annealing temperatures, and the crystalline volume ratio increases with longer annealing times. In FA, however, higher temperatures are required for further crystallization such as increasing the grain size and crystalline volume ratio. The ultimate size of nc-Si grains can be accurately determined by the thickness of the a-Si:H sublayer. Moreover, the mechanism of the constrained crystallization will also be discussed in accordance with the thermodynamic theory.

The present paper is concerned with a theoretical study of the properties of the Frenkel–Debye boundary layer at the (111) surface of pure (undoped) CaF₂, a crystal with anti-Frenkel defects, i.e. with anion vacancies and interstitials. The study is based on theoretical determinations of the adsorption energies of the fluorine ion at the terrace, step and kink sites on the surface, which are, together with the assumption of experimental step distances and theoretical kink concentrations, necessary pre-requisites for a quantitative description of the surface charge compensating that of the space-charge region. The results show that, for realistic surface conditions, the surface and space charges, and with them the potential between surface and bulk, are reduced considerably compared with those for an unlimited number of accessible surface sites. Nevertheless, on approaching the surface a strong increase in the anion vacancy concentration against that of the bulk still exists, which should lead to an increased ion conductivity in near-surface regions and thin fluorite layers.

The magnetism of Cr/W stepped surfaces is investigated using the self-consistent tight-binding method in the unrestricted Hartree–Fock approximation with a Hubbard Hamiltonian. These structures exhibit local magnetic moments with the lowest values on the kink atoms and the highest on the edge atoms, owing to the coordination. The local moments of the kink atoms are frustrated due to the ferromagnetic coupling with their nearest neighbours. The net magnetization increases as a function of the step length.

Photomodulation of the coupled plasmon–LO phonon modes has been employed to determine the change in both the surface charge density and the depletion electric field as a function of photomodulation beam (PMB) intensity. The samples are two pieces of highly doped (001) n-type GaAs. The total surface charge density has been obtained as a function of the photomodulating intensity using the dependence of the unscreened LO phonon on the depletion width. We are able to separate the impact of the PMB on the surface electric field from the impact on the depletion width. This allows a separate determination of the change in depletion electric field, which reaches ∼73% of its original value at the highest intensity used for PMB.

We study the electronic properties of ballistic thin normal metal–bulk superconductor heterojunctions by solving the Bogoliubov–de Gennes equations in the quasiclassical and microscopic 'exact' regimes. In particular, the significance of the proximity effect is examined through a series of self-consistent calculations of the space-dependent pair potential Δ(r). It is found that self-consistency cannot be neglected for normal metal layer widths smaller than the superconducting coherence length ξ₀, revealing its importance through discernible features in the subgap density of states. Furthermore, the exact self-consistent treatment yields a proximity-induced gap in the normal metal spectrum, which vanishes monotonically when the normal metal length exceeds ξ₀. Through a careful analysis of the excitation spectra, we find that quasiparticle trajectories with wavevectors oriented mainly along the interface play a critical role in the destruction of the energy gap.

The understanding of different features of the sputtering of materials lies in understanding the factors contributing to the emission of atoms from a regular crystal structure at the surface. Although the sputtering of fcc crystals has received much attention, the database on bcc materials is still scarce. We use molecular dynamics simulations to study the self-sputtering of the (100), (110), (111) and (112) surfaces of molybdenum. Single atoms as well as Mo₂ and Mo₄ clusters are used as the irradiation projectiles, in the cluster energy range of 0.125–4 keV. Contrary to the usual assumption, enhanced (nonlinear) sputtering yields are observed for the cluster bombardments at both ends of the energy range studied. The enhancements can be explained with lower threshold energies for sputtering at low energies and with a decreased fraction of channelled projectile atoms in the kiloelectronvolt energy range.

Pr overlayers, with thicknesses of about 1.8 and 0.6 nm, were deposited on 1.2 nm SiO₂/Si(100) at room temperature and x-ray photoelectron spectroscopy (XPS) was used to investigate the reactions at the Pr/SiO₂/Si interfaces as a function of annealing temperature. The results show that the Pr overlayers reduce SiO₂ at room temperature, forming Pr₂O₃ and a Pr silicate (Pr–O–Si). For the 1.8 nm Pr/SiO₂/Si system, the 1.2 nm SiO₂ layer is mostly reduced at room temperature and a Pr silicide was also observed. The Pr–O–Si silicate increases in intensity while the Pr₂O₃ intensity decreases with annealing temperature. Angle-dependent XPS, taken after annealing, indicates that the silicate is located at the top surface. In the case of the 0.6 nm Pr/SiO₂/Si system, the Pr overlayer is almost oxidized after annealing, and some SiO₂ remains even after annealing at 1090 K, which can serve as a low-defect-density buffer layer between the high-κ film (Pr₂O₃ and Pr–O–Si) and the silicon.

The recent creation of pure Bose–Einstein condensates in alkali metals and also of vortices supported by them has increased interest in these phenomena. In particular, changes observed in the topology of these vortices is a partially unsolved problem. Here we confirm Feynman's hypothesis on how circular vortices can be created from oppositely polarized pairs of linear vortices. This is done by following the transformation numerically. The circular vortices so obtained satisfy known constraining relations between radii and velocities.

We report the fabrication of opal structure using metallo-dielectric silica microspheres. Mono-dispersed silica microspheres were coated with silver using an electrode-less wet-plating technique. Thin slabs of opal were obtained by assembling the silver-coated microspheres between two glass plates using a forced-packing method. The optical properties of the resulting opal structure were studied in the infrared range. Good agreement is obtained with the predictions of a multiple scattering approach, provided that the silver layer is modelled as a silver composite.

Under a dielectric continuum approximation, by adopting the transfer matrix method, interface optical (IO) and surface optical (SO) phonon modes, as well as the Fröhlich electron–IO (or –SO) phonon interaction Hamiltonian, in a multilayer coaxial cylindrical quantum cable (QC) were deduced and investigated. Numerical calculations on a four-layer Al_xGa_1−xAs/GaAs QC have been performed. Results reveal that there are four branches of IO phonon modes and one branch of SO phonon modes. When the wavevector in the z direction k_z and the azimuthal quantum number m are small, the dispersion frequencies of IO or SO modes sensitively depend on k_z and m. When k_z and m are relatively large, with increasing k_z and m, the frequency for each mode converges to the limit frequency value of the IO (or SO) mode in a single heterostructure, and the electrostatic potential distribution of each mode tends to be more and more localized at some interface or surface. Meanwhile, the coupling between the electron–IO or –SO phonon becomes weaker and weaker. The calculation also found that the phonon modes with higher frequencies have a more significant contribution to the electron–phonon interaction.

We investigate the ground-state phase diagram of the one-dimensional 'ionic' Hubbard model with an alternating periodic potential at half-filling by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group methods. We identify an insulator–insulator phase transition from a band to a correlated insulator with simultaneous charge and bond-charge order. The transition point is characterized by the vanishing of the optical excitation gap while simultaneously the charge and spin gaps remain finite and equal. Indications for a possible second transition into a Mott-insulator phase are discussed.

We present numerical data for energies and widths of energy levels of a two-dimensional neutral donor in an electric field. The problem, formally coinciding with that of a two-dimensional hydrogen atom, is solved by a two-dimensional finite-difference method. The data are obtained for a broad range of electric field strengths and are an essential supplement and refinement to existing theoretical investigations, carried out for weak fields.

This special issue of Journal of Physics: Condensed Matter contains some of the invited papers presented at the School and Workshop on Nanotubes and Nanostructureswhich was held in Frascati, Italy in October 2001 (http://wwwsis.lnf.infn.it/conference/nn2001/). The motivation and aim of this initiative was to promote the growth and development of science at the interface between different fields, where methods in one field are used to solve problems in others, bearing in mind the need to strengthen areas of research which are between fields. The School and Workshop covered an area - that of nanotubes and nanostructures - of overlap between field theory and statistical mechanics. This area has important consequences for the study of condensed matter physics and chemistry and also has impressive potential for applications in many fields. We focussed on nanotubes because they appeared to be ideal model systems for studying the physics in one-dimensional solids and have significant potential as building blocks for various practical nanoscale devices. Nanotubes, in fact, have proved to be useful for miniaturized electronic, mechanical, electrochemical and chemical devices. Similar efforts have been devoted to growing artificially nanostructured magnetic materials. The new structural and magnetic properties of these materials are discussed with an emphasis on the correlation between structure and magnetism, which also serves as guidance for improving their magnetic properties.