Table of contents

We show theoretically that the absorption of one-dimensional metallic–dielectric photonic crystals can be enhanced considerably over the corresponding constituent metal. By properly choosing the structural and material parameters, the absorption of one-dimensional metallic–dielectric photonic crystals can be enhanced by one order of magnitude in the visible and in the near infrared regions. It is found that the absorptance of such photonic crystals increases with increasing number of periods. Rules on how to obtain a absorption enhancement in a certain frequency range are discussed.

The results of non-resonant magnetic x-ray diffraction measurements of the metamagnet FeCl₂·2H₂O subjected to an applied magnetic field, H, are reported. The material shows two successive first-order phase transitions: at H_c1 = 3.62 T from the antiferromagnetic (AF) to intermediate (I) phases, and at H_c2 = 4.67 T from phase I to a ferromagnetic phase at low temperatures. We find that the AF and I phases coexist over a wide range of H below H_c1, when H is decreased from the I phase. We were able to follow the evolution of the AF and I domains with the change of H. The coexistence appears not to come from demagnetizing effects, and instead reflects the intrinsic nature of the material.

We present a time-dependent method based on the single-particle electron density matrix that allows the electronic and ionic degrees of freedom to be modelled within the Ehrenfest approximation in the presence of open boundaries. We describe a practical implementation using tight binding, and use it to investigate steady-state conduction through a single-atom device and to perform molecular dynamics. We find that in the Ehrenfest approximation an electric current allows both ionic heating and cooling to take place, depending on the bias.

This review puts in doubt the classical description of the Verwey (metal–insulator) transition in magnetite on the basis of the wide set of experiments carried out over the last 60 years. We re-analyse here the most relevant experiments used to study the Verwey transition from the point of view of their degree of agreement with the proposed Fe²⁺–Fe³⁺ charge ordering model. We will consider three groups of experimental studies, according to their capability of detecting different ionic species and/or a charge periodicity: (1) Experiments which have been interpreted using the charge ordering model as the starting point though they are not able to demonstrate its validity. This is the case for macroscopic properties such as the electrical resistivity, the heat capacity and the magnetic properties. (2) Experiments which can distinguish different types of Fe ions, such as Mössbauer, nuclear magnetic resonance (NMR) and electronic spectroscopies. However, we show that they are not able to associate them with a specific valence (2+ or 3+ in our case) and, in some cases, they observe more than two different kinds of iron atoms. (3) Diffraction (x-ray, neutron and electron) experiments, which are the most conclusive ones for determining a periodic ordering of different entities. These experiments, instead, point to the lack of ionic charge ordering. We will focus, in particular, on the discussion of the results of some recent x-ray resonant scattering experiments carried out on magnetite that directly prove the lack of ionic charge ordering in such mixed valence oxide. Furthermore, we also reconsider some so-called Verwey-type transition metal oxides in terms of the applicability of the Verwey charge ordering model. We show that a complete charge disproportionation (δ) is not experimentally observed in any of these compounds, the maximum δ being less than 0.5 e⁻. Regarding the theoretical framework, we will outline some relevant implications for the description of the physics of 3d transition metal oxides of this critical re-examination of the experimental facts on magnetite. Electronic localization should then occur involving more than one transition metal atom, so the definition of ionic d states loses its meaning in mixed valence transition metal oxides.

In systems without inversion, symmetry spin–orbit interactions couple the charge–current and spin-polarization degrees of freedom. This review gives a brief summary of the origins of this coupling and reviews a variety of ways in which the coupling may be made evident experimentally, with emphasis on the detection of electric-field driven spin polarization.

Recent results on achieving ferromagnetism in transition-metal-doped GaN, AlN and related materials are discussed. The field of semiconductor spintronics seeks to exploit the spin of charge carriers in new generations of transistors, lasers and integrated magnetic sensors. There is strong potential for new classes of ultra-low-power, high speed memory, logic and photonic devices based on spintronics. The utility of such devices depends on the availability of materials with practical magnetic ordering temperatures and most theories predict that the Curie temperature will be a strong function of bandgap. We discuss the current state-of-the-art in producing room temperature ferromagnetism in GaN-based materials, the origins of the magnetism and its potential applications.

We calculate the electron exchange coupling for a phosphorus donor pair in silicon perturbed by a J-gate potential and the boundary effects of the silicon host geometry. In addition to the electron–electron exchange interaction we also calculate the contact hyperfine interaction between the donor nucleus and electron as a function of the varying experimental conditions. Donor separation, depth of the P nuclei below the silicon oxide layer and J-gate voltage become decisive factors in determining the strength of both the exchange coupling and hyperfine interaction—both crucial components for qubit operations in the Kane quantum computer. These calculations were performed using an anisotropic effective-mass Hamiltonian approach. The behaviour of the donor exchange coupling as a function of the parameters varied in this work provides relevant information for the experimental design of these devices.

In contrast to widely accepted view that pressure-induced amorphization arises due to kinetic hindrance of equilibrium phase transitions, here we provide evidence that the metastable pressure-amorphized state in zirconium tungstate is a precursor to decomposition of the compound into a mixture of simple oxides. This is from the volume collapse ΔV across amorphization, which is obtained for the first time by measuring linear dimensions of irreversibly amorphized samples during their recovery to the original cubic phase upon isochronal annealing up to 1000 K. The anomalously large ΔV of 25.7 ± 1.2% being the same as that expected for the decomposition indicates that this amorphous state is probably a precursor to kinetically hindered decomposition. A P–T diagram of the compound is also proposed.

The crystal structures of LiCaAlF₆-II and LiSrAlF₆-II (both P 2₁/c, Z = 4) occurring at high pressures and room temperature were studied with synchrotron angle-dispersive x-ray powder diffraction in diamond anvil cells. The structure of LiSrAlF₆-II stable between 1.6 and 3.0 GPa was solved with a global optimisation algorithm and group theory considerations, and refined with the Rietveld method in the rigid-body approximation. It is a distorted variant of the ambient pressure polymorph (LiSrAlF₆-I, ), in which each cation occupies a deformed octahedral site. LiCaAlF₆ transforms to this monoclinic polymorph II above about 7 GPa. The differences in the high-pressure behaviours of LiCaAlF₆ and LiSrAlF₆ are discussed by considering the ionic radii.

A simulation of a tensile test of copper crystal along the [001] direction is performed using the Vienna ab initio simulation package (VASP). Stability conditions for a uniaxially loaded system are presented and analysed and the ideal (theoretical) tensile strength for the loading along the [001] direction is determined to be 9.4 GPa in tension and 3.5 GPa in compression. A comparison with experimental values is performed.

We report results from density-functional plane-wave pseudopotential calculations for carbon and silicon self-interstitials in cubic silicon carbide (3C-SiC). Several initial ionic configurations are used in the search for the global total-energy minimum including tetragonal, split [100] and split [110] geometries. Neutral carbon interstitials are found to have several nearly degenerate total-energy minima configurations in split-interstitial geometries, with formation energies ranging—besides higher metastable ones—from 6.3 to 6.7 eV in stoichiometric SiC. By contrast, the neutral silicon interstitials have a clear single minimum total-energy configuration at the tetrahedral configuration with carbon nearest neighbours, exhibiting a formation energy of 6.0 eV. The split interstitial in the [110] direction at the silicon site and the tetrahedral configuration with silicon nearest neighbours are metastable and have significantly higher formation energies. The present calculations indicate that the carbon interstitial introduces deep levels in the band gap while the silicon interstitial at the tetrahedral site behaves like a shallow donor.

Measurements of bulk conductivity and dielectric relaxation processes by ac methods including complex-plane impedance analyses have been carried out on insulating , together with measurements of four-probe dc conductivity and magnetic susceptibility, so as to investigate the correlation between bulk conduction and short-range ferromagnetism due to double-exchange (DE) interactions working even above ferromagnetic ordering temperature (T_c). In bulk conduction, there are three temperature regimes where different processes dominate the electronic conduction: (i) the paramagnetic regime above the temperature at the onset of deviation of inverse magnetic susceptibility from the Curie–Weiss law, (ii) the intermediate regime and (iii) the temperature region below T_c. Though the polaronic conduction occurs in both high-temperature and intermediate regions, the hopping energy and the energy required to create a free hopping polaron differ considerably in these regimes. An anomaly emerges in both bulk conduction and dielectric relaxation processes around T_c at x = 0.20 and this anomaly becomes more clear at x = 0.25. By taking account of electron–phonon interactions, these results are discussed mainly in terms of the DE mechanism that brings about short-range ferromagnetism.

We study the properties of the localized interface plasmon modes (IPMs) arising from coupling and splitting of the plasmon eigenstates in two semi-infinite superlattices containing complex defect structures. Two kinds of defect structure are considered: one is a single quantum well, i.e., a barrier–well–barrier structure; the other is a well–barrier–well structure. The influences of the constitution of the defect structures, as well as the physical and geometric parameters of the superlattices on the characteristics of the localized IPMs are investigated in detail. The numerical results show that the localized modes exhibit peculiar features; the complexity of the dispersion of IPMs is essentially associated with the coupling of the interwell regions in the defect structure, and the barrier and well layers play different roles in the features of the localized IPMs. We also present a comparison of the dispersions of the localized IPMs between these two types of defect structure and give a physical picture to explain these results.

The bound-state problem for triaxial ellipsoidal infinite-barrier quantum dots has been solved. It is exactly solvable in terms of ellipsoidal coordinates and the eigenmodes are written in terms of Lamé wavefunctions. The need for all eight types of functions is shown. This presents a generalization over previous work on spheres and spheroids. Splitting of degeneracy and level crossing are obtained.

We present the first experimental study of the carrier density dependence of the composite fermion conductivity σ_xx^CF at Landau level filling factors ν = 1/2 and 3/2 in high-quality front-gated GaAs/Al_0.33Ga_0.67As heterostructures. Extracting α from the power law shows that . The measured is placed between the predicted value 3/4 in the strong random magnetic field regime, and 3/2 in the weak random magnetic field regime. Comparisons between our results and theory are discussed.

We have developed the potential parameters for the simulation of MgB₂ and calculated the variations of lattice parameters and volume with pressure up to 240 GPa. Our calculated results are in good agreement with experimental results below 40 GPa. By employing the McMillan expression, it is found that the lattice stiffening dominates the behaviour of the superconducting transition T_c under pressure in the scope of BCS theory. Using our calculated Grüneisen parameter γ_G, the simulated pressure effect on T_c accords well with experimental results. Our result shows that the T_c of MgB₂ can be destroyed by high pressure.

Multipod-like zinc oxide (M-ZnO) whiskers were produced in bulk quantity by the thermal oxidation of metal Zn powders that had been etched by an NiSO₄ aqueous solution. X-ray diffraction, scanning electron microscopy and energy dispersive x-ray analysis investigations showed that the M-ZnO whiskers had pure hexagonal structure with several needle-like legs united at a common junction. A weak photoluminescence emission band peak around 380 nm and an intensive one around 495 nm were observed in the M-ZnO whiskers, which have been attributed to the near band-edge emission and the deep-level emission related to oxygen vacancy, respectively.

We report on the specific heat and magnetic susceptibility of polyaniline (PANI) nanotubes, which were self-assembled by a template-free method. It is interestingly found that an electronic specific heat is not observed from present data, and the lattice specific heat of PANI nanotubes can be interpreted in terms of the contributions from both the crystalline and the amorphous phases. The magnetic susceptibility includes a Pauli-like susceptibility and a Curie-type susceptibility. The results reveal an inhomogeneous disorder picture of PANI nanotubes (i.e., PANI nanotubes are composed of crystalline regions and amorphous regions). The crystallinity fraction D_g obtained from the specific heat is about 15% for the measured PANI nanotubes, which indicates that most of the polymer chains in the nanotubes are disordered.

High resolution transmission electron microscopy (HRTEM) observations show evidence for the formation of disorder Al zones around some faceted Pb inclusions embedded in an Al matrix in immiscible lead–aluminium samples, which were prepared by a melt-spun method and then aged for about two months at room temperature. Furthermore, it is found that, both before and after ageing, all such Pb inclusions have a cube–cube orientation relationship with the Al matrix. The results of molecular dynamics (MD) simulations give an extensive analysis of the formation of the disorder Al zones and provide a reasonable explanation for the HRTEM finding: the existence of oxygen soluted in the Pb inclusions at higher temperatures may lead to the formation of the disorder Al zone in the aged sample. The dependence of the thickness of the disorder Al zone on the amount of oxygen soluted in the Pb inclusion at higher temperatures, and the average stress in the equilibrium configuration of a MD cell, is discussed.