Table of contents

Complex dielectric susceptibility (χ' + jχ'') has been studied at f = 10^-2-10⁵ Hz in the relaxor ferroelectric 0.75Pb(Mg_1/3Nb_2/3)O₃-0.25PbTiO₃. In addition to the well known relaxor dispersion observed at temperatures around and below the temperature of permittivity maximum T_m, a new and comparatively weak dispersion has been disclosed at T>T_m. It is described by the `universal relaxation law', χ'' = cot(np/2)χ'∝f^n-1. The temperature dependence of the exponent n follows the Vogel-Fulcher relation, indicating a process of freezing into the nonergodic state.

A structural disorder and melting transition of the S = 5 bicrystalline interface of B2 NiAl with a large boundary plane is investigated by molecular-dynamics simulations. The calculations have been performed at various temperatures using an embedded-atom-method potential fitted to NiAl. It is observed that the atoms in the grain-boundary region tend to form clusters in a thermal structural disorder transition, which is initiated at a temperature well below the thermodynamic melting point T_m (around 0.52T_m). The number and size of the clusters are monitored over a wide temperature range including T_m. Below T_m, the number and size of the clusters increase continuously with increasing temperature. At temperatures up to T_m, the abrupt increase in size of the clusters induces melting. At temperatures above T_m, the number and size of the clusters decrease significantly upon raising temperature. The calculations of the potential energy also indicate that the thermal disorder transition is a continuous process, in contrast to the first-order melting transformation.

Recent experiments have generated a renewed interest in the properties of the degenerate valence states in semiconductors under the influence of a uniform external electric field. In response, a number of authors have proposed that the standard Luttinger-Kohn effective-mass Hamiltonian should be modified to include the energy of interaction between the electric field and the dipole matrix elements of the relevant zone-centre Bloch functions. This article examines these proposals by comparing the proposed dipole interaction with rigorous derivations of the field-dependent Hamiltonian in the Bloch and Luttinger-Kohn representations. It is shown that the dipole matrix element of a unit cell is not a suitable foundation for a Hamiltonian because it depends on the choice of unit cell, which is arbitrary. Moreover, the correct Luttinger-Kohn Hamiltonian has no wave-vector-independent matrix elements that are linear in the applied field , except for small terms of relativistic origin. Therefore, the proposed modifications to the Luttinger-Kohn theory are not valid. The correct form of the Luttinger-Kohn Hamiltonian is derived here through terms of order k³, k, and ², along with the momentum matrix to first order in k and . In addition, the recent discovery of field-induced mixing at the Brillouin zone boundary in the Luttinger-Kohn representation is studied in detail.

Phonon softening in the Heusler alloy Ni₂MnGa crystal was investigated by neutron spectroscopy for a crystal where the martensitic phase transformation temperature is close to the Curie temperature. The main result is a softening of the TA₂-phonon branch (ξ ξ 0) of the cubic phase in a broad range between ξ = 0.1 and 0.4 when the temperature approaches the phase transformation. The behaviour of the present sample is compared with results for samples with lower transition temperature.

Zirconia aerogels are made of connected fractal clusters composed of small crystalline particles. When heated at a low temperature (i.e. 0.13 times the melting temperature), the mass transport process which predominantly occurs is surface diffusion. Due to the local character of surface diffusion, fragmentation appears in the sample during thermal annealing. This particular evolution of the structure is numerically analysed and experimentally studied by small-angle x-ray scattering.

In this work we have presented a spectroscopic study carried out to determine the room temperature crystal symmetry of LiCs_0.5Rb_0.5SO₄ crystals. On the basis of x-ray and Raman scattering measurements, we have proposed that LiCs_0.5Rb_0.5SO₄ crystallizes in a monoclinic structure belonging to the C_s point group with four molecules per unit cell, where the SO₄ groups, Rb and Cs ions, are distributed into two non-equivalent C₁(2) site symmetries.

There is currently great interest in the dynamics of electrons and phonons in low-dimensional systems, where the effects of quantum confinement cause a dramatic difference in their behaviour as compared with bulk systems. In this paper we consider a localized electronic impurity state (an electronic two-level system) linearly coupled to the vibrational modes of an isolated nanometre-scale insulating crystal, and study the phonon emission rate at frequencies less than that of the lowest internal vibrational mode, i.e., in the acoustic `gap'. We show that, at finite temperature, electronic energy relaxation below the acoustic gap can occur as a result of anharmonic broadening of vibrational modes, and we calculate the frequency and temperature dependence of the relaxation provided by this mechanism.

We study the properties of a two-dimensional interacting electronic system in the presence of a staggered magnetic field. We find that metal-insulator transitions can be achieved at both zero and non-zero temperatures. A d-wave-like pseudogap is opened on the two-dimensional square Fermi surface. The staggered magnetic flux is found to suppress the antiferromagnetic order. When random fluctuations of the perfect staggered field are turned on and increased, a finite region of Fermi surface around (±π/2,±π/2) will be gradually formed as the bandwidth decreases. We also find that the assumed disturbed staggered field in our model can actually simulate the spin fluctuations occurring in the interacting system.

Single crystals of RNi₂B₂C (R = Ho, Dy, Tb, Pr) have been grown on cold copper crucibles in a high-frequency induction furnace. As a result, shiny metallic and brittle platelike single crystals were obtained. They were examined by x-ray and scanning electron microscopy with WDX/EDX for local composition analysis and show a very good crystallographic structure and compositions. Resistivity and dc magnetic measurements were performed to study superconducting and magnetic properties. Besides known electronic properties of the RNi₂B₂C family, we report for the first time results for PrNi₂B₂C single crystals successfully obtained by this technique.

We have attributed the magnetism of PrRu₂Si₂ to the Pr ions and performed calculations of the fine electronic structure of the Pr³⁺ ion in tetragonal symmetry taking into account the crystal-field and inter-site, spin-dependent exchange interactions. The energy level scheme derived is associated with the removal of the degeneracy of the lowest multiplet given by Hund's rules, ³H₄. Magnetic and electronic properties resulting from this fine structure are compared with all known experimental results. Our calculations reproduce the zero-temperature moment, the temperature dependence of the magnetic susceptibility, the single-crystalline magnetization curves with the anisotropy field of 400 T, the specific heat with the sharp peak at T_C as well as inelastic neutron scattering data.

Using a recently developed Green's function formalism, we have calculated the spin-wave spectra and dispersions in Ni and Fe. For Ni(100), the dispersion exhibits two branches as observed experimentally. The calculated higher optical branch is found to be too high in energy when the standard local density approximation band-structure is used but a very good agreement with the measured dispersion is obtained when the exchange splitting is reduced, to correspond to the experimental value of the exchange splitting. We also found a double branch along Ni(111) which is not observed experimentally. For Fe, the calculated dispersion surprisingly exposes a gap midway along Γ-N in disagreement with experimental data. However, an analysis of the temperature-dependent magnetization has predicted a similar gap at the same wave vector, supporting the present calculations.

We investigate the elementary excitations of quasi-one-dimensional S = ½ systems built up from zigzag chains with general isotropic exchange constants, using exact (Lanczos) diagonalization for 24 spins and series expansions starting from the decoupled dimer limit. For the ideal one-dimensional zigzag chain we discuss the systematic variation of the basic (magnon) triplet excitation with general exchange parameters and in particular the presence of practically flat dispersions in certain regions of phase space. We extend the dimer expansion in order to include the effects of three-dimensional interactions on the spectra of weakly interacting zigzag chains. In an application to KCuCl₃ we show that this approach allows us to determine the exchange interactions between individual pairs of spins from the spectra as determined in recent neutron scattering experiments.

We study the electron energy-loss spectra of strongly correlated electronic systems doped away from half-filling using dynamical mean-field theory (d = ∞). The formalism can be used to study the loss spectra in the optical (q = 0) limit, where it is simply related to the optical response, and hence can be computed in an approximation-free way in d = ∞ dimensions. We apply the general formalism to the one-band Hubbard model away from n = 1, with inclusion of site-diagonal randomness to simulate effects of doping. The interplay between the coherence-induced plasmon feature and the incoherence-induced high-energy continuum is explained in terms of the evolution in the local spectral density upon hole doping. Inclusion of static disorder is shown to result in qualitative changes in the low-energy features, in particular to the overdamping of the plasmon feature, resulting in a completely incoherent response. The calculated lineshapes of electron energy-loss spectra are compared to the lineshapes of experimentally observed spectra for the normal state of the high-T_c materials near optimal doping and good qualitative agreement is found.