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The metastable states of a vortex lattice in a layered superconductor in the presence of a magnetic field parallel to the layers are shown to form structures that are modulated or commensurate to the layers. A number of commensurate structures with equivalent energies are found. The incommensurate structures can rotate abruptly when the magnetic field is varied. The structure factor and the London free energy of the incommensurate structures are calculated.

A simple model is used to calculate the tracer diffusion in a Lennard-Jones tracer-solvent system. The influence of the size ratio between tracer and solvent, and of their mass ratio on the tracer diffusivity is investigated. The adopted model, proposed by Tankeshwar et al. (1991) is based on the idea, put forward by Zwanzig (1983), of separating the configurational space of a fluid system into a vibrational part and a structural part. The results offer an interpretation of the molecular dynamics data showing a levelling of the self-diffusion coefficients for a sufficiently small size of the tracer. The levelling off is explicitly related to the length parameter of tracer-solvent interaction. The calculated values of the tracer diffusion coefficients for a large range of its size and mass values are quite satisfactory as judged by comparison with simulation results.

The authors re-investigate G.V. Chester's model (1955) for liquid ⁴He which combines Jastrow factor correlations with those of the ideal Bose gas. For the first time, they evaluate the structure factor S(k;T) in this model. This evaluation is based on the optimized random phase approximation frequently used in simple liquid theories. The calculated S(k;T) reproduces qualitatively the k- and T-dependence of the experimental structure factor. The temperature anomaly of liquid helium below the lambda -point can be explained by the Bose gas correlations.

The internal friction (IF) has been measured as a function of the H (D) content in the disordered and ordered Pd₃Mn system between 90 and 400 at frequencies in the range 180-5300 Hz. Relaxation processes due to stress-assisted motion of H (D) have been identified and their spectra of relaxation times, heights and temperatures determined as a function of H content and the state of long-range order. The main observations in the case of the alloy in the disordered state are an increase with n (n=H (or D)/(Pd+Mn) at.) of the height and width of the peak and a decrease of the peak temperatures. These effects have been attributed to progressive filling by H of higher energy I₅ sites (sites having five nearest-neighbour Pd atoms), which differ in terms of atomic arrangements in the second and more distant shells of neighbours. The partial molar enthalpy Delta H_H for H solution has also been determined both in the disordered and ordered states of the Pd₃Mn alloy. In the disordered state Delta H_H is found to increase rapidly with n starting from values of n as low as 0.01. The concentration dependence of Delta H_H as well as of the height of the IF peak reveal strong hydrogen-hydrogen repulsions.

The soliton dilute-gas phenomenology is used to study the statistical mechanics of the Phi ⁴ system with long-range interaction potential. Both the continuum and the discrete phonon excitations with their corresponding dispersion relations and non-linear excitations (walls or kinks) are investigated. In the continuum model, the authors show that the kink free energy and density decrease when the range of interaction increases. In the discrete model where the kink width is small, as a result of the collective coordinate method associated with Dirac's constrained Hamiltonian dynamics, the mass and the potential energy of kink vary periodically with the kink position in the lattice. This leads to a correction of the statistical results obtained in the continuum model.

The dipolar contribution to the spin-lattice relaxation rate Gamma ₁ of spin-carrying light interstitials is calculated assuming nearest-neighbour hopping among tetrahedral sites of a BCC lattice. As a new element, which was missing in previous treatments, the authors took into account explicitly the existence of localized excited states of the interstitials. The model considers two states per site, i.e. the ground state and one excited state, and takes into account transitions between the ground states, the excited states, and between an excited level and a ground state of two neighbouring interstitial sites as well as intrasite transitions between ground and excited states of the same site. If the intrasite transitions from the ground to the excited state are not fast compared with the tunnelling rate among the ground states of neighbouring tetrahedral sites, which is likely to be the case for hydrogen in BCC metals like Nb and Ta, the extended model predicts deviations from the results obtained for the usually considered model which describes the interstitial motion by a single effective hopping frequency among tetrahedral sites. The product alpha = Gamma ₁D, in which D is the diffusivity of the interstitial, may be considerably larger for the extended model than for the single-hopping-frequency model. Moreover the various hopping frequencies do not drop out from alpha as does the effective hopping frequency in the latter case. This may lead to a strong increase of alpha with temperature in contrast to the temperature independence of alpha for the single-frequency model. The theoretical results are compared qualitatively with NMR experiments performed on the alpha -phases of NbH_x and TaH_x.

Electron energy-loss near-edge structures at the oxygen K edges of a number of titanium(IV)-oxygen compounds have been measured with an energy resolution of 0.5 eV. A detailed interpretation of the observed structures in terms of the differing nearest-neighbour coordinations of oxygen is obtained by comparing the experimental spectra with the results of multiple-scattering calculations and some theoretical densities of states derived from the literature.

Uniaxial stress perturbations establish the 1.257 eV 'H2' zero-phonon line as an electric-dipole allowed transition at the centre of a C_2v point group. Taken in conjunction with a simple one-electron model, the magnitudes of the perturbations support recent suggestions that the H2 centre is a negative charge state of the well-known nitrogen-vacancy-nitrogen 'H3' centre. The H2 zero-phonon line is 0.9 meV higher in energy in ¹³C diamond than in ¹²C diamond. The shift is consistent, within large uncertainties, with the shift expected from the temperature dependence of the line. A local vibrational mode involving predominantly carbon atoms is identified.

The experimental Stark energy levels for the Nd³⁺ and Er³⁺ in LiRP₄O₁₂ crystals were presented in the authors previous paper. A crystal field analysis of these data is based on a Hamiltonian of C₂ point group symmetry, including J-mixing effects. They find that the twinned C₂C/ (C₂) structure is consistent with all available experimental results. A Hamiltonian parametrized in monoclinic symmetry was used to describe the observed crystal-field splittings of Nd³⁺ and Er³⁺ in LiRP₄O₁₂. Resulting RMS deviations between calculated and experimental levels range from 6.8 to 7.4 cm^-1.

Neutron spectroscopy has been applied to study the electronic ground state of NpO₂ both in the paramagnetic and in the ordered phase, below T_c=25 K. The magnetic inelastic scattering cross section shows a broad peak which is split into two components and is centred at about 55 meV. This inelastic peak is shown to originate from excitations between the Gamma ₈⁽²⁾ and Gamma ₈⁽¹⁾ crystal field quartets. Its position is in agreement with the value estimated by scaling the crystal field potential of UO₂ to the Np⁴⁺ case. Several mechanisms which could be responsible for the observed splitting are discussed. In the energy range below 15 meV the technique of polarization analysis has been used together with an incident polarized neutron beam to unambiguously separate magnetic from vibronic effects. At low temperature, below 25 K the authors see the appearance of an inelastic line (at an energy transfer of 6.4 meV) that indicates a lifting of the degeneracy of the Gamma ₈ ground state. This supports the hypothesis that the phase transition involves the quadrupolar ordering of the Np⁴⁺ ions by a collective Jahn-Teller distortion of the oxygen sublattice. The observed amplitude of the splitting is consistent with an oxygen displacement of the order of 0.02 AA, which is below the present limits of resolution of neutron diffraction experiments.

The conductivity and the Hall effect of three-dimensional Mg_xZn_100-x glasses are studied as a function of magnetic field (0<or=B<or=6 Tesla), temperature (1.5<T100 K) and concentration (x=67, 72 and 77). The magnetoconductivity Delta sigma (T,B) provides evidence for quantum corrections to the Boltzmann equation. It is in quantitative agreement with perturbation theories, when contributions due to quantum interference, electron-electron interaction and superconducting fluctuations are taken into account. The spin-orbit scattering field increases with the Zn concentration from 0.086 Tesla (Mg₇₇Zn₂₃) to 0.13 Tesla (Mg₆₇Zn₃₃). The temperature and concentration dependence of the Hall coefficient are also consistent with the nearly-free-electron model when quantum corrections are included.

Electron tunnelling in a transverse magnetic field is studied taking into account the electron-phonon interaction. When scattered on a phonon, the electron shifts the oscillator centre in the tunnelling direction, which decreases the field-related magnetic barrier and increases the probability of tunnelling considerably. For tunnelling over large distances, the most effective processes involve multiple scattering by phonons. An expression has been found for the resistivity of the Miller-Abrahams network element for hopping conductivity with regard to magnetic-barrier suppression upon scattering by phonons. The exponential temperature and voltage dependences of the tunnel junction conductance in a magnetic field parallel to a dielectric interlayer have been obtained. The contribution associated with the tunnelling processes involving scattering by two phonons to the relaxation of edge-state populations in the quantum Hall effect regime has been found.

The metal-insulator or metal-amorphous semiconductor blocking contact is still not well understood. Here, the author discusses the steady state characteristics of a non-intimate metal-insulator Schottky barrier. He considers an exponential distribution (in energy) of impurity states in addition to impurity states at a single energy level within the depletion region. He presents analytical expressions for the electrical potential, field, thickness of depletion region, capacitance, and charge accumulated in the depletion region. He also discusses ln I versus V_ap data. Finally, he compares the characteristics in three cases: (i) impurity states at only a single energy level; (ii) uniform energy distribution of impurity states; and (iii) exponential energy distribution of impurity states. In general, the electrical characteristics of Schottky barriers and metal-insulator-metal structures with Schottky barriers depend strongly on the energy distribution of impurity states.

The low-energy Hamiltonian for localized Cu spins and mobile O holes is rederived from the minimal version of the Emery model ascribed to a CuO₂ layer in high-T_c superconductors. By means of a computational algorithm the Schrieffer-Wolff transformation is performed, at large but finite Hubbard repulsion U on Cu sites, up to fourth order in the d-p (Cu-O) hopping amplitude t. The resultant interactions in O(t⁴) involve spin-spin superexchange, a Kondo-like spin-hole exchange opposite in sign to that in O(t²), and a Hubbard-type attraction between symmetrized O holes within a CuO₄ group. An effective t_pp-J_dd-J_dp-J_pp model is established for the special case U=2 epsilon , where epsilon is the charge transfer energy.

The method of construction of the wavefunction of a Cooper pair based on the Pauli exclusion principle and the Mackey-Bradley theorem is developed. Tables of symmetrized and antisymmetrized Kronecker squares of single- and double-valued irreducible representations of the groups O_h⁶ and D_6h⁴ are obtained. The tables are used to search for points in the Brillouin zone where totally symmetric Cooper pairs can exist. It is shown that, in the symmetrical points and directions in a Brillouin zone, a direct connection between the multiplicity and parity of the Cooper pair wavefunction does not exist.

With the use of a tight-binding model the authors have studied the effects due to crystallinity, i.e. the atomic structure, of the Cu-O₂ planes and Cu-O chain planes of the high-T_c superconductors on the intralayer and interlayer Coulomb interactions as well as the density-density response function. This effect is found to be appreciable since each electron is well localized around the copper site and the size of the unit cell is comparable with the lattice constant. Localization leads to poor conductivity in the normal state and is determined by the hopping between adjacent copper sites caused by the overlap of electron wavefunctions with electrons on the oxygen site. The dominant local exchange-correlation contribution is included in the local-field approximation. By taking account of the plasmon-optical phonon coupling in their calculations, the authors have demonstrated the isotope effect for acoustical plasmon-mediated pairing as well as the effect on the effective intralayer Coulomb interaction which is believed to be important for the formation of Cooper pairs in the Cu-O₂ planes. The effects due to the position of the chain planes and the number of planes in each unit cell on the plasmon excitation energies as well as the effective intralayer Coulomb interaction are also studied. The relationship between these effects and the critical temperature T_c is discussed.

The low-temperature ⁵⁷Fe Mossbauer spectra of superconducting samples of Fe-substituted YBa₂Cu₃O_{7- delta} containing less than approximately 5 at.% Fe are characterized by a broad magnetically split profile. Although there is consensus in the literature as to the shape of this spectrum, there is no consensus of interpretation. Comparison of the fits obtained for two samples with 1% Fe substitution using (i) a hyperfine-field distribution model and (ii) a dynamic relaxation model demonstrates that the distribution model is marginally better, but that the relaxation model cannot be reliably discounted on the basis of the low-temperature Mossbauer data alone.

The Sr-doped neodymium nickelates have similar (K₂NiF₄-type) structures both to the high-T_c superconductor La_1.85Sr_0.15CuO₄ and to La_2-xSr_xNiO₄ materials, in which anomalous diamagnetic signals have been observed. The authors have made AC-susceptibility, oxygen content, XRD and resistance measurements on Nd_2-xSr_xNiO_{4+ delta} samples for a wide range of Sr doping levels (0<or=x<or=1.30). Metal semiconductor transitions occur below room temperature in samples with a high Sr concentration (x>or=0.90) while more subtle changes in the nature of the electrical conductivity occur in the low-temperature resistance of samples with lower doping levels. These changes in resistance with doping and temperature are closely associated with changes in structure and with the strength of the Ni ion magnetic interactions. Similar changes in resistivity occur in the La_2-xSr_xCuO₄ system, although over a much narrower range of Sr doping levels than in the nickelates.

Information from recent X-ray and neutron diffuse scattering investigations of Fe-Al alloys and the resulting effective interaction parameters are used to construct a model in which the lattice configurational degrees of freedom of these alloys are described by Ising spins, and the magnetic moments of iron atoms are described by classical Heisenberg spins. Starting from the exact ground state of the model, information on the phase diagram at non-zero temperature is obtained from both spin wave theory, mean-field approximations and extensive Monte Carlo simulations. It is shown that use of the unrenormalized interaction parameters from the various experiments yields a phase diagram which disagrees severely with experiment even at the qualitative level. However, with a suitable renormalization of the ratio between magnetic and non-magnetic interactions, a qualitatively reasonable phase diagram of the model is obtained. The consequences for the authors' understanding of effective interaction parameters in Fe-Al alloys are spelt out, and the implications for experimental work are discussed.

Starting from the known Lieb and Wu solution of the one-dimension-Hubbard model in the U to infinity limit, the authors show how the spin-charge decoupling of the elementary excitations is responsible for several peculiar features in one-particle properties, such as momentum distribution, quasiparticle weight and the Green function. In particular they analyse in detail the structure of the one-hole Green function at half-filling, which has not been previously calculated by field theory methods due to the breakdown of conformal invariance. A rich structure is found with branch cut singularities at omega =+or-2 sin k but no simple poles. The non-trivial dependence on the momentum of the hole allows for hole propagation although the analytic structure of G(k, omega ) is quite different from that usually characterizing band insulators. These results provide a precise characterization of one-dimensional Mott insulators. The relationship between the branch cuts of the Green function and the finite-size scaling of the quasiparticle weight is also discussed together with its implications for the analysis of numerical data.

The authors discuss how to treat the magnetic properties of the ordered phase of the weak itinerant electron ferromagnets using a method based on solving a differential equation for the transverse component of the magnetic susceptibility. They show that the simple analytic continuation of the paramagnetic solution does not give the stable solution in the ordered phase below T_c. The low-temperature solutions have an ambiguity corresponding to various choices of initial conditions. The authors discuss how to choose the unique stable solution at a finite temperature below T_c.

The authors determine the ground state phase diagram of the Hubbard model on the square lattice allowing for homogeneous spiral, antiferromagnetic, ferromagnetic and paramagnetic phases. This is obtained from a saddle-point approximation of a spin-rotation-invariant form of the slave boson representation introduced by Kotliar and Ruckenstein. They obtain very good agreement in energy with exact diagonalization calculations and, concerning the magnetic structure, qualitative agreement with experimental data.

Optical Zeeman studies are reported for the ordered region of the singlet ground state dimer system Cs₃Cr₂Br₉. The results are compared with earlier neutron scattering and AC susceptibility studies, and a model is suggested which not only describes the optical data, but also produces an excellent fit to the phase diagram as defined by the susceptibility results.

A transverse Ising film, for s=1/2, with a body-centred cubic structure and two (001) surfaces is studied by means of the effective-field theory with correlations. The layer magnetizations and average magnetization of the film as functions of temperature and transverse field are calculated numerically and some interesting results are obtained.

Systematics of X-band (about 9.50 GHz) EPR measurements in the temperature range 4.2-295 K on single crystals of Gd³⁺-doped Dy(BrO₃)₃.9H₂O and Eu(BrO₃)₃.9H₂O reveal that the Dy(BrO₃)₃.9H₂O single crystal undergoes two structural phase transitions at 67+or-0.5 and 18+or-0.5 K (the phase transition detected here at 18 K being the first ever reported), while the Eu(BrO₃)₃.9H₂O single crystal undergoes only one structural phase transition at 63+or-0.5 K The Gd³⁺ EPR spin Hamiltonian parameters at 295 and 77 K have been estimated for the two host crystals. The present Gd³⁺ EPR results on the heavier R(BrO₃)₃.9H₂O (R=Dy, Eu) crystals are found to be significantly different from those on the lighter R(BrO₃)₃.9H₂O (R=Pr, Nd, Sm) single crystals; a comparison of the features of Gd³⁺ EPR data in the lighter and heavier compounds has been provided.

An investigation was carried out on both the linear and nonlinear parts of the complex dielectric susceptibility in the frequency and temperature ranges 10^-3-10³ Hz and 100-450 K, respectively. The analysis of the imaginary part of the dielectric susceptibility has revealed the existence of a broad spectrum of relaxation times in the above temperature range. The temperature dependence of the most probable frequency of the spectrum was obtained. For temperatures T