Table of contents

Molecular dynamics techniques have been used to simulate atom-atom collisional scattering of 0-5.5 eV/nucleon Ti atoms in bulk Ti. The recoiling atoms to be simulated were produced by the emission of MeV gamma -rays after the thermal neutron capture reaction ⁴⁸Ti(n, gamma )⁴⁹Ti. The information on the collisional scattering was deduced from Doppler broadened gamma -ray lineshapes, produced in the decay of excited bound states in recoiling ⁴⁹Ti. The measured gamma -ray lineshapes were simulated by combining the molecular dynamics calculation for the atomic collisions with calculations using the Monte Carlo method for the experimental conditions and decays of excited nuclear states. The dependence of the simulated gamma -ray lineshapes on the interatomic potential is demonstrated.

The perovskite crystal CsPbCl₃ is built up by a three-dimensional chain of PbCl₆ octahedra. The symmetry, the compatibility relation and the normal vibration modes of this crystal are studied by using the multiplier representation of group theory along the Gamma - Delta -X-Z-M-T-R direction in phase I and along the Gamma - Lambda -Z direction in phase II. The successive structural phase transitions (SPT) in CsPbCl₃ at T_c1=47 degrees C, T_c2=42 degrees C and T_c3=37 degrees C are induced by the condensation of the cubic zone boundary mode tau ₃ at the M point and the tetragonal zone boundary modes _{x9 chi} and _{y9 gamma} at the Z point separately. The interpretation of these successive displacive phase transitions with the normal vibration modes from the author's study is in good agreement with experimental investigation. A similar result from a previous investigation of layered perovskite RbAlF₄, which has a two-dimensional octahedral network, is also compared and discussed.

The authors investigate singularities of phonon spectra and anharmonic effects in metals due to the proximity of the Fermi level E_F to singular points E_c in the electron density of states N(E). As an example, they consider the case of the Van Hove singularity, when at small eta =E_F-E_c the singular part of N(E_F) varies as eta _+or-^1/2, where eta _+or-=+or- eta theta (+or- eta ) and theta (x) is zero at x<O and unity at x>O. The authors show that contributions to phonon frequencies omega _k that are non-analytic in eta are proportional to eta _+or-^3/2 at k>k_s and to eta _+or-^1/2 at k<k_s, where k_s approximately eta _+or-^1/2 is some characteristic value of the wavenumber k. Within the model with a weak pseudopotential, nu , an explicit expression is given for the singular part of the dynamic matrix for all not small k>k_s approximately ( upsilon eta _+or-)^1/2. In thermodynamics, low-frequency phonon dispersion at k approximately k_s results in anomalous temperature dependences at low T approximately T_s=T_D( upsilon eta _+or-)^1/2, where T_D is the Debye temperature. At T>T_s non-analytic contributions to the thermal expansion coefficients are proportional to eta _+or-^1/2, while in the temperature derivatives of the elastic constants and of the optic phonon frequencies with k=O they grow as eta _+or-^1/2. The results are compared with the available experimental data.

Band-structure and shear-constant (C_ss') calculations have been performed for the Li, Na and Ba BCC phase and the Cs, Ca and Sr BCC and FCC phases within a broad range of compressions (U) from 0 to 40-55%. To describe the 'band' contributions to C_ss', use is made of calculations based on the linear muffin-tin orbital/atomic sphere approximation (LMTO-ASA) method; the other contributions to C_ss' are described by using the semiempirical 'electrostatic' model. At U=O the calculated C_ss', values are usually close to the observed ones. The authors have revealed pronounced effects of softening for the shear constants, up to the loss of stability (C_ss'<O), when the Fermi level approaches the maximum points of the density of states n(E), as well as under certain changes of shape of n(E). The compression values, at which this softening takes place, are in good agreement with the position of structural phase transitions under pressure, observed in the metals considered. A number of anomalies in the C'(U) and C₄₄(U) dependences are predicted, in particular a sharp drop of C'(U) near the phase transition points U=U_c in BBC Li and Ba and FCC Cs, Cu and Sr, as well as a significant decrease of C₄₄(U) using U in FCC Cs and BBC Sr at U>or=0.5.

The hopping rate nu of heavy, positively charged particles (e.g. positive muons) in a type-I superlattice is calculated. The electron-electron and electron-charged-particle interactions are treated in the random-phase approximation. It is assumed that the charged particle is confined to the barrier regions between the quantum wells where there are no electrons, and hops between nearest-neighbour lattice sites R₁, R₂ only. It is shown that nu is an anisotropic function of R₁ and R₂. The contribution to nu due to particle-hole modes and plasmon excitations is obtained.

The Kapitza conductivity (thermal boundary conductivity (TBC)) between two crystals is studied within a simple three-dimensional lattice dynamical model and the role of interface roughness on an atomic scale is investigated. The interface roughness is modelled by a random alloy monolayer A_1-xB_x between the two semi-infinite crystals A and B and is treated by bulk alloy techniques adopted to the interface geometry. The dependence of the TBC on the acoustic mismatch between the two crystals and the degree of roughness is shown for a variety of parameter values. The authors results indicate in all cases an increase in the TBC with respect to the planar boundary; the increase becomes quite substantial (about 300%) for large acoustic mismatch values (about ¹/₅) and is due to diffuse phonon scattering. A comparison with two limiting models that assume only specular or completely diffuse scattering is made.

Recently, a new formalism for the temperature dependence of the anisotropy of the paramagnetic spin susceptibility of metals has been developed. The authors apply this theory to paramagnetic metals HCP scandium and yttrium. Calculated results show the paramagnetic spin susceptibility along the c axis, X_c, to be greater than that along the a axis, X_a, within the temperature range of 0-300 K for both metals. These results are consistent with experiment, when the Van Vleck orbital susceptibility is taken into account. The authors results also suggest that the magnetic moments of transition metal impurities in Sc and Y will align with the c axis.

mA^IVTe-nB₂^VTe₃ systems form well-defined compounds, characterized by a layer structure, where the Te planes are intercalated by the metallic ones. Both ordered and random distributions of A and B atoms in the cation sites have been invoked by the experimentalists. The different possible sequences of filled and unfilled sandwiches of Te planes might induce additional disorder in the unit cell when n>1. The authors' calculations for such a system, specifically for mSnTe-nBi₂Te₃, based on a tight-binding effective Hamiltonian, give a clear indication in favour of ordered structures. This would explain why these systems do not form solid solutions. The results for the sequence of Te and metallic planes suggest SnBi₂Te₄ to be the only stable compound in the system, the others quoted in the literature being just combinations of that unit cell with the Bi₂Te₃ cell.

A critical consideration of some approaches to the metal-insulator transition problem, starting from the many-electron representation, is carried out within the framework of the Hubbard and classical s-d models in the far-paramagnetic region. The analytical properties of the corresponding one-electron Green functions are discussed, the importance of terms of sufficiently high orders in l/z being demonstrated. The total energy, electronic specific heat and corrections to the local moment are calculated. The Hubbard-III approximation in the Hubbard model (but not in the s-d model) is shown to lead to difficulties when calculating thermodynamic properties.

For most theories of impurity resistivity the lowest-order results are the same for zero temperature, but for finite temperatures there are two different results. The fact that the Boltzmann results and the results from force-balance type theories disagree for finite temperatures has recently led to increased theoretical activity. In the present work the authors study how the newly proposed generalized Drude approach of Sernelius (1989) and the dynamical theory of Farvacque (1989) compare with other theories. They use the full temperature-dependent random-phase approximation screening in their numerical calculations, but also present results with the generalized temperature-dependent Thomas-Fermi screening. The authors furthermore give some analytical results where this is possible. They show that inclusion of electron-electron scattering in the solution to the Boltzmann equation brings the result closer to that from the force-balance type theories; very strong electron-electron scattering results in perfect agreement. Numerical results are presented for doped GaAs.

The electronic structures of small clusters of tungsten and rhodium on a W(110) surface have been calculated within a d-electron tight-binding approximation using the recursion method. The adatom orbital energies and the total energy calculated for three tungsten adatoms on the surface were least for a triangular cluster configuration. For three rhodium adatoms, a linear-chain arrangement gave a lower total adatom orbital energy than that given by a triangular geometry, although the system energy remained higher. The calculations demonstrate the importance of non-bonding electrons for cluster structure and partially account for stabilization of the linear-chain structure for rhodium adatom clusters on a tungsten W(110) surface seen by field ion microscopy.

This paper discusses the possibility of the transfer of electronic Aharonov-Bohm phase factors to light in double-quantum-well structures in magnetic fields. The open nature of semiconductor quantum wells permits a new type of coherent light scattering by means of certain interband transitions which preserve the positional indeterminacy of electronic states in Aharonov-Bohm superpositions. The modes of the coherently scattered light are determined by the magnetic flux per unit length between the wells. The calculation shows that the scattering mechanism can be quite efficient and may be used as an optical probe of ballistic electrons whose wavefunctions do not suffer a reduction while traversing the wells.

For weak applied magnetic fields the frequency spectrum of the voltage fluctuations was measured for Josephson ceramic samples of BaPb_0.75Bi_0.25O₃ with T_c approximately=10 K. The measurements were carried out in the resistive state. It is shown that the spectral density S_v(f) in the frequency region 20 Hz<f<500 Hz is described by a function f^{- alpha} , where alpha approximately=1 for small H and alpha approximately=2-2.5 for H>or approximately=10 Oe. For currents I>or approximately=0.8 mA through the sample, oscillations with a fundamental frequency f₁<or=100 Hz and two harmonics f₂ and f₃ were observed. For larger currents these oscillations die out and instead several new oscillations appear. Flicker noise with alpha approximately=1 is due to the existence of the superconducting glass state. The transition to a alpha >or approximately=2 and the emergence of the oscillations apparently occur owing to heating effects.

The ground-state energies and sublattice magnetizations of Heisenberg antiferromagnets with XY-like anisotropy of nearest-neighbour exchange interactions on a two-dimensional triangular lattice, as well as on a two-dimensional square lattice, are studied in the framework of spin-wave theory in the first-order perturbation approximation. To take account of the kinematic interactions the Dyson-Maleev transformation is incorporated with the method proposed by Kubo (1953). When they neglect the kinematic interactions, they obtain the ground-state energies for the S=1/2 models on the triangular lattice, namely -0.1872 for the isotropic exchange interactions and -0.1344 for the XY interactions. The kinematic interactions, however, increase the ground-state energies, and that for the XY interactions becomes -0.1293.

Measurements of the electrical resistivity, magnetization and susceptibility in an extended temperature range from 30 mK to 300 K on polycrystalline (Ce_xLa_1-x)Cu₄Ga samples (0.0<or=x<or=1.0) are presented. A crossover is observed from a spin frozen state with T_K<T_RKKY for x<or=0.85 to a heavy fermion state with T_K>T_RKKY for x approximately=1. The full formation of coherence at low temperatures seems to be prevented by lattice disorder probably caused by the arrangement of the Cu and Ga ions in the hexagonal unit cell.

The nickel-rich FCC Ni_cFe_1-c alloy system is modelled using a lattice with two Ising-like degrees of freedom at each site, one compositional and one magnetic. This system is investigated with both a mean field theory and computer simulation. Parameters of the model are obtained by fitting to the concentration dependence of Curie temperature observed. The simulations reproduce the observed compositional (LI₂) ordering in the region of interest and an associated magnetic anomaly. The authors confirm the conjecture that the Fe-Fe exchange coupling in this alloy is antiferromagnetic and provide an estimate of its strength.

The authors present investigations of the structure of liquid 3d and 4d transition metals using molecular dynamics simulations, thermodynamic perturbation theories, and integral-equation techniques. The effective pair interactions are calculated within a hybridized nearly-free-electron-tight-binding theory. The parameters determining the d electron contribution to the potential are deduced from the known electronic properties of the crystalline metals, the core radius of the pseudopotential specifying the s-electron contribution is determined by a molecular-dynamics fit of the liquid structure. The resulting pair interactions are used to test the applicability of liquid state perturbation techniques and integral equations to liquid transition metals.

The potassium-indium system has been studied using galvanic cells with potassium beta alumina as the electrolyte. The thermodynamic properties of the liquid solution have been determined between 0 and 52 at.% potassium. There is some evidence for a very weak liquid compound near 35 at.% K, but not at 50 at.% K. Changes are proposed for the potassium-indium phase diagram, including the liquidus temperature and the existence of new compounds.

The HMSA integral equation has been solved for the binary mixture He-H₂ over a range of densities and compositions at 100 K and 300 K. Comparison of the radial distribution functions and the pressures shows that these results are in good agreement with those of two-component MC simulations close to the binodal curve or melting line. The HMSA equation of state rather than the usual van de Waals one fluid approximation was used to calculate the Gibbs free enthalpy; the latter was used to determine the fluid-fluid phase separation in the mixture. The spinodal curve calculated from the composition fluctuation structure factor S_CC(k), is not consistent with the binodal curve, probably due to inconsistencies in this structure factor. The direct correlation functions that result from the integral equations were used to calculate the freezing of the mixture with the Haymet and Oxtoby version of density functional theory of freezing, where the grand potential of the solid is expanded up to second order. To force quantitative agreement with experimental results, the fourth star was omitted from the HCP reciprocal lattice. An important result is the solubility of helium in the hydrogen-rich solid of a few mole percent. The authors also find the interesting phenomenon of density inversion between the hydrogen-rich solid and the hydrogen-rich fluid.

The relationship between Wigner crystallization of the classical ionic plasma and the liquid-solid transition of alkali metals is examined within the density wave theory of freezing. Freezing of the classical plasma on a rigid neutralizing background into the BCC structure is first re-evaluated, in view of recent progress in the determination of its thermodynamic functions by simulation. Freezing into the FCC structure is also considered in this context and found to be not favoured. On allowing for long-wavelength deformability of the background, the ensuing appearance of a volume change on freezing into the BCC structure is accompanied by reduced stability of the fluid phase and by an increase in the entropy of melting. Freezing all alkali metals into the BCC structure is next evaluated, taking their ionic pair structure as that of an ionic plasma reference fluid screened by conduction electrons and requiring that the correct ionic coupling strength at liquid-solid coexistence should be approximately reproduced. The ensuing values of the volume and entropy changes across the phase transition are in reasonable agreement with experiment. The order parameters of the phase transition, excepting the (200) one, conform rather closely to Gaussian behaviour and yield a Lindemann ratio in reasonable agreement with the empirical value for melting of BCC crystals.

The translational motion of atoms in a liquid confined between two plane parallel repulsive walls is studied by computer simulation. It is shown that the motion perpendicular to the walls cannot be described in a strict sense by the conventional diffusion equation even if the diffusion constant is generalized to a space dependent diffusion tensor. Instead, for the anisotropic case, a system of linear coupled rate equations is proposed, whose time-independent rate transition matrix is shown to be necessarily spatially asymmetric, with the equilibrium mean static density as its zero eigenvalue eigenvector. This accurately describes the transverse atomic motion for times considerably larger than the velocity autocorrelation time without the need to empirically input the mean static density. This theory is tested by computer simulation and found within statistical error to be a valid quasi-microscopic description of the (slower) stochastic atomic motion. The evolution of the anisotropic self-diffusion propagators towards the quasi-periodic mean density profile is studied in detail as a function of the initial starting position.

The author investigates the disclination unbinding transition from the theoretically predicted hexatic N+6 phase to the nematic phase in discotic liquid crystals. The free energy due to an equilibrium continuous density of disclinations in the hexatic N+6 phase is derived. Two kinds of disclinations are considered, longitudinal wedge and twist disclinations, which decorrelate the sixfold orientational order, but do not break the cylindrical nematic order around the director. Two possible mechanisms for the transition are found. The longitudinal wedge free energy drives a transition of the Kosterlitz type, which is the three-dimensional equivalent of the disclination unbinding transition in two-dimensional melting theory of Halperin and Nelson (1979). Twist disclinations provide a different mechanism for the transition, which is peculiar to the hexatic phase as a quasi-two-dimensional system. The twist disclination free energy is not positive definite for some ranges of values of the Frank elastic constants. As a consequence, for strong coupling between the director distortions and the torsions around the axis of sixfold symmetry, a disclination unbinding instability, essentially due to repulsion between disclinations of opposite signs, develops in the system. In order to decide which of these two mechanisms is effective, the author should know the physical values of the Frank constants in the hexatic phase.

The phase transformations and structure characteristics of the Al₆₅Cu_17.5Co_17.5 alloy were studied by neutron diffraction. The scheme of Yamamoto and Ishihara (1988) can satisfactorily index the diffraction patterns. The Al-Cu-Co decagonal phase is stable in the temperature range between 973 and 1350 K. At the low temperature end it relaxes to a microcrystalline approximant structure. At the high temperature end, it melts directly into liquid. The scheme of Yamamoto and Ishihara can perfectly index the neutron powder diffraction patterns (a=7.212 AA, c=4.184 AA at 973 K). The authors propose that it is the average quasiperiodic sublattice that determines the powder diffraction characteristics of the microcrystalline structure at room temperature which can also be indexed in decagonal phase notation.

The authors calculate the Hall conductivity in the low frequency limit for a 2D model of independent electrons in the tails of the density of states between the disorder broadened Landau levels. It is shown that there are corrections to the quantized values of sigma _yx, which are proportional to omega ² and to the number of localized states in a given tail of the band. The authors relate their theoretical results to microwave experiments on GaAs-AlGaAs heterostructures.

It is expected that the specific heat of the SU(2)-invariant 1D Heisenberg antiferromagnet of spin S is linear to the temperature at low T with the linear coefficient gamma _s being a function of field H. The author's main result is that lim_{H to O gamma s}=¹/₃(1+ square root S Gamma (S)(e/S)^s/ pi ) not= lim_T to _O lim_{H to O gamma s}=2S/(1+S). This extends the previous result for S=¹/₂ to other spin values. The author also provides an approximate interpolation formula between these two limits as a function of H/T for very small H and T.

The authors report X-ray and UV photoemission measurements of the valence bands in equi-atomic CuAu in both the ordered (L1_o structure) and disordered (FCC) phases. They find that there are large differences in the spectra, particularly in the region of the Au d states. The measurements also suggest that the valence bandwidth in the ordered phase is greater than that in the disordered phase.