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The thermal conductivity (k_T) of the charge density wave (CDW) compounds gamma -Mo₄O₁₁, eta -Mo₄O₁₁ and KMo₆O₁₇ was measured in the range 20-300 K. At the CDW transition there is an anomalous decrease of k_T in all compounds as T decreases. In gamma -Mo₄O₁₁ the anomaly can be ascribed to the decrease of the electronic contribution due to the decrease of the carrier concentration at the transition, the lattice remaining smooth, but in eta -Mo₄O₁₁ there is evidence for an additional anomalous increase of the lattice contribution below the CDW transition. In KMo₆O₁₇ there is a very large anomaly which is ascribed almost entirely to the lattice.

The dynamical scaling behaviour of the displacement-displacement correlation function of the one-dimensional Phi ⁴ lattice model is analysed by use of molecular dynamics simulations. The existence of a crossover temperature where relaxation times drastically increase suggests possible applicability of mode coupling theory (MCT) describing freezing processes in undercooled liquids. The authors find two independent dynamical scaling regions (two master functions describing the time evolution of the correlator) in accordance with MCT. A quantitative comparison of the MCT predicted scaling laws with the calculated ones for the Phi ⁴ model shows significant non-applicability of MCT especially for the beta relaxation process.

Light scattering measurements have been carried out in molten CsCl over the temperature range 945-1245 K. Assuming the molten salt system to be a quasi-harmonic oscillator, the authors have found that the integrated intensity of the spectra increases with temperature at rates that exceed those anticipated for first-order scattering. The spectral variation of the intensity shows that the excess scattering, due mainly to second-order summation or difference bands, occurs in the low-frequency region (up to 80 cm^-1) and the high-frequency tail (above 150 cm^-1). The average depolarization ratio rho _d of the spectra remains constant with temperature, but the depolarization ratio spectrum shows that (a) rho _d( nu ) increases with T up to approximately=60/sup /cm^-1 and (b) decreases with T above 60 cm^-1. This is explained as due to a transition of the ions that contribute most strongly to the scattering from an anisotropic environment, determined by binary collisions to a more isotropic environment determined by the average local structure.

Molecular dynamics simulations are reported of the melting of small inclusions of rare gases in cavities 10-20 AA in radius in metals containing a few hundred gas atoms. Such systems enable the authors to study the melting phenomena of clusters of atoms constrained by the cavity walls. The authors have looked at the positions of the atoms, the phase diagrams, and the importance of fluctuations for bubbles of different sizes. Melting in these systems takes place over a finite temperature range as in simulations of isolated clusters. The geometric structure factors have been calculated and compared with recent X-ray diffraction measurements of Andersen et al. on krypton inclusions in aluminium. Their inability to reproduce the experimentally observed increase in the transverse diffraction peak width with temperature supports the suggestion of Andersen et al. that this increase is due to a roughening transition of the aluminium facets of the cavities.

Recently developed non-perturbative density-functional theories of freezing are applied to the classical one-component plasma (OCP). The author considers the modified weighted-density approximation (MWDA) of Denton and Ashcroft and the generalized effective-liquid approximation (GELA) of Lutsko and Baus. It is shown that both the MWDA and the GELA completely fail to predict freezing of the classical OCP in contrast to their reasonable success for the quantum freezing of the electron jellium: the MWDA does not improve over the second-order theory (SOT) and the GELA is much worse. A semi-empirical analysis is made of how and to what extent the GELA must be modified for the classical OCP: in this analysis the SOT is used for the variational liquid-like solids and the Monte Carlo results are used for the stable (or metastable) BCC solids. It is found that the two effective liquids, which are assumed to be equivalent in the GELA, must differ by about 10 % in density from each other at the freezing point.

Recently, experiments on expanded fluid mercury, and simulations of alkali metals in liquid ammonia and in molten alkali halides, have motivated interest in the possibility of a Frenkel excitonic insulator (EI) phase. In this paper, the authors discuss several aspects of a model for describing such a phase. They choose a model Hamiltonian which is the asymptotically exact low-density form for a system of atoms each possessing an sp³ basis, and present two methods of analysis. The pairing theory centres on the broadening of the S to P atomic transition into exciton bands, which, as is known, may lead to the formation of a Frenkel EI phase. They analyse two corrections to the traditional exciton picture: (i) double-excitation processes, which are responsible for the van der Waals stabilization energy, are shown to halve the density predicted for the transition to an EI phase; (ii) correct incorporation of non-boson statistics for the exciton operators is argued to drive the transition from first order to second order. The authors also analyse the model Hamiltonian via a Hartree approximation, which proves to be the more tractable method, and further allows an explicit description of the EI phase itself. The validity of the Hartree approximation is justified by comparison with the pairing theory.

The low-energy part of the phonon dispersion of the quartz isotype alpha -AlPO₄ has been investigated by inelastic neutron scattering. The experimental data are reproduced by different lattice dynamical models. Good quantitative agreement with experimental neutron, Raman and infrared data is achieved even with just a simple five-parameter Born-von Karman model. The results are compared with the lattice dynamics of quartz. As a main result, it is found that the lattice dynamics at low frequencies is essentially determined by the properties of the oxygen framework, irrespective of the specific nature of the central atoms within the oxygen tetrahedra. The dynamical stability of the lattice is discussed with regard to the incommensurate phase transition.

The authors present angle integrated isochromat spectra (11.1 eV<hv<22.5 eV) from six low-d-occupancy transition metals (Ti, V, Zr, Nb, Hf and Ta). In all cases the spectral shape basically reflects the energy distribution of the empty electron states: the agreement is, however, less satisfactory if compared with the same measurements for noble and near-noble metals and with X-ray inverse photoemission from the whole 3d and 4d series. The paper is focused on the observed discrepancies and on the possible role of secondary electron background cross section and many-body effects in determining the spectral shape.

The replica-symmetry-breaking solution is formulated for the infinite-ranged binomial spin glass model, which is a generalization of the Sherrington-Kirkpatrick (SK) model and includes the Bray-Sompolinsky-Yu (BSY) model as a limit. In the formulation it is found that the Parisi equation is the same as the original one for the SK model. This fact is used to show that the boundary between the spin glass phase and the mixed phase is independent of temperature, which is confirmed by numerical analysis of the naive mean-field equations and completes the phase diagram of the BSY model.

A series of tetragonal rare-earth-iron intermetallic carbides R(Fe₁₁Ti)C_{1- delta} of rare-earths from Nd to Lu have been prepared by reaction of intermetallic powders with butane (C₄H₁₀) at temperatures between 480 degrees C and 520 degrees C. On average, the unit cell volume increases upon carbonation by 1.8% across the series. Curie temperatures increase by about 160 K. Values of magnetization at room temperature and 4.2 K are reported. All compounds were found to have easy-axis anisotropy at room temperature, with the exception of Sm which is easy plane. Magnetization curves of aligned samples of the solid solution series (Y_1-xNd_x)(Fe₁₁Ti)C_{1- delta} give an anisotropy field for the Nd(Fe₁₁Ti)C_{1- delta} compound of 17(2) T at 4.2 K, and 6(1) T at room temperature. Structural effects of interstitial modification of the intermetallic compounds are discussed, as are the possibilities of introducing interstitial atoms other than H, C and N into these types of alloys via gas-phase reactions.

Low-temperature resistance measurements were performed on UTe crystals in the pressure range from 1 GPa to 20 GPa. The Curie temperature obtained as the maximum in the temperature derivative dR/dT, increases strongly with pressure. It has a maximum at 7.5 GPa followed by a decrease with a smaller slope at higher pressures. Another anomaly in the R-T characteristic is observed above 10 GPa and ascribed to the presence of the B2 phase, which is known to form in a very sluggish phase transformation.

Optical-RF double-resonance techniques have been used to study the superhyperfine transition frequencies in a series of Pr³⁺ centres in CaF₂:Pr³⁺ and SrF₂:Pr³⁺ crystals. In these centres Pr³⁺ substitutes for Ca²⁺ (Sr²⁺) and the excess positive charge is compensated by F^-, H^-, D^- or T^- interstitial ions along (100) directions yielding centres of C_4v symmetry. The superhyperfine structure is associated with RF-induced nuclear spin flips of the lattice ¹⁹F^- ions and of the interstitial ions that are coupled to the Pr³⁺ ion. Some qualitative conclusions are drawn about the position of the Pr³⁺ ion, the nearest-neighbour F^- ions and the interstitial ions in the lattice.

Dielectric and pyroelectric properties of single crystals of 1,6-bis(2,4-dinitrophenoxy)-2,4-hexadiyne (DNP) were measured parallel to the polar b axis. Poling experiments with static electric field strengths of up to 33 kV cm^-1 prove that, at least for parts of the crystals, the direction of the spontaneous polarization can be reversed, implying a distribution of transition temperatures. The maximum polarization measured reveals values up to 2.4*10^-7 C cm^-2 at 10 K, which is in very good agreement with X-ray studies. Within a Landau theory of phase transitions for ferroelectric materials, the experimental curves of the spontaneous polarization seem to behave tricritically in the polar phase below 46 K. Owing to the distribution of transition temperatures, the order of the phase transition cannot be determined from permittivity experiments. The influence of a small static electric field on the electric susceptibility is discussed as well.

The problem of narrowing of the homogeneous linewidth of optical impurity absorption due to the dipole-dipole interactions of magnetic moments of impurity paramagnetic ions is considered. For this purpose the use of acoherent optical pulse sequence with different polarization directions is suggested. The conditions are obtained under which the above-mentioned method can be effective.

Cathodoluminescence (CL) measurements made on natural zircon crystals are reported for the temperature range 45-300 K. Different combinations of excitation conditions used at various temperatures, coupled with information on the decay lifetimes of the emission features provided by modulation of the electron beam, enable identification of excited state transitions occurring within rare-earth impurities. A broad-band feature at 385 nm is noted, in common with other luminescence studies using different forms of excitation, and is attributed to an intrinsic decay process.

Ammonium hydrogen selenates NH₄HSeO₄ (AHSe) and ND₄DSeO₄ (ADSe) have been studied in the radiowave and microwave frequency ranges by network and impedance analysers between 200 and 410 K. Three relaxations of about 5*10^-8, 5*10^-10 and 5*10^-11 s have been observed and assigned to reorientations of ammonium ions, proton jumps and reorientations of selenate ions, respectively. The corresponding dielectric susceptibilities have been studied and mechanisms for the different phase transitions from the superionic phase to the paraelectric, incommensurate and ferroelectric phases have been proposed. The origin of metastable phases and of memory effect is discussed.

An experimental electronic bandstructure of molybdenum disulphide has been determined from angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) measurements: polar-angle-dependent ARUPS at a photon energy of 21.2 eV allows the authors to obtain an approximate plot of dispersion parallel to the basal face while photon-energy-dependent ARUPS at normal emission gives a plot of dispersion perpendicular to this face. The experimental bandstructure is compared with three calculations and, while there is a general agreement, specific aspects are better reproduced when there is self-consistency in the calculations. An analysis difficulty reported earlier for polar angle ARUPS for this material can now be understood as arising from incorrect labelling of symmetry directions in the experiment.

The authors show in this article, by using finite size scaling that the d=2 triangular lattice does not exhibit an insulator-metal transition in contrast to the claim of Srivastava.