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We have performed neutron spin-echo measurements on the supercooled glass-forming system ⁷LiCl,6D₂O above its glass transition temperature (T_g=135 K). This investigation has permitted us to determine the time dependence of the density correlation function phi (Q, t) between 2 ps and 2 ns at the maximum of the structure factor (Q=1.9 AA^-1). We have shown for the viscosity, the existence of two temperature domains limited by two critical temperatures (fixed points). This crossover from the stable liquid to the metastable liquid might be related to a change in the multiplicity of the microscopic interactions. In this study we show that the dynamics of the structural relaxation is consistent with the two scaling laws of the viscosity. Both the average relaxation time and the stretched exponential behaviour indicate a crossover regime around 210 K. However, the scaling of the density correlation function differs, in the lower-temperature regime, from the scaling of the shear viscosity. For the supercooled liquid dynamics, the results are more or less correlated to the mode-coupling theory.

The thermodynamic properties of the molten M-As₂Te₃ (M:Tl,Bi) systems have been measured by means of the electromotive force (EMF) method. The partial molar entropy and enthalpy of Tl in the molten Tl-As₂Te₃ system show a very drastic change at a composition of 54.54 at.% Tl, where the system shows a p-n transition. On the other hand, the activities of Bi show no such drastic change in the molten Bi-As₂Te₃ system, where the p-n transition and subsequent metallization occur at a much lower Bi concentration. However, the EMF data for the Bi-containing system were not accurate enough to separate the entropy and enthalpy contributions from the total free energy change. The present results indicate that strong interactions between Tl and Te in the Tl-As₂Te₃ system are responsible for the preservation of p-type semiconduction in the molten state.

Molecular dynamic computer simulations of a two-dimensional Lennard-Jones liquid, along several isotherms, have been carried out to search for a distinct transition from a compressed liquid to an amorphous solid. The dynamics of the transition is examined in terms of tagged-particle van Hove correlation functions, for times up to 600 ps. The onset of localization and the transition to non-ergodicity occur simultaneously and abruptly within a very narrow range of density. Further analysis of the data on the intermediate scattering function indicate a good fit to a stretched exponential with an exponent beta equal to 0.62 and that this exponent is nearly independent of temperature. Thus, for the time scales investigated by this simulation, it is concluded that the transition observed is a glass transition.

A simple model of atomic motion has been used to calculate the velocity autocorrelation function, its frequency spectrum, the associated memory function, and the self-diffusion coefficients for liquid alkali metals. There are no adjustable parameters in the model and the only inputs required are the interatomic potential and the pair correlation function. The predicted results are in good agreement with recent computer simulation data. The self-diffusion coefficients are found to be very close to actual experimental values for all liquid alkaline metals, including lithium, and they seem to scale, the scaling being determined by energy and length parameters for the interatomic potential and the mass.

The vibrational properties of crystalline Na⁺ beta -alumina (Na_1.22Al₁₁O_17.11) have been studied using the molecular dynamics simulation technique. The vibrational density of states was calculated from the velocity autocorrelation function, and the infrared spectrum from the dipole-dipole autocorrelation function. Knowledge of the vibrations in different crystallographic directions for the different atomic species facilitates the assignment of spectral peaks. The sodium in-plane vibrations are 59, 88 and 112 cm^-1, and the out-of-plane vibrations are at 146 cm^-1. The stoichiometric compound is also studied, and in this case the sodium in-plane vibrations are at 80 cm^-1 and the out-of-plane vibrations at 140 cm^-1. The density of states is used to calculate thermodynamic properties: heat capacity, entropy and internal and free energy. The values obtained at 300 K are C_nu =410 J K^-1 mol^-1, S_nu =300 J K^-1 mol^-1, U=370 kJ mol^-1 and F=280 kJ mol^-1. The heat capacity and entropy values are in good agreement with experiment, and thus strongly support the empirical force field used in the simulation.

Unit-cell parameters of RbTi₂(PO₄)₃, a framework structure (space group R3c) with alternating corner-linked Ti octahedra and P tetrahedra that define six-coordinated alkali cation sites, were determined at eight pressures from 0.0 to 6.2 GPa, and crystal structure refinements were obtained at 0.0 and 6.2 GPa. The structure remains dimensionally rhombohedral at all pressures, with the a axis more than four times more compressible than c. Compressibilities of both axes increase abruptly above about 1.7 GPa; the bulk modulus decreases from 104(3) GPa (K'=4) to approximately 60 GPa. Furthermore, the appearance of reflections that violate the c glide-plane symmetry above 1.7 GPa signals the occurrence of a reversible phase transition involving distortion of the framework. From the pressure dependence of the intensities of critical X-ray reflections, the phase transition is revealed as being continuous and close to second order in character. Structure refinements of the 6.2 GPa data reveal R3 symmetry, an acentric space group, for the high-pressure form. Average Ti-O-P angles decrease from 151.5 to 140 degrees between room pressure and 6.2 GPa. These changes result in a coordination change for Rb from one centric six-coordinated site (Rb-O=2.855 AA) at room pressure, to two symmetrically distinct acentric 12-coordinated sites (mean Rb-O approximately=3.01 AA) at 6.2 GPa.

Molecular-dynamics calculations have been carried out on the ordered (low-temperature) and orientationally disordered (high-temperature) phases of calcite. The potential model used is shown to reproduce the main features of the experimentally observed order-disorder transition. The significance of the results for the interpretation of recent inelastic neutron-scattering experiments is discussed with particular reference to a possible competition between different ordering processes. Comparison is made with previously reported simulations of the corresponding phase transition in sodium nitrate and differences in behaviour between the two systems are noted.

We consider a gas of fermions with parabolic dispersion and N spin components (or spin S, N=2S+1) with SU(N) symmetry in one dimension interacting via a delta -function potential. The model is integrable and its solution has been obtained by Sutherland in terms of N nested Bethe ansatze. The ground-state Bethe ansatz integral equations are solved numerically for both repulsive and attractive interactions to obtain the energy, the chemical potential, and the magnetic susceptibility as a function of the band filling and the interaction strength. For the repulsive interaction the Fermi gas has the properties of a Luttinger liquid. In the attractive case, on the other hand, the fermions in the ground-state form bound states of up to N fermions of different spin components. The spectrum of elemental charge and spin excitations is derived for the repulsive and attractive situations. The spectrum is discussed in the limits of vanishing interaction strength and very strong coupling. For the repulsive interaction the low-lying charge excitations can be characterized by the Fermi momentum and the Fermi velocity. The range of the spin-wave excitations is correlated with the Fermi momentum of the charges. The spin wave velocity is inversely proportional to the magnetic susceptibility. The spin-wave excitations become soft in the infinite-repulsive-coupling limit. In the attractive case in zero field all excitation branches except that of bound states of N fermions have an energy gap. It requires a finite energy to break these bound states and hence there is no response to a field smaller than a critical field. The low-T specific heat is proportional to the temperature.

We report measurements of the photovoltaic effect in diode structures formed with thin films (100 nm) of the conjugated polymer poly(p-phenylene vinylene), PPV, sandwiched between electrodes of indium/tin oxide, ITO, and either aluminium, magnesium or calcium. Under illumination incident through the ITO contact, large open-circuit voltages were measured, which saturated at approximately 1.2 V for Al and Mg devices, and approximately 1.7 V for Ca devices. Quantum efficiencies (short-circuit current/incident photon flux) of order 1% were measured at low intensities (0.1 mW cm^-2). The spectral response of the photocurrent demonstrates that photon absorption near the electron-collecting electrode optimizes the photocurrent, indicating that device performance is limited by low electron mobilities in the bulk PPV. The photocurrent exhibits a weak temperature dependence, with an activation energy that is a function of the electric field in the polymer. We have used these measurements to estimate an exciton binding energy in PPV of approximately 0.4 eV.

Using a method based on Lieb's theorems and SU(2) symmetry for the half-filled Hubbard model on a bipartite lattice with mod A mod = mod B mod =N (where mod A mod ( mod B mod ) is the number of sites in the A (B) sublattice), we show that the ground-state wave function has even (odd) symmetry with respect to the spin reversal operator when N is even (odd) for both U>0 and U<0 cases. These rigorous results hold in all dimensions without the necessity for a periodic boundary condition.

We study the statistical mechanics of a spin-one ferromagnet with the nearest-neighbour bilinear Heisenberg exchange constant J and the three-atom coupling constant L using the equation-of-motion method for the two-time temperature-dependent Green function. It is seen that, as the three-atom coupling parameter alpha =L/J is increased positively, the Curie temperature T_C first increases steeply and then increases very slowly and ultimately approaches the limiting value k_BT_C/Jz=⁴/₃ as alpha tends to infinity. The situation is much more complicated for negative alpha but, as alpha to infinity , T_C approaches the limiting value ⁴/₃. The temperature variation in spontaneous magnetization m and the quadrupolar ordering parameter lambda for various values of alpha are studied. It is seen that there exists a critical value of alpha (which we shall call alpha _c) beyond which the phase transition is first order for SC, BCC and FCC lattices. alpha _c decreases as the number z of nearest neighbour increases. The discontinuity in m has been found to be extremely sensitive near alpha _c. The results are discussed with reference to those obtained by earlier workers.

We present a first-principles theory of X-ray Faraday effects. Initially we calculate the difference in absorption rate for right and left circularly polarized incident radiation. From this we are able to define an effective dielectric tensor that can be used to calculate magneto X-ray properties. This work is based on a fully relativistic scattering-theory description of the electronic structure of magnetic materials. The theory is illustrated by a calculation of the Faraday rotation and induced ellipticity in linearly polarized light incident at the K edge of iron. Recent experiments compare favourably with the theory.

The ferroelectric phase transition of 1,6-bis(2,4-dinitrophenoxy)-2,4-hexadiyne (DNP) is characterized by specific heat measurements for monomer and thermally polymerized polymer single crystals. The data support a tricritical phase transition of monomer crystals, with the transition enthalpy clearly lower than that corresponding to an order-disorder transition. A lowering of the transition temperature and the transition enthalpy with increasing polymer content is observed, in agreement with the known dielectric data.

We have studied alloying effects of Al and Ge on the Si site of URu₂Si₂ using resistivity and magnetization measurements. In contrast to previous studies of substitutions on the U and Ru sites, we observe that the substitutions of Al or Ge produce only a simple dilution effect on the itinerant antiferromagnetism transition at 17.5 K in URu₂Si₂. We discuss the alloying effects on T_N, and in particular resistivity, in terms of d-f and sp-f hybridization.

Amorphous Fe_73.5-xCu₁Nb_3+xSi_13.5B₉ (x=0 and 1.5) alloys were isothermally heat treated at 550 degrees C in a vacuum for up to 16 h. ⁵⁷Fe transmission Mossbauer spectra revealed a lower rate of crystallization for x=1.5. The annealing time dependences of the hyperfine parameters and the different behaviours of the intergranular component in both samples were studied via three-dimensional projections of the hyperfine field distributions. Segregation of Fe, Si and also B, from which the crystalline phases have been created, lead to a depletion of these elements in the residual amorphous phase. As a consequence, the number of Fe atoms with Nb, Cu and possibly B atoms in their nearest surroundings increased, giving rise to changes in the chemical and topological short-range order. The amount of the residual amorphous phase as well as its average hyperfine field decreased whereas the standard deviation of the hyperfine field distribution increased with increasing annealing time.