Table of contents

The effect of the mixing of lithium triflate and sodium iodide at high and equal concentrations in a polymer-based (poly(ethylene oxide)) ionic conductor are investigated. A variety of characterisation techniques, namely conductivity, X-ray diffraction, DSC and NMR, are employed. The salient observations involve enhanced conductivities, reduced microviscosity, greatly enhanced mobility of those lithium ions observed by NMR and a recurring absence of NMR observability of a substantial fraction of the cations. The data are interpreted as indicating that the effects of mixing of the salts are to enhance greatly the volume of available amorphous phase. Another feature of the interpretation is the inhomogeneous distribution of the various cations and anions around the different phases present in the materials.

The anomalous dispersion found by inelastic neutron scattering in one of the vibron bands in solid tetracyanoethene appears to be due to the electrostatic interactions that couple the infrared-active intramolecular vibrations via the transition dipole resonance mechanism. This is demonstrated by way of both numerical lattice dynamics calculations and a simple mathematical analysis, which relates the Fourier components of the dispersion curves in specific directions of the wavevector k to two-dimensional lattice sums over the dipole-dipole interactions between layers perpendicular to k. Only for specific orientations of the molecular transition dipole moments in a solid and for specific directions of k will this lead to a dispersion curve of irregular shape.

The authors consider an SU(2) path integral formulation of a Kondo lattice model for heavy fermions that treats the RKKY interaction explicitly. At low temperatures they find the heavy Fermi liquid becomes unstable to the formation of a spin liquid amongst the f spins. Kondo coupling to the spin liquid stabilises it against antiferromagnetism, causing the resonating valence bonds of the spin liquid to occasionally escape into the conduction sea. This process induces off-diagonal resonant scattering in the conduction sea, thereby generating anisotropic superconductivity in the heavy fermion system.

A point lattice model, with parameters adjusted to fit ab initio band-structure calculations for a sodium monolayer, is used to determine the longitudinal response to an external potential as a function of frequency and wavevector. The imaginary part of the response shows loss peaks associated with both inter-band and collective excitations. An energy-loss peak observed for a sodium monolayer on an aluminium substrate is explained qualitatively by the results of this calculation.

Low-temperature specific heat and various electron transport properties have been measured for the icosahedral Mg₃₂Al₁₇Zn₃₂ and six alloys with the formula (Mg_1-xAl_x)₆₅Zn₃₅ (x=0, 0.05, 0.1, 0.15, 0.2 and 0.3), all of which were fabricated in the ribbon form by the melt-spinning technique. This series of Mg-Al-Zn alloys covers the composition range over which the amorphous single phase is gradually taken over by the icosahedral quasicrystalline phase with increasing Al content. The density of states at the Fermi level turns out to be close to the free-electron value for both amorphous and icosahedral phases. As far as the resistivity is concerned, no significant difference is observed between these two phases. However, the Hall coefficient and the thermo-electric power for the icosahedral phase exhibit unique temperature dependences apparently inherent to the quasicrystalline structure and, hence, their behaviour is sharply distinguished from that in the amorphous phase.

The electrical resistivities of monocrystalline Tm and of the solid solutions alpha -TmH_x, x<or=0.1, have been measured along the c axis and the b axis in the temperature range 2 K<or=T<or=300 K. The Neel temperature T_N for both orientations and the spin-disorder resistivity rho _mag^O,b of the b axis specimens decrease with increasing x, owing to decreasing conduction electron density and weakening RKKY interaction, while the residual resistivity rho _r^b increases linearly by 4.0(1) mu Omega cm (at.%H)^-1 in solution. For the c axis specimens, one observes a strong nonlinear increase in rho _t^c with increasing x, even suppressing the original diminution in rho in the ordered state; this is interpreted by an H-induced evolution of the superzone boundaries. The temperature dependence of the magnetic resistivity term rho _m^b(T) in the basal plane is not easily understood through a simple excitation mechanism but is best fitted through a combination: rho _m^b(T) varies as ATⁿ+BT² exp(- Delta /kT), where the power n and the gap Delta both decrease with increasing x.

The Hubbard model of an itinerant antiferromagnet (in particular, a Mott insulator) is considered. Starting from the generalised Hartree-Fock approximation, an expansion in the fluctuating part of the Coulomb interaction is developed. The magnon spectrum, corrections to the electron spectrum, the amplitude of the local moment and the total energy are calculated in the spin-wave region for arbitrary values of the Hubbard parameter U. A comparison with corresponding results within the framework of the s-d exchange model is carried out.

The author presents results obtained from a Monte Carlo simulation of the torque relaxation in a classical Ruderman-Kittel-Kasuya-Yosida spin glass in the presence of weak anisotropic Dzyaloshinskii-Moriya interactions. The simulations are carried out at zero temperature and correspond to the short-time results observed in experiments. For weak anisotropy, the results are consistent with those seen in experiments on CuMn in the short-time limit. For strong anisotropy the qualitative features seen in the simulation are interesting and should be checked in experiments.

The infrared absorption spectrum of a hydrogenated Si(100)2*1 surface is calculated with a constant-temperature molecular dynamics technique (Nose dynamics). The equations of motion embody radial and angular forces between Si atoms, encompassing previous calculations based on force constants. A novel pair potential determined by ab initio quantum chemistry techniques is used for the interactions between H and Si atoms. The spectra obtained, free of any adjustable parameters, compare well with experiment and with previous numerical works model a shift of 200 cm^-1 towards low frequencies. This was noticed previously by Tully and co-workers (1985). The origin of the shift is investigated in detail.

Neutron diffraction experiments have been conducted on 3.1 M (molar) solutions of ¹⁰⁷AgNO₃ and ¹⁰⁹AgNO₃ in heavy water, and the first-order difference method of isotopic substitution has been used to obtain the local structure around Ag_(aq)⁺. The results of this work have been used in conjunction with those from an X-ray diffraction study of 3.1 M (molar) AgNO₃ and NaNO₃, to establish the degree of structural isomorphism between Na_(aq)⁺ and Ag_(aq)⁺. It has been shown that Na_(aq)⁺ and Ag_(aq)⁺ are sufficiently isomorphic to enable useful pictures to be obtained for both Na⁺ hydration and Ag⁺-Ag⁺ coordination. Comparison with previous results for the other alkali metal ions shows that, in terms of the strength of hydration, Na⁺ is intermediate between Li⁺ and K⁺.

The structure factors S(q) of liquid alkali metals are calculated using a fluid of hard spheres as a reference system together with the Cummings potential in a random-phase approximation. For the hard-sphere reference system the direct correlation function of Colot and co-workers (1986), is used to improve the Percus-Yevick approximation at liquid-metal densities. This leads to good agreement with experimental data, particularly at low q.

In the process of vaporisation, the heat contribution from overcoming the surface tension added to the relevant volume expansion term shows a remarkable constancy with respect to temperature in the case of alkali liquid metals. This sum also exhibits a definite power dependence on the molecular weight. These features lead to direct physical interpretation of the Eotvos law.

For the model of a simple classical many-particle system generalised constitutive equations are derived which express fluctuations of arbitrary variables in terms of those for density, currents and temperature. In particular the microscopic expression for the dynamic specific heat is found. The generalised hydrodynamic equations for undercooled liquids and glasses are obtained as a special application of the theory.

Glass transition singularities as obtained by mode-coupling theory are classified within the framework of singularity theory for smooth mappings in parameter spaces. The general equations for the beta relaxation process are derived and characterised by sets of a few relevant parameters characteristic for every singularity. The simplest singularity, which is studied in detail, is specified by two relevant coordinates, the separation parameter and the exponent parameter. The scaling laws describing the dynamics and the leading corrections to scaling are discussed. It is shown that a strong asymmetry in the mode-coupling equations leads to a beta relaxation peak that can be described asymptotically by a Cole-Cole law. The corresponding dynamics is interpreted within the picture of renewal processes for motion in a high-dimensional potential landscape.

The equations for the beta relaxation dynamics as obtained within mode-coupling theory for the glass transition are solved asymptotically for parameters near Whitney cusp singularities. The solution is given by a two-parameter scaling law, where the time t enters as ln t and where the scaling times depend exponentially with a Vogel-Fulcher like form on the control parameters. The master function is given in terms of Weierstrass' elliptic function. It describes crossovers from critical relaxation Phi (t) varies as 1/ln²t to a constant f₀, to a power-law decay 1/t^a, to Phi (t) varies as -lnt, or to Phi (t) varies as ln²t depending on the sector in parameter space. The relaxation data for the Cu-Mn spin-glass alloy can be described by the theory for a time interval of eight decades.

In liquid Bi-In-Ni alloys the dependences of the Bi Knight shift K(Bi) and the In Knight shift, K(In), respectively, on the concentrations as well as the linewidths. The concentration ranges were c_Ni=0-0.20, c_Bi=0-1 and c_In=0-1, at temperatures of 1000-1300 K. The addition of Ni increases K by Gamma =c_Ni^-1. dK/K approximately=1 for In, and Gamma approximately=0.5 for Bi. The considerable nonlinear changes in K(Bi) and K(In) with c_Bi/c_In that are known to occur in the binary Bi-In system retain their general shapes on addition of Ni. The contribution of Ni d electrons to both Knight shifts has a large term proportional to c_Ni², which is negative for K(In) and positive for K(Bi). The spin polarisation is evaluated. Changes in K in the ternary liquid alloy are compared with a superposition of the changes known from the binaries; at c_Ni approximately=0.20, deviations occur for K(In).

The authors present the first microscopic investigation of the atomic and electronic structure of a molten-compound semiconductor. Their approach is based on pseudopotential-derived interatomic forces, a molecular-dynamic simulation of the atomic structure, and a supercell linearised-muffin-tin-orbital calculation of the electronic density of states. Results for the atomic structure of molten GaAs are in good agreement with recent neutron-diffraction data. They find that the complex structure of the metallic melt arises from the modulation of the random packing of the atoms by the Friedel modulations in the effective interatomic interactions. The arrangement of the atoms is chemically random and shows only weak angular correlations. This has important consequences for the electronic structure. The loss of bond-orientational order on melting leads to the disappearance of the optical covalent gap and to the semiconductor-metal transition. The large number of like-atom ('wrong') bonds causes the filling of the ionic gap characteristic of the compound semiconductors. The s bands of both As and Ga are broadened so that the As s band is no longer separated from the rest of the valence band.

The temperature dependence of the critical micelle concentration (CMC) and a closed-loop coexistence curve are obtained, via Monte Carlo simulations, in the water surfactant limit of a two-dimensional version of a statistical mechanical model for micro-emulsions, The CMC and the coexistence curve reproduce various experimental trends as functions of the couplings. In the oil-surfactant limit, there is a conventional coexistence cure with an upper consolute point that allows for a region of three-phase coexistence between oil-rich, water-rich and microemulsion phases.

A new first-order phase transition at T=92 K has been detected in single crystals of ((CH₃)₄N)₂MnCl₄ by means of several optical techniques. The phase transition displays a large thermal hysteresis, Delta T=12 K. Coexistence of phases below 92 K is also detected. The sensitivity of the optical absorption of the crystal, as well as that of emission, excitation and lifetime of MnCl^2-₄ units for detecting this phase transition is analysed in detail. The results are also compared with those corresponding to the P112₁/n-P12₁/c1 phase transition at T=171 K.

The authors present direct measurements of the electronic heat capacity in samples of metallic n-doped indium antimonide (n-InSb) with electron concentrations close to that at the metal-insulator transition (MIT). They find that far from the transition there is good agreement with the value expected from free-electron theory, but that it decreases to about a sixth of the expected value close to the transition. The heat capacity in insulating samples has been deduced from previously published work and they find the same reduction on both sides of the transition. They show that electron-electron interactions are not sufficient to account for this change and consider the role of the impurity band in n-InSb.

Using infrared absorption spectroscopy data from electrochemically deposited Si:O:F films and plasma-enhanced chemically vapour deposited Si:O:H:F alloys the authors report the presence of SiF^2-₆ ions within both fluorine-doped silicon oxide thin films.

Evidence of itinerant band-like behaviour of the Co 3d states in paramagnetic CoO has been observed in CoO(001) angle-resolved photoemission spectra. Two d bands are identified, which disperse 0.4 eV and 1.7 eV between Gamma and X. In broad terms the results are consistent with a recently proposed model of the electronic structure in which the insulating gap arises largely from an exchange potential perturbation of the one-electron band structure.