Table of contents

We report angle-dependent magnetoresistance experiments on the charge-transfer salt ( alpha -ET₂KHg(NCS)₄ using fields of up to 30 T. The data indicate that the widely reported high-field 'kink' observed in the magnetoresistance of the alpha -ET₂MHg(NCS)₄ (M=K, Tl, Rb) family of charge-transfer salts represents a transition from an imperfectly nested spin-density wave ground state, characterized by a quasi-one-dimensional Fermi surface sheet tilted with respect to the b*c plane, to a state with a cylindrical two-dimensional Fermi surface.

A large DC voltage response, which depends strongly on the number of occupied subbands, is observed when a quantum point contact is subjected to high-frequency ( approximately THz) radiation. This signal is explained by rectification caused by the non-linear one-dimensional transport properties of the point contact.

The heat capacity of the supercooled liquid phase of 3-methylpentane has been measured with a high-precision adiabatic calorimeter. Negative evidence was obtained for the occurrence of a liquid-liquid transition currently discussed in relation to the mode-coupling theory of liquid states. A search for similar transitions in 1-butene and 1-pentene also gave negative results.

We examine the surface tension and Tolman's length of a spherical drop using the simplified Cahn-Hilliard model which has been used previously by the author to calculate the homogeneous nucleation rate. It is found that the Tolman's length is given by the difference of the bulk correlation lengths of the liquid and vapour phases, and is negative for normal liquids. We discuss briefly the difficulty of estimating the Tolman's length from computer simulations.

We have studied the optical absorption of fluid Na_xNaI_1-x solutions at high temperatures up to 1073 K and in the energy range from 0.5 eV to 3.6 eV. The metal mole fraction x has been varied in situ by coulometric titration in the range from 10^-3 to 4*10^-2. This ensures a precise control of changes in the spectra as a function of x. Besides the well known F centre excitation and a Drude-like contribution the most striking characteristic of the spectra is a step-like intensity profile in the UV. Its main features and variation with x are consistently described by excitations from the F centre-like trap states into states of the conduction band tail above the Fermi energy.

The kinetics of phase ordering in binary mixtures in the presence of amphiphilic impurity molecules has received a certain amount of attention in the literature. Examples of such complex fluids include ternary microemulsions and micellar solutions. All the models proposed so far are extensions of the models used for the study of phase ordering in pure binary mixtures, e.g., binary alloys. We review the essential features of the models proposed by various groups, including ours, and summarize the main results. We compare these models and the results with the corresponding experimental observations to establish the merits and limitations of these models. We also mention the challenging open questions and the possible future directions of work in this frontier area of interdisciplinary research.

The velocity of sound in liquid sulphur-tellurium (S-Te) has been measured to investigate structural changes induced by raising the temperature. The sound velocity in a Te-rich alloy as a function of temperature shows a peaked minimum, which shifts to higher temperatures with increasing S content. The results have been used to deduce the isothermal compressibility. It has been shown that Ehrenfest's relations are satisfied among the temperature dependences of the isothermal compressibility, constant-pressure specific heat and volume expansion coefficient. The relations also explain the initial slope of the polymerization line in the pressure-temperature diagram for liquid S.

A semiphenomenological theory is proposed for the calculation of the frequency and temperature dependences of the optical properties of polyvalent metals. Utilizing the concept of parallel bands, the transverse optical conductivity is decomposed into a nearly-free-electron part and in an interband part. The theory needs only one fitting parameter, the unknown electron-electron scattering frequency above the Fermi energy. The complete set of optical quantities is calculated for aluminium for a large frequency range (0.01-50 eV) and from T=300 to 2000 K.

The X-ray crystal structure of a mixed crystal of potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP), K_1-X(NH₄)_XH₂PO₄ (KADP_X) with X=0.89, has been refined in the paraelectric phase at 154, 164, 192 and 295 K, against high-resolution data (((sin theta )/ lambda )_max approximately=1.2 AA^-1 with multipolar pseudoatom form factors. The phase transition temperature is observed to lie at T_c=132+or-1 K; a linear temperature dependence of the atomic vibration amplitudes of P and O is observed; extrapolation of U³³ curves to T=0 K suggests a distribution of P between two sites separated by 0.2 AA. The structure of KADP_0.89 is compared With ADP and KDP structural data.

Proton NMR linewidths and second moments, and spin-lattice relaxation times in the laboratory (12 and 60 MHz) and in rotating frames (H₁=9*10^-4 T) were studied for polycrystalline guanidinium nitrate over a temperature range covering three solid phases. An analysis of the experimental results enabled us to reveal a complex cation motion and derive the respective activation parameters. In the low-temperature phase a proton motion in the hydrogen bond has been demonstrated and characterized. A convergence of two reorientational correlation times of the guanidinium cation has been discovered at the first-order phase transition.

Commensurate-commensurate phase transitions are known to occur in the Ising model with competing interactions (ANNNI model). We show that an analogous scenario may occur in spin Hamiltonians with continuous symmetry and competing interactions when an external magnetic field is applied. In particular we study a planar square lattice with exchange competition up to third neighbours. We consider exchange competition suitable for giving a zero-temperature-zero-field configuration characterized by a spin-spin turn angle Q=4 pi /5. In contrast with previous conclusions, based on the hypothesis that the unit magnetic cell does not change under the effect of an external magnetic field, we find discontinuous phase transitions between a distorted-helix phase (with five spins per cell) and the spin-flop phase (with two spins per cell) and between the spin-flop phase and a fan phase (with five spins per cell). This behaviour seems to be a general rule for large enough Q as confirmed by the helix with a turn angle Q=6 pi /7 in zero field.

The critical exponents of the metal-insulator transition in disordered systems have been the subject of much published work containing often contradictory results. Values ranging between 1/2 and 2 can be found even in the recent literature. In this paper the results of a long-term study of the transition are presented. The data have been calculated with sufficient accuracy (0.2%) that the calculated exponent can be quoted as s= nu =1.54+or-0.08 with confidence. The reasons for the previous scatter of results are discussed.

The transport coefficients of the Anderson model are calculated by extending Wilson's numerical renormalization group method to finite-temperature Green functions. Accurate results for the frequency and temperature dependence of the single-particle spectral densities and transport time tau ( omega , T) are obtained and used to extract the temperature dependence of the transport coefficients in the strong-correlation limit of the Anderson model. Results are obtained for values of the local level position ranging from the Kondo regime to the mixed valency and empty orbital regimes. The low-temperature anomalies in the resistivity, rho (T), thermopower, S(T), thermal conductivity, kappa (T), and Hall coefficient, R_H(T), are discussed in terms of the behaviour of the spectral densities. At low temperature all quantities exhibit the expected Fermi liquid behaviour, rho (T)= rho ₀(1-c(T/T_K)²), S(T) approximately gamma T, kappa (T)/ alpha T=1+ beta (T/T_K)², R_H(T)=-R_infinity (1- delta (T/T_K)²). Analytic results based on Fermi liquid theory are derived here for the first time for beta and the numerical results are shown to be consistent with this coefficient. The range of temperatures over which universal behaviour extends is also discussed. Scattering of conduction electrons in higher-angular-momentum, l>0, channels is also considered and an expression is derived for the corresponding transport time and used to discuss the influence of the interference terms between the resonant l=0 and non-resonant l=1 channels on the transport properties. The presence of non-resonant scattering is shown to be particularly important for the thermopower at half filling, where the sign of the thermopower can depend sensitively on the non-resonant phase shift. Finally the relation of the results to experiment is discussed.

An exact single-particle scattering theory formulation of the problem of elastic electronic transport is employed in a 1s tight-binding implementation to study the zero-voltage, zero-temperature conductance of a two-dimensional quantum wire as a function of the Fermi energy. For a perfect wire, the conductance quantization effect is reproduced. Then, the effects on the conductance of compositional impurities and width variation in the wire are studied. It is found that the quantization effect is seriously damaged even with minimal amounts of impurity, whereas it exhibits some tolerance towards width variation, especially at low carrier concentrations. It is found that with both types of disorder the conductance is particularly strongly suppressed at Fermi energies close to the edges of the subbands for the perfect wire, which, in agreement with previous findings, shows that at those energies the localization of the electrons by the disorder is enhanced.

The conductance of the Si/p⁺-Ge/Si epitaxial heterostructure associated with hole tunnelling into isolated ultrasmall ( approximately 10 nm) quantum dots p⁺-Ge has been studied. Quantum dots have been obtained after islanding of 1.3 nm Ge film during MBE growth by the Stransky-Krastanov mode method. Conductance oscillations as a function of bias voltage were observed. The experimental data are analysed in terms of a model that involves the interplay of single-electron charging effects and resonant tunnelling through individual energy levels.

The temperature dependence of hopping conduction through a one-dimensional chain of localized states has been studied in small-area (S=2 mu m*2 mu m) thin (L=280 AA) amorphous silicon layers. If the average number (N) of states in the chain is less than five, then the conductivity was observed to obey not an exponential but a power law. From these data the values of N have been extracted in a set of microstructures. It was found that chains with different numbers of states are realized in different devices. Chain reorganization with variation in the temperature has been studied. At low temperatures (T<40-60 K), resonant tunnelling through localized states was observed. It was shown that the electronic spectrum of microstructures can be affected by external parameters: the electric field, temperature and light illumination.

The phase diagram of the Ising spin glass model in a transverse field is studied by the local mean field method. Effects of the transverse field on specific heat, irreversibility and hysteresis are elucidated.

The temperature dependence of the magnetic properties of itinerant electron systems is mainly due to the excitation of spin fluctuations. We calculate the susceptibilities of Pd, Pd_1-xAg_x, and PdH_x (0<or=x<or=0.03) with an extension of the classical Murata-Doniach model for spin fluctuations, which treats the energy functional without a Landau expansion of the magnetization-dependent ground state energy. To obtain this energy ASW (augmented spherical wave) and KKR-CPA bandstructure calculations are performed. In agreement with experiment the KKR-CPA method yields a maximum of chi (T) for Pd, which disappears with increasing Ag concentration. In the ASW calculations, low Ag and H concentrations are simulated by the supercells Pd₃₁Ag and Pd₃₂H. In addition the alloy PdH_x is simulated by calculations for pure Pd, with the valence electron concentration increased suitably. Calculations of the susceptibility of Pd within the framework of the Stoner theory and the spin fluctuation theory based upon the rigid-band model are presented. The role of the electron interaction in both theories is examined and the reason for the maximum of chi (T) for Pd is investigated. We find the different scaling of the temperature axis in the Stoner and spin fluctuation theories to have its origin in a different treatment of the electron interaction.

EPR spectra attributed to Fe³⁺ centres, produced by hole trapping at substitutional Fe²⁺ ions, are observed below 250 K, in X-ray-irradiated CdCl₂ single crystals doped with 0.1 mol% Fe. The zero-field-splitting parameter b₂⁰ decreases from 0.0818 cm^-1 at 13 K to 0.0736 cm^-1 at 225 K. Its absolute value and temperature dependence are well described by the superposition model of Newman, assuming a temperature-dependent Coulomb contraction of the Cl^- ligands sandwiching the Fe³⁺ ion.

Decay-current and conventional dielectric measurements have been performed on single-crystal samples of the disordered perovskite Pb(Mg_1/3Nb_2/3)O₃(PMN). Low-frequency dielectric data of Colla and co-workers(1992) on the same system are critically reanalysed and compared to our results. In contrast to previously proposed spin-glass or master-plot approaches, we suggest a phenomenological description in terms of alpha - and beta -relaxations: a relatively high-frequency beta -process takes over when the characteristic alpha -relaxation time becomes extremely long. The continuous slowing down of polar motion distinguishes PMN from related compounds such as Pb(Zn_1/3Nb_2/3)O₃ for which, in contrast to PMN, there is evidence of astructural phase transition.

The fraction of positrons annihilated in the Kapton support of a positron source has been measured as a function of the atomic number, Z, of the sample studied. The data have been filtered by both exponential and logarithmic laws obtained from a backscattering model. Two formulas are given to correct the source contribution in a positron lifetime experiment. The results shown that the exponential fit is satisfactory for low atomic numbers while a logarithmic fit is better for Z>30.

In the Co-Nb system, seven amorphous alloys were synthesized by room-temperature 200 keV xenon ion mixing of multilayered films with various compositions. The composition range favouring amorphization was determined to be from 23 to 80 at.% Co. In addition, two metastable crystalline (MX) phases were consecutively formed in the Co₂₃Nb₇₇ multilayered films, i.e. an HCP phase at a dose of 1*10¹⁵ Xe⁺ cm^-2 and a FCC phase at a dose of 6*10¹⁵ Xe⁺ cm^-2. The thermal stabilities of both ion-mixed amorphous alloys and the MX phases were examined by subsequent annealing. To obtain a deep insight concerning all the above observations, a free-energy diagram of the Co-Nb system was constructed by calculating the free-energy curves of the solid solutions, the amorphous phase, the equilibrium compound and the MX phases, which were considered as compound-like phases. To test the relevance of the constructed diagram in its outline, steady-state thermal annealing of the as-deposited Co₂₃Nb₇₇ multilayered films was performed and the results confirmed the calculated relative energy levels of the phases considered. The free-energy diagram can explain the amorphization range as well as the observed evolution of the ion-mixed phases.