Table of contents

We consider two perfectly smooth featureless surfaces at T = 0, defined only by their respective dielectric functions, separated by a finite distance, and ask the question of whether they can experience any friction when sheared parallel to their interface. We find large frictional effects comparable to everyday frictional forces provided that the materials have resistivities of the order of and that the surfaces are in close proximity. The friction depends solely on the reflection coefficients of the surfaces for electromagnetic waves and its detailed behaviour with shear velocity and separation is dictated by the dispersion of the reflectivity with frequency.

has been studied by means of a.c. magnetic susceptibility, x-ray and neutron diffraction from 1.5 up to 330 K. This compound shows a structural transition at 299.7 K. Below this temperature a charge ordering state was observed by both x-ray and neutron diffraction measurements. The structure is orthorhombic at temperatures higher than 300 K, but at lower temperatures the patterns show new reflection peaks that can be accounted for in a monoclinic unit cell with stripes of and in the ac-plane. Moreover, the system orders antiferromagnetically at around 120 K giving rise to a magnetic structure of CE type. The magnetic susceptibility measurement clearly shows an anomalous behaviour at both transition temperatures. The charge ordering temperature for the compound is the highest reported in the series (RE = rare earth). This is argued to be a consequence of the larger orthorhombic distortion produced in the unit cell by the lower size of ion.

Pulsed laser deposition of diamond-like carbon films under a magnetic field has been studied. The magnetic field with its direction parallel to the surfaces of substrate and target can significantly enlarge the volume of green plume, and hence improve the uniformity. It has been found that not only the uniformity of thickness but also the hardness of films grown by this method were greatly improved. These diamond-like carbon films were analysed by atomic force microscopy and micro-hardness measurements. The root-mean-squared surface roughness of the films was about 0.265 nm. The hardness of films made by this technique can reach up to .

The experimentally observed composition and uniaxial-stress dependences of the soft phonon mode in alloy have been interpreted using numerical calculations of the electronic band structure, elastic shear constant , generalized susceptibility and different sections of Fermi surface as a basis. It is shown that the main features of these dependences are attributable to two different band structure peculiarities, namely, to Fermi surface nesting and 2D van Hove singularity in the density of states. Whereas the nesting vector fixes the position of a dip (Kohn anomaly) in the phonon branch, the separation between the Fermi level and the energy of the 2D van Hove singularity governs the softening (hardening) of this branch as a whole.

A one-parameter family of models interpolates between the periodic Anderson model with infinite repulsion at half-filling and a model whose ground state is exactly the resonating-valence-bond state. It is shown numerically that the excitation gap does not collapse. Therefore the ground states of the two models are adiabatically connected.

A spin-fluctuation theory of quasi-two-dimensional itinerant-electron ferromagnets is developed. On the basis of an anisotropic spin-fluctuation spectrum, interpolating between two-dimensional (2D) and three-dimensional (3D) cases, we discuss the possibility of observing two-dimensional critical behaviours as well as the crossover phenomena between the 2D and 3D limits.

In this article we present a theoretical study on Hall transport of hot holes in the anisotropic valence band of Si at low temperatures (T = 20 K) in crossed E- and B-fields. A current in the direction of B is found, which may lead to a Hall field in the direction of B of the order of 5% of the applied electric field.

We used a valence band model including non-parabolicity and anisotropy. The scattering models employed in the Monte Carlo simulation are acoustic and optical phonon scattering.

  An approximate treatment of the transient response of an illuminated (`optically biased') amorphous semiconductor to a short light pulse is presented. The problem is formulated in terms of Rose's multiple-trapping model and is studied in two specific cases, corresponding to strongly non-equilibrium and quasi-equilibrium trapped-carrier distributions. In both approximations, the formulae describing the photocurrent and photoabsorption transients in some characteristic time intervals are derived. The accuracy of the formulae is verified by numerical calculations, performed for the exponential distribution of traps. The above results are consistent with those obtained previously by other authors, particularly by Pandya and Schiff. The given formulae make it possible to determine the trap distribution in the energy gap as well as some trap parameters from experimental data without making any model assumptions.

The c-axis resistivity of stage-2, 3, 4, 5, 6, 7, and 9 graphite intercalation compounds (GICs) has been measured in the temperature range between 4.2 and 300 K with and without an external magnetic field along the c-axis. In these compounds the c-axis conduction is dominated by the in-plane conduction because of the highly anisotropic resistivity. The temperature dependence of strongly depends on the stage number. The stage-2 and 3 GICs show a metallic behaviour: increasing with increasing temperature. The logarithmic behaviour in is observed in a limited temperature range for stages 5 and 6, and negative longitudinal magnetoresistance is observed for stages 3 to 7, indicating that the two-dimensional weak localization effect may occur mainly in the interior graphite layers with a small charge transfer. The c-axis resistivity of stage-3 to 9 GICs shows a unusual thermal hysteresis in the temperature range between 180 and 240 K. It has two local minima at critical temperatures - 210 K) and (= 223 - 231 K) depending on the stage number. The carriers in the bounding graphite layers with a large charge transfer are scattered by enhanced fluctuations due to electric dipole moments of molecules. There may occur a two-dimensional- (2D-) like dipole ordered phase below and a 3D-like dipole ordered phase below .

We propose that the normal-state transport in the cuprate superconductors can be understood in terms of a two-fluid model of spinons and holons. In our scenario, the resistivity is determined by holon dynamics while magnetotransport involves the recombination of holons and spinons to form physical electrons. Our model implies that the Hall transport time, as defined by Anderson and Ong, is a measure of the electron lifetime, which is shorter than the longitudinal transport time. We predict a strong increase in linewidth with increasing temperature in photoemission. Our model also suggests that the AC Hall effect is controlled by the transport time.

We present a numerical study of single-chain models of doped spin-1 compounds. We use low-energy effective one-dimensional models for both the cases of paramagnetic and spin-1/2 doping. In the case of paramagnetic doping, the effective model is equivalent to the bond disordered spin-1/2 chain model recently analysed by means of the real space renormalization group by Hyman and Yang. By means of exact diagonalizations in the XX limit, we confirm the stability of the Haldane phase for weak disorder. Above a critical amount of disorder, the effective model flows to the so-called random singlet fixed point. In the case of spin-1/2 doping, we argue that the Haldane phase should be destabilized even for weak disorder. This picture is not in contradiction with existing experimental data. We also discuss the possible occurrence of (unobserved) antiferromagnetically ordered phases.

The temperature-dependent electronic quasiparticle spectrum of the antiferromagnetic semiconductor EuTe is derived by use of a combination of a many-body model procedure with a tight-binding - `linear muffin tin orbital' (TB - LMTO) band structure calculation. The central part is the d - f model for a single band electron (`test electron') being exchange coupled to the antiferromagnetically ordered localized moments of the Eu ions. The single-electron Bloch energies of the d - f model are taken from a TB - LMTO calculation for paramagnetic EuTe. The d - f model is evaluated by a recently proposed moment conserving Green function technique to get the temperature-dependent sublattice - quasiparticle bandstructure (S - QBS) and sublattice - quasiparticle density of states (S - QDOS) of the unoccupied 5d - 6s energy bands. Unconventional correlation effects and the appearance of characteristic quasiparticles (`magnetic polarons') are worked out in detail. The temperature dependence of the S - QDOS and S - QBS is mainly provoked by the spectral weights of the energy dispersions. Minority- and majority-spin spectra coincide for all temperatures but with different densities of states. Upon cooling from to T = 0 K the lower conduction band edge exhibits a small blue shift of -0.025 eV in accordance with the experiment. Quasiparticle damping manifesting itself in a temperature-dependent broadening of the spectral density peaks arises from spin exchange processes between (5d - 6s) conduction band electrons and localized 4f moments.

The Heisenberg antiferromagnet on the triangular lattice with interlayer coupling is treated via a double-time Green function. The hierarchy of the equations of motion of the Green functions is decoupled by employing the Kondo - Yamaji methods. The excitation spectrum, the correlation function, the internal energy, the specific heat and the susceptibility are discussed. The results have good agreement with those of the exact diagonalization and the nearest-neighbour virtual bonding state and can be compared with some experiments.

We use the numerical renormalization group method to study an Anderson impurity in a conduction band with the density of states varying as with r > 0. We find two different fixed points: a local-moment fixed point with the impurity effectively decoupled from the band and a strong-coupling fixed point with a partially screened impurity spin. The specific heat and the spin susceptibility show power-law behaviour with different exponents in the strong-coupling and local-moment regimes. We also calculate the impurity spectral function which diverges (vanishes) with in the strong-coupling (local-moment) regime.

The low field magnetic susceptibility of samples cut from two single crystals of has been measured. Both samples show re-entrant spin glass behaviour in the antiferromagnetic state with significant differences between the samples which are related to their metallurgy. Both samples show spin freezing parallel to as well as perpendicular to the magnetic ordering direction, although the amount of spin freezing apparent with an applied field parallel to the ordering direction is less marked.

FeCrCuNbSiB nanocrystalline alloys, with low fractions of the crystalline phase (10 - 25%), have been studied by means of static magnetic measurements and Mössbauer spectroscopy over a wide temperature range (4.2 - 800 K). As a reference the temperature dependences of the hyperfine parameters measured in ordered Fe - Si alloys were used. It has been shown that at temperatures close to the Curie point of the amorphous matrix the coercivity exhibits a maximum that corresponds to a kink in the thermal evolution of the hyperfine field of the crystalline phase. These effects originate from the lessening of the interphase exchange interactions at the ferro - paramagnetic phase transition of the amorphous matrix and superparamagnetic fluctuations of the magnetization in single-domain grains. A mechanism considering different energy contributions as well as their changes with temperature in such a mesoscopic system is discussed.

Magnetic viscosity was measured for amorphous alloys in the spin-glass state as a function of magnetic field (0 < H < 500 Oe) and temperature (4.2 < T < 60 K). The viscosity field, , deduced from these measurements is independent of H and decreases with increasing temperature as for all the alloys. The activation volume, , calculated from varies with the temperature as . The reduced coercivity, being the coercivity and the saturation magnetization) as a function of , the characteristic size calculated from the activation volume, falls approximately to a common curve for all the alloys. This dependence decreases with increasing as . Such a size dependence of the coercivity hints at a curling-type nucleation mechanism of domains in the spin-glass state of the amorphous Fe-rich Fe - Zr alloys.

In this paper we study the ground-state phase diagram of the one-dimensional half-filled repulsive (U > 0) Hubbard model supplemented with the pair-hopping interaction (W) (the Penson - Kolb - Hubbard model) using the continuum-limit field theory approach. We compare the low-energy properties of the U > 0 Hubbard model and W > 0 Penson - Kolb model. We show that, despite similar excitation spectra, the character of instabilities in these models is completely different. In contrast to the Hubbard model, in the case of the Penson - Kolb model, the dynamical generation of a charge gap leads to the suppression of spin-density-wave (SDW) fluctuations. The charge-density-wave fluctuations survive and coexist with bond-located SDW (Bd-SDW) instabilities. The Bd-SDW corresponds to a magnetically ordered state with staggered magnetization located on bonds between sites. The possibility of bond-located ordering is connected with the site-off-diagonal nature of the pair-hopping interaction. In the case of PKH the bond-ordered states exist at . For W > U/2 the Bd-SDW is realized while for the dimerized phase is realized.

The relaxation dynamics of a catalytically polymerizing, initially molecular liquid was studied by dielectric spectroscopy in the 1 MHz - 10 GHz frequency range until a macromolecular network structure formed and the liquid became a solid isothermally. Further, its dynamic heat capacity for a fixed 5 mHz frequency, and the heat evolved during the growth of the molecular network, were measured. These are related to the number of new covalent links or bonds, n, formed. In the molecular liquid at 335.4 K, one relaxation peak with a spectral half-width of 2.4 decades appears at 0.8 GHz frequency. All polarization of the liquid at 335.4 K relaxes by this relatively broad relaxation process. As the liquid's viscosity increases and the volume and configurational entropy, , decrease isothermally with increase in n in the macromolecule, the strength of this relaxation decreases, but its characteristic time does not change. Both effects are analogous to the effects of the increase in viscosity and density, and decrease in , on the Johari - Goldstein relaxation during supercooling a chemically stable liquid and physically ageing a glass. The rate of orientational diffusion observed in the GHz range is not affected by a decrease in or increase in the density.

The elpasolite compound lead magnesium tellurate undergoes a structural phase transition at about 194 K. This phase transition has been characterized by calorimetry, by dielectric permittivity measurements and by Raman spectroscopy. The main features of the Raman spectra, collected between 35 K and room temperature, are consistent with group-theoretical predictions, though the onset of the incommensurate superstructure gives rise to a very complex splitting of the cubic lines. From the comparison between these results and the structures, it is concluded that the phase transition is only weakly first order and has a mainly displacive character. The thermal evolution of the Raman spectra and of the dielectric permittivity support the existence of an additional phase transition at about 142 K.

We report on the polarized IR and Raman study of the misfit layer compound . The obtained data were compared with those obtained on the parent layer compounds LaSe and . On an assumption of a charge transfer of 1.14 electrons per formula unit from the LaSe to the layers the decrease of the plasma frequency in the misfit compound with respect to that in the parent layer compounds is qualitatively explained. A qualitative model of electronic structure formation of the misfit layer compound from electronic bands of the parent layer compounds is discussed. An in-plane optical anisotropy related to the small misfit between the LaSe and layer structures has been detected.