Table of contents

A new scaling effect, connected with the sound absorption in compensated piezosemiconductors, is established. The values of critical indexes both below and above the threshold are found to be equal and depend on effective conductivity critical index above the percolation threshold.

The authors have carried out a high-resolution X-ray scattering study of the first-order paraelectric (PE) to ferroelectric (FE) transition of BaTiO₃. Using a pulsed electric field to cross the PE-FE phase boundary, both the X-ray scattering and bulk polarisation associated with the equilibrium FE phase appear only after a well defined delay relative to the pulse onset. Subsequently, the polarisation evolves much more rapidly than the X-ray intensity.

Raman spectra are reported for single Se and Te chains stabilised in the regular system of parallel channels of a mordenite dielectric matrix. The symmetric bond-stretching mode frequency is determined for Se chains (260 cm^-1) and for Te chains (164 cm^-1). The distinct bond-bending mode bands associated with ring-like and chain-like fragments and the Fermi resonance structure are found in the Se chain spectrum. A comparison of the spectra of chains with those of bulk materials is made. It is shown that the interchain interaction in trigonal Se and Te crystals weakens significantly the intrachain bonding. Confirmation is given for the amorphous Se and Te models that are based on slightly interacting chains with varying signs and magnitudes of dihedral angles.

A semi-empirical model is presented for describing the interionic interactions in molten salts using the experimentally available structure data. An extension of Bertaut's method of non-overlapping charges is used to estimate the electrostatic interaction energy in ionic melts. It is shown, in agreement with earlier computer simulation studies, that this energy increases when an ionic salt melts. The repulsion between ions is described using a compressible ion theory which uses structure-independent parameters. The van der Waals interactions and the thermal free energy are also included in the total energy, which is minimised with respect to isostructural volume variations to calculate the equilibrium density. Detailed results are presented for three molten systems, NaCl, CaCl₂ and ZnCl₂, and are shown to be in satisfactory agreement with experiments. With reliable structural data now being reported for several other molten salts, the present study gains relevance.

A phase transition appearing in systems of weakly coupled magnetic sine-Gordon chains with pi -kink solitons is analysed by the use of the transfer operator method and the pseudo-spin approach. It is shown that the crucial role for this phase transition is played by solitons. A phenomenological picture is proposed in which the phase transition considered is driven by a pairing of kinks below T_c. This picture is confirmed by the transfer operator result for the free energy of the system analysed below T_c.

Phonons in the mixed crystal system CdBr_2(1-x)I_2x have been studied by infrared absorption and Raman spectroscopies for selected concentrations in the range O<x<1 at liquid air temperature. For x=0.43 the Raman spectra at 300, 85 and 10K are compared. The shear-type rigid-layer E₂phonon characteristic of the 4H polytype C_6v⁴ CdI₂ structure to the D_3d⁵ structure of CdBr₂ is evident. The concentration dependence of the multi-mode behaviour observed for the two Raman-active modes common to the two structures is discussed in terms of an averaged nearest-neighbour interaction between cadmium and halogen layers. The appearance of weak features in the Raman spectra associated with disorder-induced scattering from acoustic and polar optical phonons is also discussed.

The previous experiment of high-angle double-crystal X-ray diffraction (HADOX) performed on silicon at low temperatures showed a very anomalous behaviour of a(T), the lattice constant as a function of temperature. An extensive study has since been made under various conditions of specimen crystals. The results are in contradiction with the common belief that the silicon crystal with the diamond structure is rigid and stable: it is found that the anomaly is rather sensitive to external stresses and that the behaviour of a(T) can therefore be very complicated. It is also found that the anomaly is anisotropic, namely largest in (111) direction and smallest in (110) direction. The apparent irreproducible behaviour of a(T) is attributed to an atomic rearrangement with a long relaxation time.

The ground states of both the fully frustrated XY model in two dimensions and ³He-A films have Z₂*U(1) symmetries. Both line defects (domain walls) and point defects (vortices) can disrupt ordering in these systems at finite temperatures. The presence of line defects induces a breakdown on vortex charge quantisation in both of these systems. In the fully frustrated XY model on a square lattice, integral quantisation of charge is replaced by quarter-integral quantisation; in ³He-A, quantisation of charge is completely destroyed. It follows that in both of these systems phase ordering cannot coexist with Z₂ disorder. The fully frustrated XY model is relevant to Josephson junction arrays in a transverse magnetic field, at a value of the field where half of a magnetic flux quantum threads each cell of the array.

The authors discuss the applicability of the Tchebycheff iteration scheme and a simplified derivative of this scheme in electronic calculations. Both methods are very simple and lead to a strongly accelerated convergence especially in cases where the simple mixing scheme converges quite slowly.

Several liquid alloys of alkali metals (A=Li, Na, K, Rb, Cs) with group-IV (B=Si, Ge, Sn, Pb) alloys show strongly anomalous electrical transport properties for compositions AB and/or A₄B. In some cases a nearly complete charge transfer leads to a nearly non-metallic behaviour (for example in Li₄Sn and KPb). These anomalies are explained in terms of calculations of the electronic structure, using a tight-binding model and taking into account on-site and nearest-neighbour Coulomb interactions, clustering of B atoms and short-range order. In particular near the composition AB the presence of tetrahedral clusters B₄^4- has a large influence on the properties on the liquid alloy.

The interaction between electrons in Landau levels (separated by the cyclotron energy h(cross) omega _c) and phonons is highly anisotropic. A study of the anisotropy of the phonon output created by hot electrons at an effective temperature T_E in Landau levels is presented. An exact solution to a simplified model shows oscillations in the phonon power output as a function of omega _c^-1 or T_E^x (x=¹/₂ or 1) when one goes from the classical (k_BT_E>>h(cross) omega _c) to the quantum (k_BT_E<<h(cross) omega _c) regime. Experiments are suggested.

A direct diagonalisation scheme is used to determine the wavefunctions of one-, two- and three-dimensional samples with weak gaussian disorder. The inverse participation number is investigated for different energy ranges. Its distinct dependence on the sample size allows the authors to draw conclusions about the localisation of the states considered: in one dimension all states, and in two and three dimensions only the energetically extreme states, are strongly localised on few sites. At the band centre and in its vicinity in three dimensions the states are extended. For all other states in two and three dimensions the wavefunctions are characterised by a power-law dependence.

A previously obtained generalised field-dependent screening function involving one-electron Wigner distributions and spectral functions is studied under various model approximations. The field dependence of the screening arises from the action of a (constant) electric field within duration of a collision process (estimated from the uncertainty relations) and has the effect of reducing the screening efficiency. This field-induced descreening effect is studied analytically and numerically for a range of electric field strengths and electron temperatures and densities in both three- and two-dimensional non-degenerate electron assemblies. The model assumed allows the influences of displaced Maxwellian distribution functions and finite electron quasiparticle lifetimes to be considered.

The electronic properties of liquid, amorphous and trigonal Se have been studied by photoemission and reflectance spectroscopy near the energy gap. Sharp valence band edges with no observable tailing were found for the amorphous and liquid phases. On liquid Se a surface photovoltage of 0.4 eV was observed upon secondary illumination. This corresponds to a pinning of the surface Fermi level by mid-gap states and provides the first evidence for 'surface states' on a liquid semiconductor. It is proposed to relate them to an accumulation of intrinsic structural defects at the surface.

When niobium counter-electrode is sputtered onto a tunnel barrier oxide grown on a niobium base electrode, the result is normally a superconductively shorted junction. However, the authors find that a thin (0.3 nm) gold layer deposited on top of the oxide prior to counter-electrode deposition protects the junction oxide and prevents shorting. The authors give a brief summary of junction electrical and mechanical characteristics, and propose a mechanism for the shorting and the protection behaviour which is based on electrochemical enhancement of the Cabrera-Mott oxidation barrier potential. They also observe a large increase in tunnelling current in junctions with niobium top electrodes compared with junctions with lead top electrodes, and propose a tunnelling barrier potential model to account for this.

Neutron diffraction investigations have been performed on ZnCr_2xAl_2-2xS₄ in the range 0.5<or=x<or=1.0. For x>or=0.85 a helimagnetic ordering builds up below T_N which is transformed at T₀ into a mixed phase. Below x<or=0.80 no long-range magnetic ordering was detected, reflecting the presence of a spin glass (SG) state. Strong spin correlations occur below 100K giving rise to important diffuse scattering contributions. The short-range ordering in the SG state is of the same nature as that in concentrated samples above T_N: locally the magnetic moments are arranged in a ferromagnetic (001) plane and turn by an angle of about 71 degrees from plane to plane. The correlation function gamma _i increases continuously with decreasing temperature. No anomaly or saturation was observed at the freezing temperature T_f below which the correlation length still increases, gamma ₁ scales characteristically with x² which agrees fairly well with a high-temperature series expansion approach above T=20K including first- and second-nearest-neighbour interactions (J₁=2K, J₂ approximately=-2K). The inverse correlation length 1/ xi decreases linearly with temperature leading to a finite value at T=0 in the SG state.

Total current spectra obtained in the energy range from 0 to 50 eV of incident electrons for highly oriented pyrolitic graphite, polycrystalline graphite and glassy carbon are presented. For highly oriented pyrolitic graphite the total current spectrum shows a complex and angle-dependent structure. It was found that the spectrum exhibits two types of structure: the first obeys the dispersion relationE(k/sub ///) and we ascribe it to pi to pi transitions; the second type of structure does not exhibit significant dispersion and is thought to be due to empty sigma bands. The authors discuss the observed features in the light of the principle of the total current spectroscopy method, selection rules and recent theoretical band calculations for graphite.

The surface compositions and chemical states of an amorphous and a crystalline Cu₅₀Ti₅₀ alloy have been analysed by XPS. The Ti element was oxidised on the surface of the as-quenched Cu₅₀Ti₅₀ alloy. The binding energies of the Ti 2p_1/2 and Ti 2p_3/2 peaks of the as-quenched amorphous Cu₅₀Ti₅₀ were 464.8 and 459.2 eV; these values correspond to 464.1 and 458.6 eV for TiO₂. The titanium was thought to be Ti⁴⁺ present on the surface of the sample because of the value of the shift in binding energy, Delta (BE). A Ti2p_3/2 shoulder peak of the sample appeared at 457.8 eV. The Ti here was thought to be Ti³⁺, because of the value of Delta (BE). The O 1s_1/2 peak showed a double structure, whereas after sputtering only a single peak was observed. The Cu peaks were barely detectable on the surface of the as-quenched sample, but they could be observed on the sputtered surface.

The Raman spectra of GaP microcrystals are decomposed into TO, LO an surface phonon (SP) peaks. The deconvoluted SP peak is broad and asymmetric. Its intensity increases rapidly as the particle size decreases. The experimental size dependence of the ratio of the integrated intensities I_SP/I_TO is compared with theoretical calculation and the importance of the shape effect on this ratio is suggested. The lineshape of the SP is calculated by introducing a simple equation based on the effective-medium theory. By assuming a shape close to ellipsoidal a lineshape is obtained that reproduces the observed asymmetric SP lineshape fairly well.