Table of contents

The thermal conductivity was found to correlate with the maximum absorption near 8.4 mu m in diamonds with A and/or B features. This result can be interpreted simply by correlating this absorption with the total nitrogen content. This interpretation and a more complicated one are relevant to the current debate about differences between A and B features.

The vibrational density of states of a model amorphous Cu₅₇Zr₄₃ alloy was calculated by the recursion method. A general agreement was obtained between the calculated result and the experimental one reported for amorphous Cu₄₄Zr₅₆ alloy. The wavenumber-dependent spectra for both longitudinal and transverse excitations were also calculated. The elastic constants estimated from the sound velocity (Young's modulus E=0.80 Mbar, shear modulus G=0.30 Mbar) agreed fairly well with the experimental results (E=0.745 Mbar, G=0.244 Mbar). The Debye temperature was estimated to be 327K, which is 30K higher than that determined from experimental DOS spectra.

An anomaly in the c₃₃ elastic stiffness function has been observed at the previously reported phase transition temperature T_c=38.6K in trigonal ammonium fluosilicate. Results from thermal expansion measurements also show anomalies at T_c. The acoustic behaviour near T_c is discussed in terms of possible domain formation. Orientational ordering of NH₄ groups near T_c is briefly considered.

Exact numerical solutions of the magnetic susceptibility, the specific heat, the spin and the charge correlation functions are obtained for a Hubbard tetrahedron which contains a high degree of frustration. Complete ferromagnetism shows up in the ground state when the system is more than half-filled, in agreement with Nagaoka's exact solution (1966) for the bulk FCC lattice.

The authors have investigated the logarithmic regime of transport of the two-dimensional electron gas in the Si inversion layer between 0.05K and 1K. There was little band tailing in the specimens used, and it was possible to achieve the k_Fl_EL approximately 1 condition at carrier concentrations as low as approximately 2*10¹⁵ m^-2. The behaviours of the conductance and the Hall effect suggest that in the presence of a magnetic field the logarithmic corrections are caused by electron-electron interaction. In particular, the authors confirm the prediction of this theory that the Hall coefficient varies at twice the rate of the resistance. However, at low values of magnetic field it appears that the transport behaviour arises from localisation rather than interactions. At particular values of magnetic field these two mechanisms are both present, but have a different stability against an increase in temperature. Consequently, at a constant lattice temperature it is possible to observe a transition between them as the electron temperature is increased with an electric field.

The critical exponents alpha , beta and gamma of a Heisenberg ferromagnet are calculated in the reaction field approximation, the results showing agreement with the spherical model.

In ion-implanted ZnSe the g value of the donor resonance is found to shift from the value observed in unimplanted material and this shift is accompanied by a broadening of the resonance line, which in some cases is asymmetric. The behaviour is explained in terms of the mixing of the donor wavefunction with that of defect centres lying within the donor volume. The model allows an estimate to be made of the defect concentration within the implanted layer.

The g-values of the GR1-GR8 levels are obtained from axial Zeeman and magnetic circular dichroism (MCD) measurements using a diamond sample which has minimal strain. The presence of large Zeeman interactions and the occurrence of doublets which are mixed by the magnetic field are not understood.

Point contacts between normal metals at low temperatures show very interesting non-linear phenomena in the current-voltage characteristics. The observed deviations from Ohm's law in metallic constrictions can be used for an energy-resolved spectral analysis of the interaction mechanisms of the conduction electrons with elementary excitations in a metal. A review is given on the theoretical and experimental work dealing with this spectroscopic method. A theoretical analysis, involving an iterative solution of the full non-linear Boltzmann equation for the transport problem of a current through a point-contact geometry, is able to show that the observed second-derivative signal is proportional to Eliashberg's form of the electron-phonon interaction alpha ²F( omega ), with a slight modification due to a transport efficiency function. Non-equilibrium effects of the phonon system are discussed in relation to the observed background signal on which the alpha ²F signal is thought to be superimposed. The theory can easily be extended to other scattering mechanisms than the electron-phonon interaction, and some experiments show that the interactions of conduction electrons with magnons and paramagnetic impurities can be detected using point contacts between ferromagnetic metals and between magnetically dilute alloys.

The fourth moments of current spectra have been calculated for liquid lead using the potential and the corresponding molecular dynamics g(r) obtained by Mitra (1977). These results and the memory function approach are used to calculate the viscosities and dynamical structure function. An estimate of the shear viscosity is found to be in good agreement with the experimental value. The results for the symmetrised dynamical structure function obtained from different models are compared among themselves and with experiment.

The eigenvectors of a temperature-independent and a strongly temperature-dependent M-point mode have been determined. Near the alpha - beta transition they exchange their eigenvectors. In both modes the SiO₄ tetrahedra stay rigid. In addition, in the temperature-dependent mode, all Si-Si distances (Si-O-Si bond angle) are unaffected, similar to the well known soft mode. In the beta phase a flat low-frequency dispersion branch in the Gamma -M direction connects the temperature-dependent mode at M with the soft mode of the Gamma point. This connection is permitted by group theory for the Gamma -M direction but not for the Gamma -K-M direction. The results are consistent with a domain model of the beta phase.

The one-electron energy band structure for YSb and a series of La pnictides, i.e. LaN, LaP, LaAs and LaSb, which attract particular interest as a proper reference material for the study of various anomalous properties of Ce pnictides, is calculated self-consistently by an APW method with the local-spin-density approximation exchange potential. In all La pnictides, narrow 4f bands lie a few eV above the Fermi energy, and distort the valence band appreciably. The calculated bands are metallic for all pnictides, though the density of states at the Fermi energy for LaN is far smaller than for other pnictides, in qualitative agreement with the observed transition from a semiconductor to a metal through the La pnictides. The calculated density of states at the Fermi energy for all pnictides except LaN agrees quantitatively well with the experimental result. The calculated valence band density of states for each La pnictide accounts qualitatively well for the valence band part of the recent experimental XPS spectrum of the corresponding Ce pnictide.

The kinetics of carrier capture, emission and recombination at a deep trap are studied using a model in which these processes go via an excited state. It is shown that non-exponential transients can result and that detailed balance interpretations should be used with care. The model is used to interpret experiments on Si:Se and Si:S.

One of the few direct means of identifying the position of localised states within the band gap of semiconductors entails the measurement of radiative transitions between bound states and the host crystal bands. Studies of the temperature dependence of the spectra supply information from which the phonon contribution can be extracted, but the electronic contribution to the transition probability and its physical signature remain poorly understood. Accordingly, the authors have employed their Green function technique to calculate the electronic components of the optical cross section for deep impurity states in direct (GaAs) and indirect (GaP) gap materials. Localised states transforming as the a₁ and t₂ representations of the T_d point group were considered at several positions in the forbidden gap. A large sampling (compared with that needed for the calculation of binding energies) was employed in order to capture accurately the angular properties of the momentum wavefunction and to ensure convergence of the optical sums. Transitions involving the conduction and valence bands were computed, and some simple trends discussed.

The Auger transition at a He-type defect, deeply binding two electrons, is investigated, with the aim of assessing the dominant factors affecting the magnitude of Auger cross sections. The usual approach is to borrow the key approximations of the effective-mass theory and simplify the evaluation of the transition matrix. In the present study, more detailed knowledge of deep levels is exploited. The results indicate that the standard effective-mass model does not seem to be suitable for the assessment of Auger recombination at deep centres.

The previous determination of the Jahn-Teller parameters related to the KCl:Au^- system has allowed the authors to perform a complete investigation of the B band. Absorption and stress linear dichroism measurements lead them to the conclusion that the B band is due to a E_u state arising from the cubic crystal-field splitting of the ³P₂ state. In view of this assignment, most of the features connected with the absorption spectrum of KCl:Au^- are interpreted.

Measurements of the dependence of conductivity on area fraction has been made on macroscopically anisotropic, two-dimensional systems. The systems consist of squares of anisotropic conducting paper into which circular, square, or elliptical holes are punched. Measurements of both principal conductivities were made on each sample. The percolation threshold for the circles and ellipses is 0.4, that for the squares 0.6. The latter difference appears to be due to a difference in coordination number between the squares and the other systems. The ratio of principal conductivities appears to agree with the predictions of Shklovskii (1978) but, because of large statistical fluctuations, this conclusion is tentative. The data on all samples can be fitted by an expression of the form sigma varies as (p-p_c)^t. Because of statistical fluctuation the fit is poor for p near p_c, but is good for larger values of p up to p=1. Values of t range from 1.1 to 1.6. For the circles and squares both principal conductivities can be fitted with the same value of t, but for the ellipses the two principal conductivities have different exponents.

The temperature dependence of the infrared spectra (4000-200 cm^-1) of powder samples of the highly conducting mixed valence TTF compounds (TTF)Cl_0.68, (TTF)Br_0.76, (TTF)I_0.71, (TTF)(SCN)_0.58 and (TTF)(SeCN)_0.58 is investigated. At temperatures below the metal-insulator phase transitions induced by three-dimensional ordering of charge density waves (CDW) the spectra show additional strong absorptions and antiresonance dips. The plots of their intensity against temperature display abrupt changes of slope in correspondence with the transition temperatures and, for the case of (TTF)(SCN)_0.58, a close resemblance to that reported for the X-ray superlattice intensity. These results are interpreted by applying the phase phonon model to a CDW system displaying a three-dimensional ordering transition. The additional absorptions are assigned to the excitation of phase phonons originating in the coupling between electrons and some intramolecular vibrations of TTF. The antiresonance dips are attributed to Fano-like effects. The relative intensities of the phase phonon absorptions related to totally symmetric vibrational modes compare well with the corresponding electron-molecular-vibration (EMV) coupling constants calculated a priori. The infrared spectra suggest also that the EMV interaction is not limited to totally symmetric modes.

Expressions for the first and second strain derivatives of the single-ion crystal-field Hamiltonian have been systematically calculated for the rare earth intermetallic compounds with the CsCl structure. The calculations were performed on the basis of the point-charge model considering the contributions of the rare earth and metal ions. The Hamiltonian is used to calculate the crystal-field contribution to the elastic constants and to the equilibrium strains.

Reviews some early results that have been developed for the biquadratic Heisenberg ferromagnet. A new generalised decoupling scheme is presented for all S>¹/₂ and several different decoupling choices are discussed. The authors present analytic and numerical results for some of these and compare them with exact low-temperature and high-temperature results.

The X-band ESR spectrum of vanadium-doped alpha cristobalite AlPO₄ has been observed at room temperature. The spectrum, attributed to V⁴⁺ in a distorted tetrahedral site, can be described by a spin Hamiltonian of axial symmetry. The Hamiltonian parameters are: g/sub ///=1.9168, g_{perpendicular to} =1.9835, mod A/sub /// mod =188*10^-4 cm^-1, mod A_{perpendicular to} mod =75*10^-4 cm^-1. The wavefunction of the ground state is of the d_x2-y2 type.

Trigonally symmetric electron spin resonance spectra have been observed for Co²⁺, Ni²⁺ and Mn⁴⁺ ions occupying single sites, and Cr³⁺ ions occupying two sites in ilmenite-structure MgTiO₃. The fine-structure parameters at 295K for Cr³⁺(1), Cr³⁺(2), Mn⁴⁺ and Ni²⁺ ions are D=-1672, +364, -1769 and 1370*10^-4 cm^-1 respectively, whilst for Co²⁺ ions g/sub ///=2.867 and g_{perpendicular to} =4.99. Previous results for Fe³⁺ and Mn²⁺ ions are substantially confirmed. Oxidation and reduction experiments show that the valence state of iron in MgTiO₃ may be readily changed; the observed effects are consistent with the earlier proposal that Fe³⁺ ions are located at both the Mg²⁺ and Ti⁴⁺ sites. A comparison of experimental and calculated fine-structure splittings for Cr³⁺ ions suggests that Cr³⁺ (1) and Cr³⁺ (2) ions are located at sites which involve Cr³⁺-Ti⁴⁺ and Cr³⁺-Mg²⁺ ion pairs in nearest-neighbour cation sites respectively, although whether the Cr³⁺ ion in each pair occupies the Mg²⁺ or Ti⁴⁺ host lattice site is indeterminate. The calculations are based on the superposition model of crystal fields and existing data for ruby. The ESR data for MgTiO₃ and Al₂O₃ containing Mn²⁺, Co²⁺ and Ni²⁺ ions suggests that the single-ion anisotropy in the layered antiferromagnets MnTiO₃, CoTiO₃ and NiTiO₃ may be comparable with the dipolar anisotropy.

The EPR linewidth dependence on temperature and impurities concentration in KMn_1-xMe_xF₃ single crystals (where Me is Ni²⁺, Co²⁺, Fe²⁺) is investigated both experimentally and theoretically. The theoretical investigation is performed with the use of a nonequilibrium statistical operator method proposed by Zubarev (1971). The microscopic kinetic equations of motion for the magnetisation of the host and impurities subsystems are derived and the width of the resonance line is calculated. The theoretical results and compared with the experimental data.

The Mossbauer conversion electron process is investigated. The formula of the intensity of the Mossbauer conversion electrons is obtained using density matrix of an electron-nuclear system. It is shown that selective Mossbauer conversion electron spectroscopy may be applicable in gamma magnetic resonance studies.

If a dielectric obeys the Collide, Hasted and Ritson equation (1948) rather than the Debye equation, the parameters obtained for it on the assumption that it obeys the latter equation will be slightly in error. The experimental values of epsilon will lie just within a semicircle. The effects on the apparent values of the distribution parameter, the high-frequency permittivity and the relaxation time are calculated.

The electrical conductivity as a function of the frequency at zero temperature is studied for donor concentrations in Si-P near the critical value for the metal-insulator transition on the insulating side. The enhancement of the dielectric constant is obtained using the Kramers-Kronig relations. The dielectric constant diverges near the transition as (E_c-E_F)^-1.

It is demonstrated that runaway of electrons in polar semiconductors in a strong magnetic field H begins when the value of the total electric field E_tot^BR exceeds the critical value E_tot* for which an electron velocity in crossed fields is given by nu _c=CE_tot/H=¹/₂( square root 2-1) nu ₀ ( nu ₀=(2h(cross) omega ₀/m*)^1/2 is an electron velocity at an optical phonon energy). This result was found on the basis of experimental investigations of avalanche breakdown in n-InSb.

By studying resonant Raman scattering in AgBr excited at 1.8K in the indirect exciton absorption, various exciton relaxation processes are revealed. They are identified by analysing both the Raman shifts and intensities of the scattering lines as a function of the kinetic energy of the exciton. Intravalley scattering by acoustic phonons near the zone centre, intervalley scattering by TA(X) and LA(X) phonons and trapping by impurities are found to be important relaxation mechanisms. Their relative scattering probabilities are determined. From the analysis of intravalley scattering involving dispersive phonons the effective exciton mass is determined as M*=(1.4+or-0.2) electron masses. A model is developed on the basis of the authors results that allows reproduction of the essential features of the experimental scattering spectra demonstrating the consistency of their analysis.