Table of contents

The authors report the effects of uniaxial stresses on the 875 meV line in irradiated Ga-doped Si. The line is shown to occur at an optical centre with the C_2v point group.

The temperature dependences of the transport parameters-Hall coefficient (R), electroconductivity ( sigma ) and thermoelectric power ( alpha )-in high-resistivity PbTe:Cr have been studied. It has been concluded from the experimental results that the Cr impurities act as electron trapping centres in PbTe.

A general theory of phase conjugation of bulk acoustic waves in piezoelectric crystals is proposed. The formulae obtained for the ratio of amplitudes in two cases of special boundary conditions allow a quantitative prediction for the so-called 'space resonance'. The theory is in good agreement with the experiments.

Neutron inelastic scattering experiments have been carried out to measure the phonon energies of deuterated naphthalene C₁₀D₈ for the lowest 12 dispersion branches along ( xi 00) and (00 xi ) directions at various temperatures ranging from 6 to 300K. Using measured mode Gruneisen parameters the quasiharmonic shifts related to the deformation of the lattice and to the temperature-dependent anharmonic self-energy shift were obtained separately. For calculations the authors used the rigid molecule model based on the '6-exp' potential.

Closed nonlinear equations are derived for a self-consistent treatment of density propagation, self-diffusion and current relaxation in a classical monatomic fluid. The solution for a hard-sphere model system brings out a phase transition to a glass at the packing fraction 0.516. Approaching the transition from the glass side the particle mean-square displacement increases to a finite value. A simplified model is analysed in detail. Approaching the transition from the liquid side the diffusivity is predicted to decrease to zero with a power law with exponent 1.76 which the authors find to agree well with some experimental data. The low-frequency density spectrum is found to consist of two contributions; one is an elastic line of the frozen structure on the glass side, which then decays to a narrow diffusion broadened quasielastic peak on the fluid side; the other part is described by a dynamical scaling law and it yields in particular a spectrum diverging at the glass point with certain exponents.

A theoretical and experimental investigation of the low-temperature thermal conductivity of heavily doped p-type semiconductors has been carried out. The concentration dependence of the thermal conductivity for GaSb and Si obtained here is found to be similar to that for other heavily doped p-type semiconductors. Calculations of the thermal conductivity for various p-type semiconductors including GaSb and Si have been done by using the expression for the phonon relaxation rate obtained in the authors' previous paper (see ibid. vol.17, p.2661 (1984)). They can semiquantitatively explain the thermal conductivity of semiconductors with the large spin-orbit splitting. It is shown that the essential features of the concentration dependence of the thermal conductivity are due to the concentration dependence of the shear terms arising from a multiband structure of the valence bands and of the cut-off frequency in the hole-phonon interaction.

The authors present a new method for calculating the density of electronic states on a Bethe lattice which makes use of real-space renormalisation-group techniques. This new method is very simple from the computational point of view and can be used to investigate both ordered and disordered systems. They consider here random binary alloys A_xB_1-x and show that the results they get for the density of electronic states of those systems using renormalisation-group techniques exhibit structures which are not revealed by calculations based on the coherent-potential approximation.

For any lattice temperature T<or approximately=10³K and different charge states Z of the centre the authors describe the dependences of non-radiative multiphonon carrier capture coefficients C^Z(T; E) on the carrier energy E by products of the familiar Sommerfeld factors s^Z(E) with corresponding energy-loss factors C⁰(T; E). As the carrier energy E is increased these 'neutral parts' C⁰(T; E) of capture coefficients decrease or increase (exponentially) in the alternative regimes of small and large lattice relaxation. Their maximum enhancements C_max⁰/C⁰(0;0), which can be produced in the latter regime, at low lattice temperatures, are shown to be approximately exp(S(1+ gamma ln gamma - gamma )) (approaching e^S in the gamma to 0 limit) where S represents the Huang-Rhys factor and gamma the ratio between thermal depth and lattice relaxation energy. For values of S of the order of 10 this 'enhanced hot-carrier capture due to large lattice relaxation' effect can thus amount to several orders of magnitude and should hence be of considerable importance for interpretations of corresponding hot-carrier capture data.

It is now generally accepted that the description of solitons in a classical easy-plane ferromagnetic chain in terms of a sine-Gordon theory is inadequate. The structural and dynamic properties of these solitons are not very clear. The authors present here results of a numerical simulation of the dynamics of a single soliton as well as collisions between a soliton-antisoliton pair. The dynamics of a single soliton appears to be consistent with variational method calculations. The energy dispersion E(u), where u is the propagation velocity, consists of three continuously connected branches. Only the first branch is sine-Gordon-like with an effective soliton mass. A soliton-antisoliton pair collision leads to a variety of final states. As a function of magnetic field B, there are four major regimes. At very low fields, the pair transmit through each other similar to a pair collision for true sine-Gordon solitons. For somewhat higher fields, the pair forms a bound state (breather mode) on collision. Further increase in magnetic field leads to reflection of the soliton-antisoliton pair. As a function of increasing collision velocity u_sG for an initial sine-Gordon pair, the various critical fields in general decrease. Furthermore, there are details in the final state diagram (in the u_sG-B plane) that correspond to resonance scattering (for breather modes) and branch transfer (in the pair collision leading to reflection). Implications of these results for quasi-one-dimensional ferromagnets such as CsNiF₃ and CHAB ((C₆H₁₁NH₃)CuBr₃) are suggested. In particular, they suggest that non-linear elementary excitations in these chains may be breathers rather than isolated solitons.

Mossbauer and X-ray diffraction studies of the tin-substituted zinc ferrites Zn_1+xSn_xFe_2-2xO₄ have been carried out and the cation distribution Zn(Zn_xSn_xFe_2-2x)O₄ has been established using both the methods. ⁵⁷Fe Mossbauer spectra in these spinel solid solutions with x=0.0 to 0.9 are reported at 77K and 295K. The ¹¹⁹Sn Mossbauer spectra have been used to identify the site occupancy of tin and show its presence at octahedral sites only. ⁵⁷Fe Mossbauer spectra show well resolved quadrupole doublets for all the compositions studied. A monotonic increase in the value of quadrupole splitting with increasing tin content is observed. This is attributed to the change in the oxygen parameter of the spinel structure with addition of tin. No detectable change in the value of quadrupole splitting is observed with the change in temperature, which suggests that the quadrupole splitting is mainly of lattice origin. The value of the isomer shift does not change with tin concentration, which indicates that the s-electron charge distribution of Fe³⁺ ions is not influenced by tin substitution. An increase in the value of the isomer shift with decreasing temperature is attributed to the second-order Doppler shift.

The dielectric relaxation data for a range of vanadium oxide/tellurium oxide glasses recently published by Mansingh et al. (1983) has been re-examined and shown to be consistent with the observation of a DC conductivity and a single process of relaxation over the entire ranges of composition, temperature and frequency which have been reported. The relaxation process however has been found not to be of the loss peak form but an example of the low-frequency dispersion process which has been commonly observed in materials containing quasi-free ions, such as beta -alumina. Detailed examination of the temperature dependences of both the dispersion and conductivity processes supports the original proposal that thermally assisted tunnelling of ions governs both the relaxation rate and the conductivity.

The one-magnon Raman spectrum of FeF₂ has been measured as a function of temperature and exciting wavelength. From the acoustic-magnon integrated intensities the authors deduced the ratio r of the quadratic-to-linear magneto-optic coupling coefficients: r was found to increase with wavelength from 0.40+or-0.02 at 457.9 nm to 0.58+or-0.03 at 647.1 nm. The origin of this dependence is unclear. Previous observations of an asymmetry in the acoustic-magnon lineshape have been extended to include measurements at higher temperatures. The asymmetry was found to be independent of the exciting light wavelength and scattering geometry, and increased markedly for temperatures near T_N=78K. A theoretical model based on a frequency-dependent damping mechanism satisfactorily accounts for the observed lineshape for T<or approximately=0.8 T_N. An alternative mechanism, based on light scattering from magnons at surfaces, was concluded to be unimportant for FeF₂ at these wavelengths. Theory and experiment for the temperature dependence of the acoustic-magnon peak frequency were found to be in remarkably good agreement. The theory predicts the behaviour of the three optic modes, as well as the acoustic mode. Two of these modes, near 49 and 59 cm^-1 at 63K, have been observed for the first time and their temperature dependences and intensities were found to be in accordance with theory.

Raman and far-infrared (FIR) measurements have been made on a series of amorphous Ge_xSe_1-x alloys with 0.20<or=x<or=0.52. The concentration dependence of the very strong FIR (Raman) feature at 260(198) cm^-1 is consistent with that predicted by a network of fourfold-(twofold-) coordinated Ge(Se) atoms, with heteropolar bonds favoured over homopolar bonds. Some evidence for intermediate-range order has also been found; in particular it would appear that in a-GeSe (x=¹/₂) the density of GeSe₄ tetrahedra exceeds that predicted by a random-network model.

Velocity-field characteristics for electrons in a quantum well two-dimensional GaAs system have been calculated by the Monte Carlo method. The characteristics are different from those obtained earlier by using the displaced-Maxwellian approximation, but no negative differential resistance is exhibited.

Oscillatory effects in a strong magnetic field B and magnetic susceptibility are investigated, as applied to 2D systems, in which the twofold spin degeneracy is lifted by the spin-orbit-interaction Hamiltonian H_SO= alpha ( sigma *k). nu . The term H_SO is shown to change greatly the usual patterns of B^-1-periodic oscillations; some oscillations are strongly suppressed due to the diminishing of the gaps between adjacent levels, and new oscillations appear due to intersections of levels.