Table of contents

Type IIa diamonds were irradiated below 250K with electrons of 1.50, 0.60 or 0.35 MeV. The radiation-induced thermal resistivity is shown to correlate with the concentration of neutral vacancies as determined by the strength of the GR 1 optical absorption band. The production of vacancies was found to saturate at high doses and to be as large as previously observed after irradiation at about 500K.

The temperature dependences of the elastic stiffness moduli of La₃S₄ have been determined by pulse echo overlap measurements of ultrasonic wave velocities. While the elastic anisotropy is close to unity at room temperature, the shear modulus C'(=¹/₂(C₁₁-C₁₂)) decreases as the cubic to tetragonal phase transition temperature. T₀ is approached: the phase transition is accompanied by substantial softening of the transverse (110), polarised (110) acoustic phonon mode as the Brillouin zone centre. A decrease in C₁₁ as the temperature is reduced towards T₀ shows that the longitudinal (100) mode also softens slightly. Hydrostatic pressure dependences of the elastic constants (including delta C'/ delta P) measured at room temperature are all positive: at temperatures well above T₀ the application of pressure does not markedly enhance the mode softening.

The authors discuss a general formulation for self-trapped excitations in condensed matter physics. Previously known aspects include lattice polarons, spin polarons, bubblons, etc. New aspects involve collective excitation self-trapping; solidons in liquid He have been suggested as the correct description for rotons. They emphasise the difference of self-trapped excitations with order parameter fluctuations.

Results are presented on the ballistic injection of electrons into n⁺ silicon from aluminium point contacts. The principle phonons responsible for intervalley scattering are identified, and it is shown that new scattering modes are introduced by impurities. The introduction of these modes, and the enhancement of the electron coupling to modes always present, depends on the impurity species. Higher-energy structure is observed and is attributed to two closely separated plasmons. The origin of this separation is unclear.

For pt.I see ibid., vol.13, no.26, p.L709 (1980). Ballistic injection of electrons into Al reveals the maxima in the phonon density of states arising from longitudinal and transverse phonons. Weaker coupling to phonons is also found, possibly indicating scattering between bands near a critical point.

For pt.II see ibid., vol.13, no.26, p.L717 (1980). Results are presented on the ballistic injection of electrons into an Si inversion layer at 77K. The results illustrate the expected quenching of low-energy f scattering by the lifting of the valley degeneracy. Phonon series associated with different sub-bands are revealed and the separation of fourfold degenerate sub-bands can be detected.

The authors discuss the low-frequency response of the classical sine-Gordon chain with respect to the lowest-order corrections to the non-interacting soliton/magnon picture. They show that the strength of the soliton-induced central peak is considerably reduced by the interference with magnons. In addition, an appreciable contribution to the central peak is obtained from terms analytic in temperature, which may be attributed to breathers or to anharmonic magnon processes. Quantitative estimates of the results are given and compared to experimental results on CsNiF₃ in an external magnetic field.

The crystal structure if RbH₂PO₄ has been determined by neutron diffraction at room temperature and T_c+5K in the tetragonal (paraelectric) phase using single-crystal samples. The techniques of constrained refinement and statistical testing have been used to examine carefully the significance of some pertinent structural features. The crystal structure has also been determined at 77K in the orthorhombic (ferroelectric) phase using neutron powder-diffraction methods. The results for both phases are compared with those obtained from similar studies of KH₂PO₄. Whilst it is shown that the crystal structures of RbH₂PO₄ and KH₂PO₄ (and the way they change through the transition) are closely similar, some small but important differences are revealed. In particular, the separation of the 'disordered' proton sites in the tetragonal phase is significantly larger in RbH₂PO₄ (by approximately 0.05 AA); yet the hydrogen-bond lengths differ by only 0.003(2) AA, and the transition temperatures by only approximately 20K.

It is shown that the crystal structures of the binary IV-VI compounds can be classified by parameters derived from a Pauli-force model potential, which has previously been used successfully for the octet and suboctet materials (A^NB^P-N with P=8 and 2<or=P<or=6). Trends in the transverse effective charges, in the heats of formation, in the energy gap between bonding and antibonding states, and in bond angles and bond lengths can also be explained with these parameters. The author discusses the origin of these correlations in terms of the band structure and atomic pseudopotentials, and demonstrates how a microscopic analysis of the crystal structure leads towards a quantitative understanding of the cubic to rhombohedral phase transition in the (Ge, Sn)Te system.

For pt.I see ibid., vol.13, no.26, p.4855 (1980). A semi-empirical theory is developed for the cubic to rhombohedral phase transition in A^NB^10-N compounds, based on the heats of formation and pseudopotential band-structure parameters. Calculations of the rhombohedral shear angle and the internal sublattice displacement in the rhombohedral phase of GeTe, SnTe as well as the group V materials As, Sb and Bi are in good agreement with experiment. A mean-field theory is employed to calculate the phase transition temperature T_c in the IV-VI materials, and the pressure coefficient dT_c/dp.

The author derives a simple expression for the average polarisation P induced by an optic phonon within the Born-Oppenheimer approximation. The calculation for P requires a knowledge of the electronic wavefunctions and energies in both the undistorted crystal and the distorted crystal with a 'frozen-in' optic phonon. The expression is correctly boundary-insensitive and is the proper starting point for the calculation of both linear and non-linear contributions to the polarisation, i.e. effective charges. Specifying a pseudopotential formalism and taking the small displacement limit the author recovers the formula of Vogl (1978) for Z_T^*. Finally a simple model of a diatomic chain of atoms is discussed to show explicity that anharmonic effects are important if Z_T^* is large.

Thermal expansions have been measured from 2 to 90K on four compounds of fluorite structure. Data at the lowest temperatures lead to the following limiting values of the Gruneisen parameter: gamma ₀^thermal=1.0 (CaF₂), 0.64 (SrF₂), 0.23 (BaF₂) and 0.5-0.8 (PbF₂). These agree within limits of error with those calculated from the pressure dependence of elastic moduli. For the three alkaline earth fluorides gamma (T) increases monotonically with temperature from gamma ₀ to gamma ₂₉₃ approximately 1.5-1.8, but for PbF₂ there is a marked maximum near 25K where gamma approximately=3.5, consistent with the effect of an optic mode of characteristic temperature theta _E approximately=68K.

The master-equation approach to transport in disordered systems is considered. The diffusion of excitations is treated within a self-consistent scheme. This has been shown previously to lead to very satisfactory results when exponentially varying jump rates are used. The effective-medium idea is extended to treat diffusion., limiting trapping or relaxation in amorphous networks. The theory is compared with some of the predictions made using the 'continuous time random walk' model of trapping. The method is also applied to the problem of spin relaxation in amorphous semiconductors in the hopping regime.

The diffusion and trapping of excitations in disordered systems is considered. The problem is treated in the framework of the master equation using the Forster transfer mechanism. The diffusion and trapping equations are solved using a self-consistent effective-medium approach for which analytic results can be presented. The results are compared with experimental data on fluorescence quenching in solutions of chlorophyll.

SnO₂ single crystals have been grown by use of vapour-phase reaction method. The crystals obtained are characterised by chemical analysis and electrical measurements. The density of the crystals was determined as 6.994+or-0.001 g cm^-3. The thermal conductivity measured is discussed for different orientations of the crystals and as a function of temperature. The thermal conductivity parallel to the c axis is 0.98 WK^-1 cm^-1 at room temperature and 17.5 WK^-1 cm^-1 at 26K: the ratio of thermal conductivities parallel and perpendicular to the c axis is lambda /sub //// lambda _{perpendicular to} =1.78 at 300K and varies with temperature.

The pressure dependence of the energy spectrum (bands, density of states (DOS) and core levels) of trigonal Se up to 140 kbar is calculated within the X_alpha scheme ( alpha =1 and ²/₃) using the self-consistent symmetrised orthogonalised-plane-wave (SCSOPW) method. The minimum optical gap at zero pressure is from M₂ to H₁ and the only pressure coefficient ( delta E_g/ delta P)_P=0 agreeing with experiments is associated with the same transition. The valence band maximum shifts at 33 (23) kbar for alpha =1(²/₃) from M₂ to H₃ and the pressure coefficient doubles, which unambiguously explains the anomaly found to the experimental absorption edge at 35 kbar. At zero pressure the SCSOPW DOS and the core level spectrum agree well with the X-ray photoemission results. The pressure-dependent behaviour of the energy spectrum is presented and analysed.

Theoretical calculations are presented of the hyperfine and superhyperfine interactions of H_s,a⁰ centres (atomic hydrogen at anion site) in KCl in various models. Ligand polarisation by unscreened charges proves very important for the interpretation of the experimental data. The experimental hyperfine and superhyperfine interactions can be explained satisfactorily by a configuration mixing of the states H⁰-Cl^--K⁺, H^--Cl⁰-K⁺ and H^--Cl^--K²⁺, in which ligand polarisation effects, localised vibrations of hydrogen and the displacements of nearest neighbours due to the charge of the anion vacancy are taken into account. The results illustrate what may be a general phenomenon: modest charge transfers from weak covalency can have their main effect by polarising neighbouring ions.

A theory of impurity-related transient photoconductivity in semi-insulating semiconductors is developed. When a photoconductor is illuminated in an applied electric field, the initial sweep-out of carriers generates space charge at the active centres within the crystal. This space charge is neutralised by injection from contacts, but the injected carriers are partially trapped leading to the possibility of current oscillation with frequency determined by a dielectric relaxation time and a trapping time. The theory of this effect is developed for a simple model involving a deep donor and compensating shallow acceptor. Effects of additional traps on the transients are also discussed. If extra traps with large capture cross sections are present near the quasi-Fermi energy, they can damp the oscillations. Such traps can also strongly affect the transient shape, introducing dependence on optical chopping frequency and temperature.

The random-bond mixture of the Ising model on the FCC lattice is treated by the tetrahedron approximation. The phase diagram of the binary mixture of the ferromagnetic (F) and antiferromagnetic (AF) bonds are determined. The phase diagram of the ternary mixture of F, AF and the non-magnetic (N) bonds is also calculated by the simple version of the tetrahedron approximation. It is shown that they have the paramagnetic, ferromagnetic and spin glass (G) phases. In particular it is shown that the diluted antiferromagnet has no AF phase but a G phase. The phase diagram of the binary mixture is qualitatively similar to those for Eu_xSr_1-xS, Co(S_xSe_1-x)₂ and (Ti_1-xV_x)₂O₃.

The static two- and four-spin correlations for the one-dimensional easy-plane Heisenberg spin system with a ferromagnetic coupling, like CsNiF₃, are investigated in the classical limit by means of the transfer matrix method. They are found to consist essentially of two components which both depend exponentially on the distance between sites. Both in the high- and low-temperature regions, only one component remains and this component still dominates at intermediate temperatures. Analytic forms for the correlation functions are given for asymptotically low temperatures. These are obtained by calculating the second-frequency moment of the dynamical structure factor in three different ways, and by requiring the T=0 limit of the moment to coincide with the square of the spin wave frequency. Generalisations of the Tomita-Mashiyama recursion relations (1972) for the isotropic case are given, valid for low temperatures, Monte Carlo calculations of the correlation functions have also been performed and some results of these are presented as well.

The replica method is applied to Ising spin systems with random interactions. An effective Hamiltonian is derived for an arbitrary distribution of random bonds and the thermodynamic variational principle is used to evaluate the mean-field free energy. In the general case, the free energy depends explicitly on an infinite set of momenta ((s_i)^k). For specific types of bond randomness results obtained previously are recovered. The phase diagram of the dilute Ising model with competing nearest-neighbour and next-nearest-neighbour interactions is worked out in detail. A spin glass phase is predicted at large enough dilution.

For pt.I see ibid., vol.13, no.26, p.5033 (1980). The replica method is used to study the behaviour of random Ising models in the Bethe approximation. In the limit n to 0 the physical quantities depend on an infinite set of effective fields which have to be determined self-consistently. Explicit conditions are obtained for the critical temperature and agree with previous results for specific types of randomness. The dilute Ising model with competing interactions is discussed in detail. Unphysical results are obtained in a range of values of the ratio R between next-nearest-neighbour and nearest-neighbour interactions.

The authors report novel ultra-sharp satellite luminescence lines when excited states of neutral-donor bound excitons (BE) are resonantly excited in ZnTe by a narrow laser line. The spectral position and magnetic characteristics indicate 'two-electron' transitions, leaving the donor in a series of ns and np excited orbital states. Optical pumping of the inhomogeneously broadened lines in the best crystal provides an almost tenfold linewidth reduction compared with normal BE transitions, toward a value comparable with both the laser line and the homogeneous lifetime broadening of the BE. This narrowing is greatly advantageous in the difficult task of distinction between donor species in medium to narrow direct-gap semiconductors. Orbital and spin magnetic splittings provide accurate values for electron effective mass and g value. The intensity ratio of transitions to s and p orbital states yield information on the character of the BE excited states, in agreement with recently published data of Romestain and Magnea (1979).