Table of contents

The authors report molecular dynamics simulations of structure, fluctuations and diffusion in a crystal, modelled by pairwise forces, in which a symmetry-breaking phase transformation, resembling certain martensitic processes, occurs as the temperature is lowered.

The matrix reduction method developed in an earlier paper is recast in a form useful for the study of layered superlattice structure. In particular, a rather general theorem relating the eigenvalues of a superlattice with those of the component lattice is proved for the model system of layered structure. Its most interesting feature is that the vibrational frequencies of the component lattice remain as those of the superlattice, consisting of repeated components, independent of the coupling strength between adjacent components. This analytical result is confirmed numerically by a series of frequency computations for the layered structures of type AB, ABAB and ABABAB, carried out without invoking the theorem.

The authors have measured the density of states in the tails of the Landau levels of GaAs-Al_xGa_1-xAs heterostructures by analysing the variation of the Fermi level position as a function of the magnetic field. The experimental data are best described by a gaussian-like density of states superimposed on a constant background density of states.

The authors report the results of variational estimates of the binding energies and oscillator strengths of excitons in GaAs-Ga_1-xAl_xAs quantum wells.

Highly excited states of the 0.79 eV luminescent defect observed in photoluminescence excitation (PLE) measurements are interpreted as effective-mass (EMT) states of a pseudo-donor with E_i=38.3 meV. The interpretation implies that the 1s ground state of the donor electron is fivefold split in the C_1h symmetric strain field of the defect. Modelling of the internal deformation around the defect by a compressive uniaxial field of 80 MPa along (001) allows the authors to explain the ground state splitting quantitatively and gives rise to a re-interpretation of published uniaxial stress data. Transient decay data suggest that the lowest excited defect state is a split singlet-triplet bound exciton, the 0.79 eV line being emitted by the higher-energy singlet.

A relation between two alternative parametrisations describing anisotropic two-electron interactions is obtained. This clarifies the role of the spin-correlated crystal field parametrisation, allowing the authors to express it as a simple truncation of one of the alternative two-electron parametrisations.

Zero-point oscillation anharmonicity induces the softening and then occurrence of the instability of the low-frequency mode omega _- (at long-wave limit) of 2D electron crystal in quantised magnetic fields. 2D electron and hole crystals melt at nu _e^c=0.12(6) and nu _h^c=0.87(4) respectively ( nu is Landau level filling factor). The region where the fractional quantised Hall effect may exist is 0.13<or= nu <or=0.87 and is consistent with experimental data. The softening of the shear modulus mu ( nu ) and modified melting parameter gamma ( nu ) is linear at nu << nu _e^c( mu = mu ₀(1-1.866 nu )), near nu = nu _e^c the dependences is more rapid and at the instability point mu ( nu _c)/ mu ₀=0.543, gamma ( nu _c)=0.08.

The authors show that the quasiparticle scattering time, due to nonmagnetic impurities diverges at zero energy in anisotropic superconductors with zeroes in the order parameter at points and on lines on the Fermi surface. They assume S-wave scattering from the impurities and carry out the calculations in the weak coupling limit. As a result of the energy dependence of the scattering time, the quasiparticle mean free path diverges at zero energy. This is in contrast to the BCS result where the mean free path is equal to its constant normal state value. The consequences of this for the ultrasonic attenuation are discussed.

An exact proof is given that Edwards-Anderson order exists at T=0K in any number of dimensions and for all lattices for a very wide class of continuous coupling distribution functions. The difference between continuous and discrete distribution functions is discussed.

The authors report ⁷⁷Se NMR data on (TMTSF)₂ClO₄ showing that the field-induced spin density wave state is pushed to higher fields under pressure. The pressure dependence of the FISDW state suggests a common phase diagram for all (TMTSF)₂X systems. The change of the 1/T₁ temperature dependence occurring under pressure can be understood as a sizeable growth of longitudinal and transverse bandwidths which decreases the effects of Coulomb correlations.

The high-frequency limit of the correlation functions which determine the nuclear spin relaxation rates due to magnetic dipolar coupling between diffusing spins in a crystal has recently been calculated by MacGillivray and Sholl (1985) for the simple hopping model. It is shown that a modification of the analysis produces more accurate results for the high-frequency limit.

A proton magnetic resonance experiment known as spin polarisation torsional spectroscopy is reported for a sample of beta -phase palladium hydride with a hydrogen concentration of 0.71+or-0.015. The experiment was carried out at a resonance frequency of 30 MHz and at a temperature of 40+or-0.5K. The transient signal detected in the time domain after the spin-locking sequence and the resulting frequency spectrum obtained by Fourier transformation give definitive proof of the occurrence of low-frequency tunnelling involving groups of two or more protons in this metal hydride sample.

The authors report the first measurements of the dielectric function for well characterised metal-insulator composite materials. These composite materials were prepared by a novel sputtering method and consist of uniformly sized (nanometre diameter) metal spheres embedded in the insulator matrix. The dielectric functions were measured by electron energy loss spectroscopy (EELS) and optical methods over the energy range 0.4 to 40 eV. No existing theory satisfactorily explains the results.

Near-band-edge luminescence in polythiophene has a short lifetime of <or approximately=9 ps at 4K owing, it is suggested, to competition by fast nonradiative processes. The decay of luminescence in polydiacetylene (1OH) at 4K is somewhat slower, and much more complex.

The theoretical model and computer program (AMPITS) of Ashworth and coworkers (1984) for the calculation of projected range distributions of implanted ions in multielement multilayer amorphous targets are extended to provide a new method for the derivation of three-dimensional implantation distributions in single-layer targets. The method involves the computation of the chord range distribution and this, together with the projected range distribution, enables the lateral standard deviation to be evaluated. Computed values of lateral standard deviation are found to agree well with those derived previously by Furukawa and Ishiwara (1982). Three-dimensional distributions are constructed using the projected range and chord range distributions together with the lateral standard deviation in the computer program AMPITS-3D.

The random-field Ising model is analysed using a self-consistent approximation. With this approximation there is no phase transition in the presence of a random field and the inverse correlation length kappa remains finite upon lowering the temperature below the transition temperature in d<4 dimensions. The inverse correlation length has the form kappa varies as H^{nu (H)} with nu _(H)=2 in d=3 dimensions for temperature T to 0(d=2: nu _(H)=1) and nu _(H)=²/₃ for T=T_N(d=2: nu _(H)=¹/₂). These results are compared with the results of neutron scattering measurements on three- and two dimensional diluted Ising antiferromagnets.

It is shown that in the limit of a very dilute gas the approaches of Kosterlitz and Thouless (1973), Kosterlitz (1974) and Jose et al. (1977) to topological order in two dimensions lead to the same scaling equations. Young's proof (1978) is thus concluded.

The electron-vibronic band with its no-phonon (NP) transition at 0.9351 eV which was recently detected in heat-treated silicon is strong after irradiation of silicon and has been studied in high-resolution photoluminescence. The defect exhibits a quasi-local mode of quantum energy h(cross) omega =66.3 meV which is observed as one- and two-phonon vibronic sidebands. The existence of an electronic excited state at an energy of 1.8 meV in excess of the upper level of the NP transition is demonstrated by temperature-controlled experiments. Uniaxial stress and Zeeman measurements show that the defect possesses rhombic I (C_2v) point group symmetry, and that the NP line is a 'spin-1/2 to spin-1/2' ( Gamma ₅ to Gamma ₅) transition. The stress response is nonlinear due to the mixing of the two electronic states by the perturbation. Piezo-optical splitting and interaction parameters are determined. The g-values are g_isotropic=2 for one level of the NP line and g_x=3.65, g_y=1.5 and g_z=0.8 for the other level suggesting that the defect is a pseudo-donor with a tightly bound hole and a loosely localised electron. A carbon isotope effect is observed in the NP transition and in the local-mode satellite giving evidence that one carbon atom per optical centre is involved. Sample statistics and a boron-specific vibronic satellite (h(cross) omega =29.4 meV) suggest that boron is also involved.

Analytical approximations for the ground state energy of the polaron and for its effective mass are obtained throughout the entire range of the electron-phonon interaction constant. The detailed structure of the phase transition in the polaron system is considered on the basis of these results. Numerical calculations corroborate the analytical conclusions.

The photoionisation energies of thermally induced levels (H1 and H2) in a quenched n+p silicon diode are measured by the DLOS technique. The photoionisation cross sections are well described by the Lucovsky model. The measured photoionisation energies (E_v+0.42 eV and E_v+0.52 eV) confirm the thermal activation energy determined by the DLTS method.

The no-phonon structure of ⁵E to or from ³T₁ absorption and luminescence transitions within an Fe²⁺ ion in ZnS was measured. Both the temperature and uniaxial stress effects were investigated. The model involving dominant Jahn-Teller coupling of the ³T₁ term with the epsilon mode of vibrations describes correctly observed positions, shifts under uniaxial stress and selection rules of no-phonon lines.

The Boltzmann equation description of transport in an electron gas in a narrow-channel system when many sub-bands are occupied is derived from a Green function calculation. The approximations implicit in the Boltzmann equation are exposed and discussed. It is shown that the approximations fail when the Fermi energy is near the origin of any one of the sub-bands. An improved calculation indicates why the discontinuities predicted by the Boltzmann equation (quantum-size effects) are rounded in experimental results. The calculations suggest that for systems with k_Fl>or approximately=4-5 in the electric quantum limit dips in the conductivity occur as new sub-bands are populated. Below k_Fl approximately=4 the dips are broadened into plateaus which are expected to be difficult to observe.

The energy loss of a slow charged particle in a semidegenerate plasma is calculated using analytical approximations. Previous results obtained by various authors are compared with these calculations. An analytical expression describes the stopping power for low velocities, over a wide range of plasma densities and temperatures. The description applies from the cold limit of solid state plasmas to the hot and dilute extreme of Tokamak plasmas.

The electron states of a thin isolated layer of HgTe in CdTe are considered using a many-band k.p Hamiltonian. These states are obtained by a transfer matrix technique and the bound states identified by the associated singularities of this transfer matrix. It is found that the heavy hole states are similar to those of GaAs/GaAlAs quantum wells and that the interfaces introduce strong mixing of heavy and light hole states. However, in the present case, single interfaces support bound states which in the quantum well lead to light particle bound states with large amplitude at the interfaces. The implications for quantum well mobilities and interface stability are pointed out.

Duality and scaling techniques are applied to a N*N Josephson junction array to calculate the static Josephson current response at a junction ij, due to an applied transverse vector potential at kl. The long-wavelength response function vanishes suddenly above a Kosterlitz-Thouless transition temperature T_KT, due to the onset of incoherence in the effective Josephson phase. The zero-frequency inverse kinetic inductance and the helicity modulus thus also drop at T_KT, consistent with recent Monte Carlo calculations.

The author calculates the response of an N*N Josephson junction array to an oscillatory transverse vector potential. Duality transformations, and a dynamic version of Kosterlitz real space scaling, are used with the resistivity shunted junction model as a starting point. The long-wavelength low-frequency conductivity sigma (q to 0, omega , T) identical to sigma ₁+i sigma ₂ is calculated, for T near T_KT, the Kosterlitz-Thouless transition temperature. For fixed frequency, sigma ₁( omega ,T) peaks at, and sigma ₂( omega ,T) falls off at, a frequency-dependent T=T_w>T_KT. In the limit of mod T-T_KT mod /T_KT<<1,(1n( Gamma ₀/ omega ))^-1<<1, dispersion relations and a sum rule are approximately satisfied. The sigma _1,2( omega ,T) behaviour is similar to the (phenomenological) response function of helium films to an oscillating substrate, with the difference that the quantum phase relaxation rate Gamma ₀ is not a fitted parameter, but is related to the isolated-junction resistance.

A Monte Carlo calculation of the temperature-dependent inelastic neutron scattering from Ising systems composed of mixtures of spin-¹/₂ and spin-1 or spin-⁵/₂ constituents is made. The phase boundary of critical temperature versus spin-¹/₂ concentration is determined. Experimental data on the mixed systems Rb₂Co_xNi_1-xF₄ and Rb₂Co_xMn_1-xF₄ for 0.41<x<1.0 are found to be in good agreement with the results of this simulation.

A generalisation of the Sherrington-Kirkpatrick Ising spin glass model to spins S^z=0,+or-1, . . . +or-S with exchange J_ijS_i^zS_j^z, where the J_ij are randomly distributed, and crystal field D(S_i^z)², is examined with regard to the character of its transitions and the stability of its phases, in order to clarify and correct earlier work. Both continuous and discontinuous transitions are found. Conventional Parisi theory with renormalised parameters suffices near the continuous transition away from the tricritical point, but near the tricritical point and the first-order transitions, new effects are predicted.

Precise measurements of the X-ray attenuation coefficient of crystalline silicon have been made in the energy range 25 to 50 keV. The results are compared with theoretical predictions and earlier measurements. A systematic discrepancy between theory and experiment is observed. The most likely cause of the disagreement is thought to be the uncertainty in the estimation of the thermal diffuse scattering cross section.

As a method of studying the phonon scattering mechanisms that occur at crystal boundaries and interfaces, the thermal conductivity of water geometry sapphire substrates has been measured. A model has been developed that enables a phonon specular reflection coefficient to be determined for a particular surface when the sample surfaces are inequivalent. This model has been applied to sapphire surfaces that have undergone three types of treatment; highly polished, roughened and coated with epitaxial silicon. The authors conclude that for the most carefully prepared sapphire surface the dominant mechanism of phonon scattering is from surface asperities of near atomic dimensions while the coated surfaces show scattering from vibrating dislocations within the silicon layer. They estimate the mean dislocation pinning lengths to be of the order of 10^-6 cm with a resolved shear stress factor Omega approximately 10^-4.