Table of contents

Several author have used the isotropic Heisenberg Hamiltonian to model spin-glass behaviour, the purpose of the model being to produce mod (S_i^z) mod not=0 without the presence of long-range spatial order. The letter proves that there can be no spin-glass in one or two dimensions for isotropic Heisenberg Hamiltonians for T not=0 under certain conditions.

The results of low temperature measurements of inversion layer conductance suggest that there are positive and negative charges in the form of pairs close to the Si-SiO₂ interface. The negative centres trap holes created in the SiO₂ by the irradiation of MOS structures. The annealing treatments developed to 'harden' the interface, by minimising the hole trapping are interpreted as resulting in a reduction in the total interfacial charge, which is not apparent from measurements of the net charge. It is suggested that the dependence of the localisation effects on the substrate bias may be useful as a diagnostic, pre-irradiation, screening test. By using various interface preparation treatments an exercise in interface engineering is now possible, in which the total interfacial charge, and the form of the random fluctuations in potential, can be altered in a controllable manner.

The magnetic susceptibility of LiH has been calculated employing linear combinations of gauge invariant atomic orbitals. It is shown that a proper account of overlap effects leads to a magnetic susceptibility in good agreement with experiments.

The temperature renormalisation of the spin wave stiffness is shown, within the non-linear sigma -model of a ferromagnet, not to be modified at order epsilon by the inclusion of mode-coupling terms.

Mossbauer spectra of K₂FeF₅ show that it undergoes an ordering to an antiferromagnetic state at 11.2+or-0.5K. An anomalously low value (41.0+or-0.5T) of the saturation hyperfine field for a high-spin ferric ion is observed. This is interpreted as resulting from zero-point spin reduction.

The origin of thermoluminescence in dysprosium doped CaSO₄ phosphor has been attributed to the reduction of RE³⁺ to RE²⁺ by the capture of an electron in the irradiation process and its reconversion back to RE³⁺ in the heating process of the thermoluminescence, the reconversion being due to a thermally released hole combining with the electron at the RE²⁺ and leaving the residual RE³⁺ in excited states. The author attempts to verify the above hypothesis by studying the fluorescence of CaSO₄:Eu since Eu exists in both the divalent and trivalent states in the CaSO lattice and emits fluorescence in both states.

The random ordered phase (ROP) proposed in a previous paper by the authors (J. Phys. Soc. Japan, vol.40, p.1513 (1976)) is investigated in a Bethe lattice. The critical temperature T_c, the random susceptibility chi _r and the uniform susceptibility chi _u are obtained rigorously by use of the effective Hamiltonian method. chi _r diverges at T_c, while chi _u makes a cusp at T_c. The spin glass phase in the Bethe lattice is discussed. It is concluded that the phase transition takes place between paramagnetic phase and ROP, and neither ferromagnetic nor spin glass phase can exist in the Bethe lattice.

The fixed points of the exact renormalisation group equations with smooth cut-off are determined in the epsilon -expansion for systems characterised by mn-component vector order-parameters. The critical behaviour of quenched random n-component spin systems is obtained by taking the limit m=O. The critical exponent is obtained to order epsilon ², and the other exponents to order epsilon . A derivation of Nelson's trajectory-integral expression for the free energy, appropriate for the case of smooth cut-off renormalisation group equations, is presented.

For pt.I see ibid., vol.10, p.2985 (1977). The crossover function for the specific heat of a disordered material is constructed to order epsilon by solving the renormalisation group equations for the m n model (m to O,n>1). From this an effective exponent is obtained which provides a local measure of the degree of singularity of the specific heat in the critical region. It is seen that as the critical region is approached from above the critical temperature the effective exponent behaves initially as if no disorder were present. But as the critical temperature is more closely approached the effective exponent crosses over to the value predicted for the disordered material.

The macroscopic conductivity defined by De Gennes is calculated for a Cayley tree of random resistors. The conductivity exponents are t=3 and s=(log). The implications for the dimensionality dependence of the conductivity exponents is discussed.

Molecular dynamics simulations of molten caesium halides based on polarisable ion interionic potentials which are derived from solid state data are presented. The most interesting feature of the partial radial distribution functions is the emergence of a subsidiary peak in the like-particle functions for CsCl, CsBr and CsI. It is argued that this structure is due to ion-size effects and, although related to the differences found in the stable crystal structures, does not arise from this directly.

The effect of thermal treatment on the thermoluminescence peaks of X-ray irradiated KCl:Sr²⁺ and their isothermal decay have been investigated. A new characteristic decay behaviour has been found for the curves of the main peak (408K). It is shown to be sensitive to impurity aggregation which can be dispersed by thermal treatment at 393K. From the optical absorption and dielectric loss measurements, it is suggested that interstitial traps are responsible for the decay behaviour. A quantitative explanation of the decay behaviour is obtained by using rate equations based on the two types of the trapping centre of interstitials. An emission band at 430 nm was observed, its spectrum being unchanged by thermal treatments.

The defects are observed in the optical spectrum of irradiated diamond. TR12 has many very sharp phonon replicas in absorption, but fewer in photoluminescence. During electron irradiation at room temperature the defect is produced initially as the cube of the dose. It can be induced only in type II diamonds, and is probably an interstitial complex. The photoluminescence spectrum of 3H has been recorded for the first time. The defect is quite distinct from the H3 and S1 centres, in spite of the similarity in energy of their zero phonon lines.

The group theoretical analysis of lattice vibrations in GaS and neutron inelastic scattering measurements of acoustic phonons propagating along the Delta , Sigma and T directions are presented. The elastic constants C₁₁, C₃₃, and C₄₄ are derived and the results for the Delta direction are analysed to determine interlayer force constants. The frequencies of the B₁ modes along Sigma and T show a weak quadratic dependence on wavevector. A simple interatomic force model is fitted to the phonon frequencies, and the frequency distribution function is calculated. The temperature dependence of the lattice specific heat and of the corresponding Debye temperature is calculated; over a small temperature range the variation of the specific heat is characteristic of a pseudo-two-dimensional solid.

For pt.I see ibid., vol.10, p.1641 (1977). A recent theory for the heat of transport in solids is applied to the case of vacancy diffusion in the simple cubic lattice. The harmonic theory of diffusion is used. The theory proceeds via the calculation of the energy moment and current in the limit of large times starting from a specified initial condition. An additional insight into the dynamical processes involved is given by computing the moment and current for general times. Accurate numerical values (within the harmonic approximation) for these quantities and for the model heat of transport Q are obtained. One contribution to Q is the three-dimensional analogue of Schottky's linear chain result. The contribution is an important one. This result contradicts Huntington's argument, according to which the Schottky contribution is insignificant in three dimensions. Huntington's argument (1968) may be fallacious.

A closed expression for the energy of a fermion system is found using a Jastrow product wavefunction. The energy is expressed in terms of the two-particle and three-particle distribution functions. Using the Kirkwood superposition approximation for the three particle distribution functions and a cluster expansion to derive the fermion two-particle distribution function from that of mass-three bosons the energy of ³He has been evaluated. The results are in reasonable agreement with experiment and with those obtained by the cluster expansion technique. An attempt is made to assess the error introduced by the super-position approximation by making the same approximation for a boson system.

A closed expression for the energy of a fermion liquid is used to derive the surface energy of liquid ³He. The wavefunction of the ³He particles is taken to be a completely antisymmetric Jastrow product describing the uniform system and a variational function which modulates it at the surface. The Fermi statistics have the effect of thinning the surface relative to that of an equivalent mass-three boson system in contrast to the thickening which results from the change of mass from mass-four to mass-three. The surface energy found for ³He is 0.150 erg cm^-2, in reasonable agreement with experiment. The surface thickness is predicted to be approximately 3.7 AA, thinner than the equivalent result for ⁴He (about 5 AA).

Distortions in the superfluid order parameter around a small object in ³He are calculated, together with the supercurrents and the angular momentum induced by it in the liquid. The forces acting on the impurity due to the liquid texture structure are also considered.

A technique for reduction of the Dirac equation, which initially omits the spin-orbit interaction (thus keeping spin as a good quantum number), but retains all other relativistic kinematic effects such as mass-velocity, Darwin, and higher order terms is presented. The spin-orbit interaction can be included as a perturbation once the 'relativistic' spin-polarised bands and wavefunctions have been obtained. The technique is used together with the local spin density approximation for exchange and correlation to calculate the self-consistent charge and spin density of a neutral Gd atom. The calculated magnetic form factor agrees extremely well with experiment. Comparison with a paramagnetic RAPW calculation shows the procedure should be accurate and fast for general band structure determinations.

Measurements are reported of low-field resistivity and Gunn effect threshold field as a function of (100) and (111) uniaxial stress for a number of epitaxial and bulk GaAs samples. The results provide the first direct evidence that the L_1c minima are situated lower in energy than the X_1c minima and are therefore involved in the operation of GaAs transferred-electron devices. Intervalley separations are obtained at 300K, but these are dependent on density-of-states values. Further information on band structure parameters such as intervalley coupling constants should now be obtainable by fitting the high-field data to Monte-Carlo calculations currently in progress. Deformation potentials for the satellite minima have been determined.

The band structure and wavefunctions of TiC are calculated in the HFS model using the discrete variational method and several of the Slater exchange parameter alpha . These wavefunctions are then used to compute the momentum density and directional Compton profiles of TiC. Principal deviations from spherical symmetry in the Compton profile are found in the (100) direction and good agreement between theoretical and experimental Compton profiles along the (100), (110) and (111) directions is obtained when alpha =1.0. The band structure for alpha =1.0 is also in good agreement with other experimental data and APW calculations. Finally, the spherically averaged Compton profiles and momentum densities of the valence electrons of TiC and several other materials with the NaCl structure are compared.

The Green function for a crystal can be found by matching the KKR Green function onto an atomic Green function over the surface of an atomic cell. The band structure can be found by this method. The atomic sphere approximation which uses an energy-independent KKR Green function can be applied to this method.

The Edwards-Anderson mean-field theory for a spin glass is extended to the case of a general S Ising model with random exchange and uniform but variable crystal field energy. The model exhibits anisotropic behaviour of the static susceptibility.

A study has been made of the neutron magnetic scattering by single crystal FeCO₃ in the antiferromagnetic phase at 4.2K. Polarisation analysis has been used to determine the absolute direction of the magnetic interaction vector for several low angle (hhl) reflections. The presence of equal proportions of the two 180 degrees domain types in the crystal used has prevented a direct observation of deviations in the directions of the interaction vectors due to spin-orbit coupling. However, an indirect measure of these deviations could be obtained from the degree of depolarisation introduced by scattering. The results obtained are in qualitative agreement with theoretical predictions. Uncertainties intrinsic to the technique used and the sample have prevented quantitative comparison, but it is suggested that more precise results could be obtained using a development of the present technique. Some features of the occurrence of different kinds of antiferromagnetic domains are discussed.

As a step in the calculation of gradient terms to the exchange and correlation energy in the spin-density functional theory, the authors consider the perturbation expansion of this energy for a paramagnetic electron gas. It is shown that the two relevant kernels can be expressed exactly in terms of the density and magnetisation dependence of the pair functions for the inhomogeneous gas. A method based upon the fitting of the third frequency moments of the density and spin-density response functions is used to calculate the perturbation kernels for a range of electron density. It is shown that the correlation contribution to these kernels can be substantial.

The single-particle phonon propagator is calculated for a longitudinal acoustic mode in an extended Kobayashi model of KH₂PO₄, for temperature just above T_c. The equations of motion for the Green function are decoupled by expressing the four-spin correlation in terms of pair correlations. A second approximation involves use of a previous result for the two-spin Green function which invoked random phase decoupling, so that the results are valid only outside the critical region. With these approximations, we calculate the phonon self-energy to O(H_d²) where H_d, the anharmonic term in the Hamiltonian which gives rise to three-phonon damping, stems from the strain-dependence of proton pseudo-spin coupling. Both real and imaginary parts of the phonon self-energy are obtained, yielding an absorption coefficient for longitudinal sound which is proportional to ((T-T₀)/(T-T_c))⁵2/, where T₀ is the temperature at which the soft polar mode frequency extrapolates to zero. The sound velocity should increase as T to T_c+, although rough numerical estimates suggest the increase may not be large outside the critical region.

Families of superconvergent relations for the normal reflectivity function are given. Sum rules connecting the difference of phases of the reflectivities of two materials are also considered. Finally superconvergence relations and sum rules for magneto-reflectivity in the Faraday and Voigt regimes are also studied.

A simple Fourier series algorithm is presented for the determination of the phase, and hence the other optical constants from measurements of the reflectance. The advantages of the proposed procedure and its mathematical equivalence with the Kramers-Kronig relations are discussed.

Thermoluminescence experiments on KBr, electron irradiated between 80K and room temperature, yield thermoluminescence activation energies which increase with increasing irradiation temperature. All specimens show the same optical F-centre absorption spectra but the interstitial defect absorption spectra varied with irradiation temperature. Contrary to Hageseth, who suggests electron transfer from the F-centre as the initial activated step, the authors propose that the liberation of mobile interstitials from interstitial aggregate centres is the rate-controlling, thermally activated step and that the different activation energies observed are due to the different interstitial aggregates produced at the different irradiation temperatures.

In Lytle's theory the observed absorption maxima correspond to the allowed energy states E of the ejected electron in a spherical well of infinite depth; E=const Q, where Q are the zero roots of the spherical Bessel function. The observed plot of E against Q fails to pass through the origin of the non-metallic compounds. This departure is accounted for by modifying the Lytle potential to include the effect of coulomb attraction felt by the escaping electron in the vicinity of the absorbing atom bound in a molecule. The depth of the appended spherical well, representing the effective coulomb force, is correlated with the bonding energy through electronegativity considerations.