Table of contents

In order to describe the electron motion in deformed lattices, the Frohlich-Mitra modified tight-binding approximation (1968) has been generalised. The unperturbed electronic wavefunction is written as a Bloch sum of displaced and suitably orthonormalised Wannier functions so as to take partly into account the adiabatic followup of the electron upon deformation of the lattice. It is then shown that the electron-phonon interaction matrix elements in this scheme are identical with those given by the ordinary Bloch theory.

The average kinetic energy per atom, (KE), is determined for liquid ⁴He from an analysis of low-order frequency moments of the dynamic structure factor, S(Q, omega ), obtained from neutron-scattering experiments with wavevector transfers in the range 60<or=Q<or=80 nm^-1. It is found that (KE)/k_B has the value 13.5+or-1.2K at T=1.1K and 14.1+or-1.0K at T=4.2K.

The electronic structure of FeAl, CoAl and NiAl is discussed in terms of cluster approaches. For NiAl a comparison to a self-consistent APW band structure calculation is shown.

Using a formulation of the dilute bond Ising model in terms of an equivalent Hamiltonian described recently by Bidaux, Carton and Sarma (1976), the authors study the case of an antiferromagnetic nearest-neighbour interaction. The appearance of a spin-glass phase is observed if the antiferromagnetic structure at temperature T=O, concentration c=1 is 'non-trivial', i.e., when the ground state does not allow minimisation of the energy for each bond, due to geometric hindrances. Such a situation, with only one type of interaction, contrasts with the case of oscillating interactions in dilute systems usually invoked to explain spin-glasses.

The difficulties of straining a thin sample such as mica have been overcome by applying to the sample a 40 kHz standing strain wave. ESR measurements have been made in mica using magnetic field modulations and ultrasonic strain modulation (UMER). By separating the contribution of different defects the UMER results the underlying complexity of the spectra.

Optically detected magnetic resonance of donors has been observed by monitoring the edge emission in ZnSe. The results are explained using a distant donor-acceptor pair model and the shallow donor g-value is g_e=1.115+or-0.010.

Far-infrared transmittance spectra of Mn_1-xZn_xF₂ have been obtained at 1.2K in the frequency region from 3-15 cm^-1 for x values from 0 to 0.49. The antiferromagnetic resonance frequency is observed to drop nearly linearly from the 8.7 cm^-1 value for MnF₂ to 4.0 cm^-1 at 40% ZnF₂. The linewidth broadened from <<1 cm^-1 to approximately 2 cm^-1. Resonance frequencies obtained are compared with those from recent neutron scattering measurements.

Recent computer simulation studies of the one-component plasma have indicated that for large values of the plasma parameter the single-particle motion is strongly influenced by the density fluctuations. By making use of a form of the random phase approximation an expression is derived for the memory function associated with velocity correlation in which the coupling of the single particle and 'plasmon' modes is clearly exposed. Satisfactory quantitative agreement is obtained with the recent molecular dynamics studies for both the memory function and frequency spectrum.

A new method based on the WCA perturbation approach (Anderson, Week and Chandler, 1972) is proposed for calculating the effective pair potential from the measured structure factor of simple liquids and applied to liquid neon. The calculated potential gives satisfactory values for the pressure and internal energy, and when used in molecular dynamics calculations it reproduces with high accuracy the pair correlation function.

A conventional implantation accelerator producing light ions M<40 and generally used for ¹⁸O⁺ implantation is described. Oxygen implantations along channelling directions have been performed in copper with low energies (11-43 keV). The values of stopping power of copper for oxygen ions are deduced from these experiments by measuring maximum ranges of oxygen ion implantation profiles.

CSEM (coherent self-energy method, Slechta, 1975) is a self-consistent continued-fraction method for the calculation of the density of states. It has a coarse-grained structure. Results are given of its application to the calculation of the phonon density of states in linear disordered systems with short-range order which are models of real linear polymers or biological compounds such as proteins. The results are compared with similar calculations.

The Wagner-Horner theory (1974) of the liquid-gas transition in coherent metal-hydrogen systems is used to study the case of thin cubic crystals in detail. All density modes, elastic energy eigenvalues and spinodal temperatures are found for the case of a two-dimensional sheet. The effect of surface loading is explicitly demonstrated, and the lowest-order corrections for nonzero thickness are discussed.

By means of an AC calorimetry technique the heat capacity has been measured in several Sn_xGe_1-xTe crystals with x ranging from 0.62 to 0.91. The anomalous specific heat near the transition temperature can be separated into two parts. One shows a step-like behaviour; the height of the step becomes larger with decreasing x. The other contribution is a logarithmic critical behaviour; its critical region becomes broader with decreasing x as well. These characteristics might be related to a tricritical point. It has been found that in Sn_0.79Ge_0.21Te crystals the anomalous specific heat is proportional to the anomalous elastic stiffness.

Atomic (s- and d-) orbitals and an atomic approximation to the potential are used in the chemical pseudopotential secular equation to calculate the covalent binding of an adsorbed monolayer on a transition-metal slab. After showing that the method provides a realistic description of the d-bands in bulk W and Pt the changes which accompany adsorption of hydrogen on W(100) in various surface arrangements are examined. Maximum coverage is found to correspond to occupation of all bridge sites on the W(100) surface ( beta ₁ phase). Bridge sites also provide maximum covalent binding at lower coverage but energy differences between alternative sites are small ( approximately 0.2 eV per H-atom) and ionic effects may stabilize adsorption above atoms in the beta ₂ phase.

For pt.I see ibid., vol.10, p.2083 (1977). A localized orbital method is used to calculate the covalent binding energy of H, O and CO overlayers in various sites on the (001) surface of Ni or Pt. All three adsorbates bind most strongly above four-fold hole sites, but for hydrogen, energy differences between alternative sites are particularly small ( approximately 0.1 eV on Pt). Local densities of states on the surface orbitals are presented and discussed in relation to photoemission spectra and the mechanism of chemisorptive bonding.

Photocapacitance measurements of ZnSe:Ni reveal a level about 0.96 eV below the conduction band at room temperature. The spectral distribution of the photoionization cross-section for transitions from this level to the conduction band was obtained at liquid nitrogen temperatures from a transient photocapacitance method. It shows a narrow band (0.12 eV width) superimposed on a broad continuum. The spectrum is interpreted in terms of a resonance between a localized excited state of nickel and the continuum of the conduction band. The oscillator strength in the narrow band is 10^-2. The threshold at liquid nitrogen temperatures was estimated to be 1.05 eV. There is evidence that when the level is filled, nickel is in the d⁹ configuration.

The classical equations of motion and continuity are used to find the response to an external charge density of a two-dimensional charge sheet between media of different permittivities; the response function is used to discuss the dispersion relation, screening and radiation by an oscillating charge. The dispersion relation is found for an isolated charge line. For a lattice of charge lines the dispersion relation omega = omega _p cos theta , where theta is the angle between the direction of propagation and the chain axes, arises in the long-wavelength limit.

For pt.I see ibid., vol.10, p.211 (1977). Quantum transport theory formulated in the previous paper in the coherent-potential approximation on the basis of the effective-medium approach is reformulated on the basis of the diagrammatic expansion and resummation approach due to Yonezawa (1968). It is shown that the new formulation is equivalent to the previous one.

For pt.II see ibid., vol.10, p.2131 (1977). It is shown that, when a random substitutional alloy has a small carrier density, a large cancellation occurs between two terms of its electrical conductivity in the quantum transport theory. This cancellation must be treated carefully when the reduction of some quantum transport theory to the Boltzmann transport theory is made in the weak-scattering or the dilute limit. It is shown that the quantum transport theory in the coherent-potential approximation is consistent with the Ward identity and a similar identity and that this consistency is essential to obtaining the correct result in that reduction.

The spectral density is shown in order to investigate more closely the origin of the changing sign of the Hall coefficient of binary alloys, which has been reported recently by Niizeki and Hoshino, (1976).

Formulae describing the electron transport coefficients as functions of the conductivity tensor are derived on the basis of the linear response theory. It is shown that the Wiedemann-Franz law and the Mott rule are obeyed even in the presence of a quantizing magnetic field omega tau >1 if the scattering of electrons is elastic and if h(cross) omega >>kT. As no assumptions about the form of the one-electron potential are made, the conclusions are valid not only in the case of the Shubnikov-de Haas oscillations but also if effects such as magnetic breakdown and the size-effect appear.

The metal-semiconductor interface is analysed by means of a simple method, which includes both the effects of virtual surface states and the many-electron interaction. This model is used to obtain the barrier height phi _Bn for the junctions of Si (111) with Al and Na; the results are in good agreement with experimental data. The trend on going from covalent to increasingly ionic semiconductors is also studied for junctions of zincblende (110) compounds for different metals. The results do not show the transition from Barden-like to Schottky-like behaviour displayed by experimental data.

The Edwards and Anderson model (1975) for spin glasses is investigated by a real space rescaling transformation on an S=¹/₂ Ising Hamiltonian with nearest-neighbour-only interactions in three dimensions. A transition to a spin glass ordered phase is found at about half the mean transition temperature, in contrast to earlier work on a two-dimensional model where no transition was found. The results suggest that the lower critical dimensionality, d_c, for spin glass order is slightly greater than 2, although possibly d_c=2 exactly. The specific heat is calculated and found to exhibit a broad maximum above T_c, with no discernable singularity at the transition since the exponent alpha is large and negative.

In the study of the temperature variation of the quadrupolar ordering parameter B=((S^z)²)-(1/3)S(S+1), using the Green function method, discontinuities are observed indicating the possible occurrence of a first order quadrupolar transition. Utilizing some suitable decoupling approximations, the equation of motion is linearized and the energy spectrum is determined which is then employed to derive an equation for B. Numerical computations are carried out for a spin-1 BBC lattice and it is seen that when the biquadratic parameter alpha exceeds a certain critical value alpha _c, the variation of B with temperature exhibits a new feature near the ferromagnetic-paramagnetic (FP) transition point. Instead of going continuously to zero at the Curie temperature, B abruptly falls to zero from some large positive value, showing thus a discontinuous jump.

The authors study an ordering binary alloy with magnetic and spatial interactions. Solutions for a two sub-lattice model are obtained within a combination of Bragg-Williams and the molecular-field approximations. The presence of magnetic interactions can have far-reaching effects on the spatial ordering characteristics of the alloy. In particular, the spatial long-range order (LRO) parameter can be substantially renormalized and the location of the order-disorder transition temperature can be appreciably shifted. Similarly, the presence of spatial LRO, in general, causes the occurrence of more than one type of magnetization for each magnetic species. Moreover, it can also seriously renormalize the temperature dependence of the magnetizations as well as shift the location of the magnetic transition temperature.

A renormalisation group approach developed for the percolation problem by Young and Stinchcombe (1975) is extended to yield magnetic indices. A real-space cluster calculation is carried out for the Ashkin-Teller-Potts model (Ashkin & Teller 1964, Potts 1952) with general S, allowing contact with results for the Ising model (S to 2) as well as with the previously published thermal indices of the percolation problem (S to 1). For the percolation problem, the magnetic index obtained is 2.06, about 10% larger than current series estimates, and results in gamma =2.87.

An EPR study has been carried out on Mn²⁺-doped single crystals of hexaquonitrates of Mg and Ni, Mg(OH₂)₆.(NO₃)₂ and Ni(OH₂)₆.(NO₃)₂, at room temperature ( approximately 300K) and at X-band. Mn²⁺ substituting for Mg²⁺ in Mg(OH₂)₆.(NO₃)₂ exhibits two identical, but differently oriented, magnetic complexes, whose z axes make an angle of approximately 15+or-1 degrees with each other in the zx plane. However, Mn²⁺-doped Ni(OH₂)₆.(NO₃)₂ crystals exhibit only one single thirty-line spectrum indicating the equivalency of two Ni(OH₂)₆²⁺ complexes per unit cell. The EPR spectra are analysed using a spin-Hamiltonian of orthorhombic symmetry to get the best fit parameters. These studies reveal that the two crystals are not isomorphous and that the site symmetry at the cation sites is very low.

Raman spectra of crystals of TTF-TCNQ as well as of powdered samples of TTF, TCNQ and (TTF)₇I₅ are reported for the temperature range from 4 to 77K. An assignment of the observed lines in TTF-TCNQ to the intramolecular lines in TTF and TCNQ is given. By comparing the frequency shift of Raman lines between neutral and charged TTF and TCNQ and the corresponding lines in TTF-TCNQ a charge transfer of roughly 0.5 electrons in TTF-TCNQ is obtained. In the low frequency range five out of six lattice modes are found and assigned to their symmetry species.

The magnetic circular dichroism (MCD) spectra of type I and type II diamonds at low temperatures have been measured. The MCD spectra of the GR1 band, produced by irradiation, were the same in both types of diamond and support its assignment to the ¹E to ¹T₂ transition at a neutral vacancy. Values for the MCD parameters A₁/D₀ and B₀/D₀ have been calculated from a moments analysis of the spectrum and a value of g=1.68*10^-2 has been obtained for the excited state ¹T₂.