Table of contents

Phonons in the cubic phase of KCN have been investigated by coherent inelastic neutron scattering at temperatures of 215K and 175K. Acoustic phonons were observed in the three principal axes of symmetry whereas optical phonons could be seen in the LO (100) branch only.

It is shown that one cannot explain the phase transition at 97.7 degrees C in squaric acid by using a planar eight- and sixteen-vertex model. It is argued that the inclusion of dipole forces between hydrogen bonds will yield a satisfactory explanation for the occurrence of the phase transition.

The authors have used a torsion pendulum to measure the superfluid density and viscosity of liquid ³He at pressures of 18.2, 26.0 and 29.7 bar. It was found that rho _s in the A-phase is not reproducible; the range of variation is about a factor of two, and large amplitudes tend to give low rho _s, as would be expected for orientation of the l-vector by flow. In the B-phase, rho _s is about 20% greater than any value found in the A-phase at the same reduced temperature. The viscosity below about 0.9 T_c is almost independent of pressure at a given value of (T/T_c); it is slightly lower in the B-phase than in the A-phase.

The authors present a theory for treating randomly diluted antiferromagnets with uniaxial anisotropy in the presence of an arbitrary external field along the symmetry axis. In particular, they examine the behaviour of the interface between the antiferromagnetic and the spin-flop phases as a function of the dilution. It is found that the dependence of the critical field on magnetic concentration is steeper than indicated by virtual-crystal-mean-field arguments. Moreover, this dependence is found to be a strong function of the relative size of the anisotropy. The present theory uses a coherent potential ansatz for treating randomness in the exchange interactions but ignores dynamical effects occasioned by fluctuations in the local anisotropy field. Thus, its relevance would be limited to systems where the average anisotropy, D(2S-1), acting on a magnetic ion is substantially smaller than the average exchange, 2SzJ, experienced by the same.

A new method of spin-density imaging by NMR is described which allows simultaneous observation and differentiation of signals arising from spins distributed throughout a thin layer or plane within the specimen. The method, which is based on selective RF irradiation of the sample in switched magnetic-field gradients, can produce visual pictures considerably faster than previously described line-scan imaging methods. A simple example of a two-dimensional image obtained by the method is presented.

The exact statistical mechanics of a one-dimensional gas of particles is presented. Each particle has a classical vector spin, and nearest neighbour particles interact via spin-independent and spin-dependent forces. In zero external magnetic field, the resultant eigenvalue problem is solved and, for a particular choice of interactions, the model is studied in some detail, including an analysis of a variety of possible ground states.

The coherent scattering function S(k, omega ) is calculated for density fluctuations in spherical, fluid particles for wavelengths long compared with the interatomic spacing (small k). A model with rigid boundary conditions displays distinct peaks at non-zero frequencies omega , which arise from the excitation of the compressional normal modes of the particle. Surface tension effects are shown to be minimal in the range of frequencies of interest in neutron scattering, and a droplet behaves therefore as a particle with a free surface. A third example studied should be a reasonable model of a simple, spherical virus. The protein shell is modelled by an elastic shell and the effect on the response of the nucleic acid is calculated. It is concluded that neutron scattering can be used to study the internal motions of viruses, and measurements should provide information on the physical properties of the nucleic acid and protein shell.

The motion of an atom across a (100) surface of a simple cubic solid of static atoms is investigated by numerical integration of its classical equations of motion. It is found that over short times the atom moves as a free particle but that over long times it behaves like a diffusing particle executing a random walk. It is shown that these results can be fitted to a random walk which has some correlation between successive steps.

The infrared absorption is evaluated theoretically for a one-dimensional conductor with half-filled tight-binding band by incorporating the band symmetry into Sadovskii's highly renormalized theory (1974) of electrons interacting with the soft phonon of twice the Fermi wavevector. Closed analytic expressions are given for Re sigma ( omega ) for finite bandwidth and finite temperature. The final result is evaluated numerically. Re sigma ( omega ) is derived within a conserving approximation from a one-electron Green function qualitatively different from that found by Sadovskii. The spectral function, density of states and breakdown of the quasi-particle picture are also discussed.

Three single-site structure-independent approximations for the electronic band structure of a condensed material are considered. The results of these methods when applied to copper are compared with the known results belonging to the Burdick potential as calculated for crystalline copper. In each case the dispersion relation may be said to be 'fair' but the approximate single-particle density of eigenstates is completely unsatisfactory.

A model for the electronic state of amorphous evaporated arsenic is proposed which is based on the interpretation of its electrical and optical properties. The semiconducting behaviour of amorphous arsenic is analysed using the general density of states model for amorphous semiconductors (localized states at the band edges and in the energy gap). The optical properties are interpreted in agreement with the non-direct transitions model. Taking into account the valence band density of states given by XPS measurements, the authors try to deduce from the experimental optical density of states epsilon ₂ omega ², the profile of the conduction band density of states and the average matrix elements for transitions from initial states of p, s and d character.

The electrical properties of semi-insulating n-type indium phosphide single crystals which have been pulled from the melt by the liquid-encapsulation technique are reported. Measurements of thermal activation energies for both ohmic and space-charge limited conduction over a broad temperature range have enabled the deduction of both depth and concentration of the localized levels in Cr-doped material. One dominant level is situated approximately 0.20 eV from the conduction band. A deeper level at 0.54 eV is attributed to the Cr impurity in the crystals. Data for iron-doped InP show that this type of crystal may also be suitable as an epitaxial substrate material.

In a model of diatomic molecules in which the long-range part of the interatomic potential energy curve is due to the London non-retarded dispersion interaction between the constituent atoms, it is shown that the proximity of a metal surface weakens the strength of the chemical bond when the molecule is parallel to the metal surface. The theory is based on the nature of the dyadic Green function connecting the electric field with a dipole source outside the metal surface-this is established by use of image theory. For a Lennard-Jones-type interatomic potential it is shown that the maximum reduction of the bond strength in the parallel configuration is 4/9.

A biquadratic Heisenberg ferromagnet with Hamiltonian H=- mu H Sigma _bS_b^z- Sigma _abJ_ab(S_a.S_b)- Sigma _ab alpha J_ab(S_a.S_b)² is investigated with a simple Green function approach. Some invalid results of earlier treatments are noted and magnetization curves are presented for spin-1 in a simple cubic lattice. Comparison is made with mean field and constant coupling results. The phase transition is found to be second order for alpha <0.48 and first order for alpha >0.48.

The authors consider a lattice of localized electronic spins coupled by an exchange interaction. These spins may be either ferromagnetically or spirally ordered (as in heavy rare-earth metals) and each electronic spin is coupled via a hyperfine interaction to the nuclear spin on the same lattice site: a model is used in which only a fraction c of the sites are occupied by nuclear spins. The coupled mode spectrum is calculated for a nuclear spin concentration c<<1 for both types of magnetic ordering, and the results are shown to be those yielded by a 'virtual-crystal' treatment of the nuclear spins. An analysis of the self-energy shows that because the hyperfine coupling is very much less than the exchange coupling in any real system, the results obtained must hold for arbitrary c. The resonance strengths of the zero-wavevector modes are calculated in this approximation and again display the virtual-crystal behaviour of the nuclear spins for all concentrations.

The author studies the modes of magnons in a cubic ferromagnetic material with very narrow domain walls. In the case of a domain wall having a width of only one interatomic distance, a band of localized magnons whose bottom is inside the gap of free magnons is found. This calculation can easily be generalized to the case of a domain wall several interatomic distances thick.

Using classical equations of motion together with Maxwell's equations, the authors calculate the response of a ferromagnet to an applied RF magnetic field, including the effects of retardation. With the aid of the fluctuation-dissipation theorem the response functions are used to find the correlation functions required for an evaluation of the cross section for inelastic light scattering. The dispersion relation and cross section are discussed in detail at zero temperature, and some discussion is given of the corresponding results at low, but non-zero, temperature.

Contributions to the Raman scattering intensity, calculated in the bond polarizability approximation, are presented for vitreous SiO₂, GeO₂ and BeF₂. The computed frequency spectrum-scattering intensity relationship for SiO₂ shows only partial agreement with that derived from experiment.

The excitation spectra for intrinsic luminescence due to recombination of self-trapped excitons at liquid helium temperature have been measured in NaCl, NaBr, RbCl and RbBr. The observed sequence of thresholds is explained in terms of multi-exciton production processes involving both valence and core excitons. Results are compared with published optical and photoelectron data and found to be in reasonable accord.

Phonon scattering by electrons in the simultaneous presence of intense laser and magnetic fields is discussed. A kinetic equation is derived, and the rate of change of the phonon population is calculated. For laser radiation propagating parallel to the magnetic field, it is found that multiphoton processes are dominant when the laser frequency is near the electron cyclotron frequency. Furthermore, the damping rate is found to decrease as the laser frequency approaches the electron cyclotron frequency.