Table of contents

The optical spectrum of MnO₄^- ions substituted for Br^- in KBr has been measured down to approximately 4K. The ordering of the absorption bands is as expected for MnO₄^- in a site of cubic symmetry. Rich vibrational structure is apparent on the band at approximately 520 nm, due to coupling with both local modes of MnO₄^- and the lattice modes of KBr.

Parapositronium lifetime has been determined in water and heavy water at different temperature settings around 20 degrees C using a fast time measuring set up with a prompt resolution (FWHM) approximately 200 ps and slopes of approximately 35 ps at the experimental side channel settings. The para lifetime, for all the cases except for water at 20 degrees C, is the same ( approximately 0.125 ns) within errors as expected from theoretical predictions. At 20 degrees C in water, the para component cannot be separated from the free annihilation component indicating that the para life is the same as the free annihilation lifetime. This is consistent with the swollen positronium concept as suggested by Goldanskii for explaining the magnetic quenching data for water at 20 degrees C.

The hydrodynamic expression for the intermediate scattering function (including the heat diffusion term) has been generalized, using memory function approach and the dynamic structure factor, S(q, omega ), calculated for the q and omega domain investigated in neutron scattering experiments, that is q approximately 1AA^-1 and omega approximately 10¹³s^-1. Good agreement is obtained with the spectral function of the longitudinal current correlation, omega ²S(q, omega ), produced in molecular dynamics studies of argon at 76K. The formation of the three-peak structure around q approximately=0.3AA^-1 has been investigated and appears to be correlated with the shape of the static structure factor in the small q region.

A model calculation is made for a monatomic face centred cubic crystal, with interatomic potential phi (r)=-4 epsilon (( sigma /r)⁶-( sigma /r)¹²). The calculation is based on a comparison between the macroscopic secular equation for plane waves propagating in a medium, described by the above equation of state, and the phonon secular equation at long wavelengths. The results are valid only in the neighbourhood 0K.

A computer simulation of the migration of interstitials in a Lennard-Jones model solid of FCC structure was made, using the molecular dynamics method. The interstitial system was displayed on a graphic data screen so that the interstitial configuration and path of migration could be identified visually. The 100 split was found to be the most stable form of interstitial, and it was observed to migrate by a cooperative mechanism. At high temperatures, near the melting point, the interstitial was observed in two forms: the 100 split configuration was still predominant but for about 30 per cent of the time the interstitial adopted the octahedral configuration. The diffusion of the interstitial was studied at temperatures from one half the melting temperature upwards, and the energy and entropy of migration were obtained from an Arrhenius plot.

Dispersive curves for crystalline HCN have been calculated on the basis of a theoretical intermolecular potential which had previously been used to study the lattice statics of this crystal. Good agreement is obtained with measured Raman frequencies, Debye-Waller factors and specific heat. The phase change from the orthorhombic to the tetragonal form can be understood in terms of the 'softening' of a transverse acoustic mode whose wavevector is close to the (110) direction.

The results are presented of calculations, for a model three dimensional system, of the effect of the introduction of various types and amounts of disorder into a crystal on the shape of the electronic density of states curve. It is found that even a small amount of disorder is sufficient to remove the van Hove singularities in the centre of the band, while in general the amount of distortion introduced depends on the amount of irregularity in the density of states curve for the perfect crystal, the amount of disorder, and whether this disorder resembles that appropriate to an amorphous system or that appropriate to an alloy.

The magnetophonon effect in p-tellurium is extensively studied over the temperature range from 20 to 90K in magnetic fields of up to 300 kOe. High resolution of the peaks is achieved by means of a double differentiation technique. When the magnetic field is applied perpendicular to the c axis, the position of the magnetophonon peaks in the transverse magnetoresistance differs according to whether the current direction (J) is parallel or perpendicular to the c axis. When the magnetic field is applied parallel to the c axis, the oscillations in the longitudinal magnetoresistance are almost the same order of magnitude as those observed in the transverse magnetoresistance. This rather large amplitude in the longitudinal magnetoresistance is ascribed to the double minimum structure of the energy of the Landau levels.

A displaced maxwellian model for hot electrons in polar semiconductors is used to analyse the detailed time dependence of optical and acoustic phonon disturbances in n-InSb at low temperatures. No nonlinear phonon processes are considered, restricting the applicability of the present theory to small phonon disturbances (optical phonons) or to the initial stages of strong acoustic phonon instabilities.

The paper considers the I(¹/₂), H(¹/₂), and H( infinity ) ferromagnetic models in which there are nonmagnetic impurities distributed at random on the sites of the underlying physical lattice. High temperature power series expansions, which are presented, are analysed by conventional methods to find the Curie temperature T_c(p) and susceptibility exponent gamma (p), where p is the concentration of magnetic elements. The series are analysed for p>1, as well as p<1, thus making contact with the nonselfintersecting chain problem. At p=1, gradients of T_C(p)/T_C(1) against p, are found, for the face centred cubic lattice, to be 1.05+or-0.02 for I(¹/₂), 1.15+or-0.01 for H( infinity ) and 1.36+or-0.03 for H(¹/₂). For the plane triangular lattice, the initial gradient for the I(¹/₂) model is 1.45+or-0.05. The behaviour found for gamma (p) is rejected as physically implausible, and the implications of its anomalous behaviour are discussed.

A high temperature expansion is obtained for the zero field susceptibility of a generalized Ising model which contains a single-ion anisotropy term commuting with Ising hamiltonian. The first five coefficients of the series are obtained for general spin and all three cubic lattices. Estimates of the critical temperature are made and compared with the predictions of molecular field theory and previous results. Estimations of the critical exponent of the susceptibility are in favour of the hypothesis that it is the same as for the pure Ising model.

Measurements have been made of the acoustic paramagnetic resonance spectra of both Cr²⁺ and Mn³⁺ in Al₂O₃ at frequencies between 8.4 and 9.5 GHz. Results have been obtained as the direction of the magnetic field H is verified in the ac plane, the bc plane and in a cone around the c axis. Two types of lines are in evidence: type 1 lines have an edge and type 2 lines a peak. The shapes are explained by assuming that random internal strains and electric fields of E type symmetry exist in the crystal. The variations of the positions of the peak or edge as H is varied are readily explained. This analysis shows that (3d)⁴ ions in Al₂O₃ are at a site which has predominantly trigonal symmetry and that the superimposed random tetragonal components are small.

For pt. I, see ibid., vol.5, 3397 (1972). The shape of APR lines from (3d)⁴ ions in Al₂O₃ has been investigated when the direction of the applied magnetic field and constant electric fields up to 70 kV cm^-1 are varied. It is found that the spectrum is much more sensitive to an electric field applied along the a axis than the same field along the b axis. A detailed semiquantitative discussion of the intensities of the edges and peaks in the spectrum is given. From the results, information is obtained on the E type components of the random internal strains and electric field distributions experienced by the Cr²⁺ and Mn³⁺ ion impurities in Al₂O₃.