Table of contents

The authors show that, in a perfect solid with a certain type of overlapping potential, the original KKR equations determine completely the band structure, but determine only a part of the wavefunction.

For Pt. I see abstr. A68718 of 1971. Calculations are reported of the diffusion coefficient and frequency spectrum of the velocity autocorrelation function under conditions of temperature and density appropriate to liquid argon at the triple point. They are based on a theory of the associated memory function, in which the latter is given in terms of the radial distribution function and the selfcorrelation function of an atom in the liquid. Numerical values of the memory function are also given and compared with results obtained from computer experiments.

Liquid selenium shows a reversible switching to high conductance, which is effected by an electric field. A further increase of the electric field produces a reversible switching to low conductance. After switching to low conductance a voltage controlled negative resistance was observed. In a certain temperature range (dependent on the shape of the sample) a positive resistance after switching to low conductance was measured and a memory on switching back. In order to explain these switchings and the negative resistance, the authors suggest a model by assuming localized states at the top and bottom, and extended states in the middle of the conduction band. The measurements are extended up to 1000 degrees C.

In continuum elasticity theory the strain field around a point defect in a cubic medium and hence the interaction between point defects can be calculated analytically only for directions in a symmetry plane. This interaction is calculated for the (110) direction. The results are applied to the interaction of O₂^- centres in KCl, KBr, KI and NaI.

Silicon containing high concentrations of up to 10²⁰ atom cm^-3 of boron alone, or doubly doped with boron and a group V donor impurity, have been irradiated by 1.5 or 2 MeV electrons at 300K. Infrared local mode absorption measurements have shown that this treatment can lead to the removal of a very large fraction of boron from unperturbed substitutional sites, and there is correlated growth of other local mode bands. Certain bands have been identified with a defect that appears to involve two boron atoms, while other bands are thought to arise from boron in interstitial sites. Annealing studies, involving both optical and electrical measurements, are consistent with this latter interpretation. The initial removal rate of boron from normal sites is very high and reaches a limiting value of about 3 atom cm^-3 per electron cm^-2, when the impurity concentration exceeds 10¹⁹ cm^-3. It is concluded that recombination of primary vacancies and intrinsic interstitials is almost completely inhibited in these samples. In the doubly doped crystals, it is suggested that the group V donors trap the vacancies, while boron traps the silicon interstitials. It is inferred that large vacancy clusters form in samples containing only boron impurities. These results show that the large changes observed in the conductivity of degenerate p type material may be interpreted as due to the removal of the acceptor impurities.

The interaction of a cation vacancy and an anion vacancy in a CsCl lattice at the nearest neighbour separation is investigated using the Dienes mode for the core distortion surrounding the vacancy pair. The formation energies of vacancy pairs in a family of seven crystals of CsCl type are evaluated to be 1.315, 1.208, 0.993, 1.007, 0.944, 0.691 and 0.698 eV for CsCl, CsBr, CsI, TlCl, TlBr, NH₄Cl and NH₄Br respectively. The results suggest that at elevated temperatures, vacancy pairs may occur in concentrations comparable to those of Schottky defects.

According to the theory of Gor'kov and Eliashbeg (1965), the electric polarizability of very small metal particles should be greatly enhanced because of quantization effects. The dielectric constant of glass samples containing large concentrations of small (<or approximately=100AA) silver particles has been measured to test this prediction. No enhancement of the dielectric constant of the composite material was observed, hence it is concluded that the electric polarizability of small metal particles is not enhanced. The reason for this is uncertain. It may be that the electron energy level separation assumed by Gor'kov and Eliashberg is incorrect. Alternatively, as suggested by Kubo (1962), it may be that the effect is suppressed because the oscillator strength at low frequency is insufficient.

It is shown that the spectral density used by Scaife (1971) in his theory of complex permittivity is closely related to the autocorrelation function used by other authors and that both Scaife's equation and the equation in terms of autocorrelation functions follow immediately from the superposition principle. The origins of the different expressions obtained when distortion polarization is present are discussed.

From transport theory in relaxation time approximation, analytic expressions are derived for the thermomagnetic power tensor alpha (B) in terms of the magnetoconductivity tensor components and partial Seebeck coefficients for materials with multivalley Fermi surfaces. Thereby, the thermomagnetic power in bismuth is explicitly connected with the carrier densities and mobility tensor components in the two band model. To obtain these model parameters, a least mean squares procedure has been adopted to analyse experimental results of alpha ₂₂(B) for bismuth at liquid nitrogen temperatures. The solutions found are in agreement with the model parameters computed from measurements of the twelve coefficients that describe the low field, magnetoresistivity tensor: a finding that attests to the validity of the theory developed for the thermomagnetic effects. The Umkehr effect and sign reversal observed in the thermomagnetic power of bismuth are shown to follow from the nature of the Fermi surface. Predictions are made for the occurrence of the Umkehr effect in other components of the thermomagnetic tensor.

A high temperature expansion is obtained for the zero field susceptibility of a generalized Ising model, which can be used to describe the critical behaviour of various multicomponent systems. The first seven terms of the series are obtained for the face centred cubic lattice. Estimates of the critical temperature obtained from this series are considerably more precise than those obtained previously from the zero field free energy. Estimates are also obtained for the critical exponents of the susceptibility and the derivatives of the zero field free energy. The results are compared with the predictions of mean field theory.

The results of measurements of the magnetothermal resistance of DyPO₄ are presented in the form of isothermal plots of thermal resistivity against magnetic field at selected temperatures in the range 0.26 K-1 K. These results are correlated with the magnetic phase diagram obtained from other experiments, and the form of the thermal resistivity variations with field is explained with reference to various phonon scattering processes.

For Pt. I see ibid., Vol. 5, 450 (1972). The measured magnetothermal resistivity of GdVO₄ at low temperatures is correlated with the spin wave dispersion curves calculated for this substance using experimentally derived parameters. Such a procedure yields strong evidence for a spin wave contribution to the thermal conductivity, which is at least partially dependent on point defect scattering. This is supported by simple calculations and also by experiments on crystals containing nonmagnetic impurities.

For Pt. II see ibid., Vol. 5, 459 (1972). Thermal resistivity measurements in the temperature range 0.1K-1.2K and in magnetic fields up to about 30 kOe have led to tentative suggestions as to the magnetic behaviour of ErVO₄ at low temperatures. The results indicate that this substance becomes antiferromagnetic below T_N=0.4+or-0.1K, while the assumption that certain peaks in the thermal resistivity correspond to a magnetic phase transition, if correct, leads to a value of about 2.7 kOe for the effective exchange field.

Linear dichroism measurements are reported on a specimen of silicon doped yttrium iron garnet (YIG) with composition Y₃Fe_5-xSi_xO₁₂ where x=0.08. Weak spontaneous dichroism ( approximately 5%) was observed when a specimen was placed in a saturating magnetic field at 300K. The magnitude of the dichroism, which was determined using a near infrared beam directed normal to the (110) plane, was strongly dependent on the orientation of the magnetic field with respect to the crystallographic axes. This field was confined to the (110) plane. On decreasing the temperature the spontaneous dichroism increased to reach a maximum ( approximately 14%) at about 70K. Below 70K photoinduced linear dichroism was observed; this also increased with decreasing temperature, though the maximum photoinduced dichroism, at 1.9K, was only 3%. The variation of the magnitude and the axis of the induced dichroism with the direction of the E vector of the radiation employed to induce it was examined, as was the effect of changing the wavelength of the inducing and sensing radiation. The optical absorption coefficient for both doped and undoped YIG in the wavelength range 0.7 to 2.0 mu m was also measured. The results of the experiments are interpreted in terms of a simple model which involves photon stimulated valence exchange between Fe²⁺ and Fe³⁺ ions. It is deduced that the optical absorption probability of an Fe²⁺ ion is about 35% smaller when the E vector of the radiation is along the local trigonal axis than when E is perpendicular to it.

The impurity induced first order Raman spectra are calculated for CsCl. The perfect crystal is described by a breathing shell model. The impurities are described by a model involving changed central and noncentral force constants between the defect and its nearest neighbours and between the nearest neighbours themselves. The calculation shows that there is relatively little change in the frequency dependence of the scattered intensity for extreme variations in the force constants describing the defects. Therefore, Raman scattering experiments should give useful information about the phonons of the perfect crystal without detailed knowledge of the defect model.

For Pt. I see ibid., Vol. 4, 2962 (1971). The authors report on an experimental study of the excitation of Er³⁺ emission in ZnSe. It is shown that two rare earth luminescent centres both emitting from the ⁴S_3/2 level are excited by the resonant transfer of energy from two different broad bands. It is thus concluded that the transfer of energy from a broad band emission to any rare earth ion in the II-VI lattice, simply because of an energy overlap, does not take place as the local environment of the rare earth ion plays a significant part in the transfer process.

Comparison of the emission spectra produced under photoexcitation and cathodoexcitation reveals a peak wavelength difference. This is interpreted in terms of a difference between the effective electron temperature of free electrons for the two modes of excitation and leads to a brief discussion of possible complications in the interpretations of radiative recombination processes.

At low temperatures in molecular crystals, hindered methyl group torsional oscillator-nuclear spin states exhibit an energy splitting which is simply related to the rate of tunnelling rotation of the group through +or-120 degrees about the threefold axis. In free radicals in radiation damaged crystals where there exists a large hyperfine interaction between the methyl group protons and the unpaired electron spin, the tunnelling splitting may be measured from the electron nuclear double resonance (ENDOR) transition frequencies. A study at 4.2K of the free radical CH₃C(COOH)₂ formed by heat treatment of irradiated methyl malonic acid crystals yields a value of the splitting equivalent in frequency units to 160+or-30 MHz. It is found that those ENDOR transitions whose frequencies depend most upon the tunnelling splitting are anomalously broad. This implies that the splitting has a wide though strongly peaked distribution. Some methyl groups are found to be in states in which the tunnelling splitting is inverted, providing experimental support for theoretical models which have invoked low lying states of this kind to account for the averaging out of the tunnelling splitting at those higher temperatures where classical models for methyl group rotation apply.