Table of contents

In contrast with previous reports, an important negative phonon drag hump is observed in a large binary bismuth single crystal. Without questioning the theory, this result is compared with the existing data showing the extreme sensitivity of phonon drag to size in the liquid helium range. This, together with the marked anisotropy of the transport properties, should be taken into account to explain the apparent discrepancy between the published experimental data. It is suggested that the phonon drag hump is negative for an infinite sample.

Magnetic anisotropy in the symmetry plane has been examined in the temperature interval 100-300K under different uniaxial pressures. Results show that the phase transition temperature increases linearly with increase of pressure. dT_c/dP calculated from the curves gives a value of 22K kbar^-1 for CoSiF₆.6H₂O and 18K kbar^-1 for CoZrF₆.6H₂O.

Electric susceptibility measurements on cerium ethylsulphate show that the splitting between the lowest cerium crystal field levels varies with temperature as proposed by Fletcher and Sheard (1971). The ion-ion interactions can be represented by an Ising model in a longitudinal field, as expected for coupling of the Gamma ₁ uniform lattice mode to the cerium ions. The data fitted equally well the susceptibility calculated using the mean-field approximation or using an exact expression valid for nearest-neighbour interactions only.

Measurements have been made of the shear viscosity eta and the positive ion mobility mu along the melting curve in liquid ⁴He, passing through the lambda triple point. The effective radius of the ion, defined by Stokes' law, shows a maximum 40 mK below the triple point. This and similar behaviour observed previously by Ahlers and Gamota at saturated vapour pressure are explained by the presence of normal fluid in the immediate vicinity of the ion due to the electrostrictive increase in pressure. The maximum observed at the melting curve is evidence that the lambda line continues into the region of supercooled liquid.

It is shown, within the dielectric matrix formalism, that in the presence of free electrons, the rapidly changing component of the phonon potential is not screened but produces an additional slowly changing potential component. Moreover, within the approximation of spherically symmetric valleys, the dilatational part of the acoustic deformation potential is screened by free carriers (i.e. the electron matrix element of this part of the phonon potential is decreased), the shear part remains unchanged; the optical deformation potential (in Ge) is not screened.

Variations in the conductivity of QN(TCNQ)₂ depending on sample preparation are correlated with the presence of doubly ionised TCNQ in the samples. Increasing TCNQ^2- concentration results in decreasing conductivity and increasing activation energy. The influence of D⁺ impurities compensating the excess charge is discussed.

The theory of thermal dielectric relaxation for MIM structures is modified by considering distributed traps. It is found that the value of the temperature T_m can increase with increase in the value of applied potential for an insulator containing a large number of distributed traps below the Fermi level. This physical situation can explain the recent experimental results of Thurzo and Dezal (1976).

Considering a stack of superconducting and insulating layers, the penetration of a magnetic field parallel to the layers into the insulating junctions is calculated.

The absorption spectrum of LiF single crystals doped with UO₂²⁺ ion is measured in the region extending into the vacuum ultraviolet. It is proposed that the UO₂²⁺ ion enters the lattice substitutionally at the Li⁺ site with the oxygen directed along (111). The observed spectrum is interpreted on the basis of the molecular orbital scheme given by McGlynn and Smith (1961).

For pt.II see ibid., vol.3, p.619, 1970. Published sets of tables of the irreducible representations of the space groups are restricted to k vectors within Omega , the basic domain, whereas for a complete identification of all the irreducible representations it is necessary to consider phi , the representation domain, rather than just Omega . The representation domain, phi , is identified for every space group. A general procedure is described that enables one to identify all those additional special k vectors in ( phi - Omega ) that must be included in order to ensure that one, and only one, k vector from each star is identified.

An alternate calculation to an earlier one (see ibid., vol.8, p.1322, 1975) on the dispersion binding energy of the rare gas solids is presented. the calculation neglects the self-polarisation terms previously included but retains the finite size of the atoms. The calculations give good agreement with other calculations using point-dipoles, but as with the earlier work, the authors are able to compute a more stable FCC structure than HCP. They are able to show that many-body contributions differ for the two structures, and are larger for HCP than for FCC especially for large atomic polarisabilities.

The temperature dependence of the lattice parameters, from 21 degrees C up to temperatures close to the melting point of each material, has been obtained from neutron diffraction measurements on powder samples.

For pt.I see ibid., vol.9, p.4237, 1976. The phonon scattering by paramagnetic ions having an effective spin of unity in a crystal field of uniaxial symmetry is calculated for arbitrary concentrations of ions. The calculation employs a Green function technique based on the use of semi-invariants. To evaluate the semi-invariants a Wick's theorem is introduced that allows for the presence of crystal-field terms in the static spin Hamiltonian. The lifetime of the coupled spin-phonon excitations is calculated in the phonon regime and the frequency, temperature and concentration dependence is examined in detail. Comparison with the Born approximation in the dilute limit is also made as it provides a valuable check on the accuracy of the expressions and gives additional physical insight into the origin of the scattering processes.

The acoustical phonon frequencies for Ta alloyed with 22% D and with 18% H have been measured at room temperature. For comparison the phonon dispersion curves of pure Ta (including the previously unknown (110) T₁ mode) have also been determined. The observed frequency shifts are qualitatively the same as those for the system Nb-D. For the optical branches in Ta-D two energy levels could be found at 20.3 THz and at 28.5 THz independent of the phonon wavevector.

The thermodynamic perturbation theory of Weeks, Chandler and Andersen (1971) is used to derive a physically transparent expression for the excess free energy of a simple liquid in coexistence with its vapour. The expression is minimised with respect to a single parameter specifying the width of the density profile, to give the surface tension directly. The theory gives useful insight into the thermodynamic and microscopic factors which determine the equilibrium density profile and the magnitude and temperature dependence of the surface tension. Calculated density profiles for argon and results for the surface tension of argon, nitrogen and methane, modelled by a Lennard-Jones 12-6 potential, are presented. Near the triple point, these are in reasonable agreement with experiment and with the results of more sophisticated theories.

For pt.I see ibid., vol.8, p.814 (1975). The author puts forward a method of determining the dielectric response function of an insulator which is valid in the region of the fundamental absorption edge and which includes the effects of electron-hole correlation (exciton effects) and the broadening due to scattering of the electron and hole (by phonons or impurities). In the limit of zero electron-hole correlation the method yields the convoluted density of states for the imaginary part of the dielectric function, and in the limit of no scattering it yields the conventional exciton result. The scattering is included by means of the electron and hole propagators, and several phenomenological models for these are considered. An important result which the calculations reveal is that the shape of the long-wavelength absorption edge is hardly affected by the electron-hole correlation.

The E_g modes of a simple cubic lattice are described in detail for the case when all ions have the same mass and the elastic forces are of a simple nearest-neighbour form. The density of modes is then computed. A T_g ion is assumed to be coupled to such modes via an interaction linear in the E_g displacements of its nearest neighbours. The resulting Jahn-Teller effect is studied by applying to the Hamiltonian a unitary transformation chosen so that the ion-lattice coupling is replaced by a constant. The Jahn-Teller energy E_JT and a first-order reduction factor gamma can then be computed, and it is found that contributions to E_JT and gamma are important over a wide range of frequencies and that the major contribution to gamma is due to the motion of the ion's nearest neighbours. The displacement of lattice points which results from the Jahn-Teller interaction is shown to vary approximately as the inverse square of the distance from the magnetic ion.

The exact form of the valence bands in III-V compounds has long been controversial and it is demonstrated that quantitative information concerning the valence band structure can be obtained from the magnetophonon effect. The valence band anisotropy deduced for InSb from the low-field peaks is comparable with that of Ge. For GaAs the heavy hole peaks were interspersed with light hole peaks and were insufficiently numerous to determine the exact valence band anisotropy. The high field peaks of Ge showed a peak splitting and a reduction in anisotropy attributable to the unequal spacing of Landau levels close to the valence band edge which is caused by an admixture of light and heavy hole states. The high-field peaks of InSb showed a similar reduction in anisotropy but no peak splitting. The light hole series in both InSb and GaAs showed an additional peak arising from the admixture of light and heavy hole bands.

The shape of the coexistence curve in the vicinity of the critical consolute point has been determined by measuring the phase separation temperature of 34 different samples. The measurements indicate that the asymptotic behaviour of the coexistence curve is characterised by the critical exponent beta =0.344. The linewidth of scattered laser light has also been measured by intensity correlation techniques along the critical isochore. The data were fitted to the unmodified and modified Kawasaki equation to determine the critical exponent nu of the long-range correlation length.

A theoretical feasibility study into the possibility of using magnetic impurity relaxation of Fiske modes in Josephson junctions as a spectroscopic probe for type I superconductors is described. It is estimated that concentrations of less than 100 ppm magnetic impurities might be measured.

The local single-particle and density correlation functions for the symmetric Wolff model are derived analytically. The scattering channels are separated by means of the Stratonovich-Hubbard transformation and the associated one-body problem is solved explicitly in the long-time limit. A stationary phase argument yields expressions for the correlation functions in the weak-coupling limit. The single-particle correlation function acquires a time-dependent phase shift due to the readjustment of the spin-up and spin-down Fermi systems. The spin-up and spin-down Fermi systems relax on a timescale determined by the inverse bandwidth. In the intermediate- and strong-coupling regimes many-body effects intervene and the calculation breaks down.

The theory of the glass-like phase (GLP) for a random mixture of ferromagnetic and antiferromagnetic interactions is proved to be valid on the basis of computer experiments. It is shown that the distribution function of the thermal averages of spins has a single peak in the paramagnetic and ferromagnetic phases, while it has double peaks in the GLP. It is also shown that the specific heat and the susceptibility seem to have a cusp singularity at the transition temperature T_g. A comment for the phase transition is also given.

Previous ESR studies of AgCl:Ni and AgBr:Ni at low temperatures have identified divalent nickel ions in octahedral and axial sites, which were attributed to substitutional nickel ions without and with charge-compensating vacancies in next-nearest cation positions. Another angular dependent ESR spectrum, present with lower intensity in both crystals, has now been found to have orthorhombic symmetry. This centre is attributed to a substitutional divalent nickel ion with a charge-compensating vacancy in a nearest cation location. A microwave power-dependent line with resolved super-hyperfine structure superimposed on the octahedral nickel ESR line is believed to be a double quantum transition. Other more complex centres, produced by low-temperature annealing, have been observed but not identified.

A model explaining the temperature dependence of the efficiency of F-centre accumulation (as well as F₂-centre destruction), observed in the course of steady-state experiments, has been developed. The model takes into account three- (or one-) dimensional continuous diffusion of H centres, resulting in a primary process, and their decay due to the diffusion-controlled: (i) annihilation, (ii) tunnelling recombination with F centres and (iii) localization by pre-existing defects (or recombination with F₂ centres).

In two dilute paramagnets (insulating solids containing both dipolar nuclei and paramagnetic centres) the nuclear magnetisation is found to decay surprisingly rapidly in the presence of a weak radiofrequency field distant from the nuclear resonance by many linewidths. The decay is shown to be the result of a direct saturation by the RF field of those nuclei through which the majority communicate with the electronic reservoirs; its study confirms that nuclear spin diffusion is possibly well into the field gradient surrounding each paramagnetic centre, providing the dipolar energy generated can be relaxed sufficiently quickly. The extra Zeeman splitting of nuclei within these gradients, up to a certain limit, contributes to the nuclear dipolar reservoir, greatly enhancing its heat capacity in these materials at low temperature. Direct measurements of the nuclear dipolar relaxation rate are also presented.

Measurements of optical linear birefringence have been used to study the cooperative Jahn-Teller phase transition in the mixed crystals Tb_CGd_1-cVO₄ for 0.34<or=c<or=1. For two concentrations (c=0.34 and c=0.36) a double phase transition was observed as a function of temperature, in which an orthorhombic phase exists between two tetragonal phases. This is in agreement with earlier Raman studies. The origin of the birefringence and its relationship to the order parameter is discussed. A mean-field theory of the variation of the order parameter (i.e. the orthorhombic strain) with temperature is found to give a satisfactory explanation of the observed birefringence. Measurements of the critical behaviour of the order parameter are discussed.

Results of infrared (IR) absorption experiments on some H^--impurity complexes in alkaline earth fluoride crystals, doped with divalent foreign cations are reported. Two different centres have been observed; one of them shows axial symmetry (C_3v oscillators); the symmetry of the other centre is lower. It appears that the first excited state of the trigonal centres is split by an amount of energy which depends upon the difference of the ionic radii of the divalent impurities and the cations of the host material. In order to explain the positions of the IR absorption bands it was assumed that the H^- ions move in a rigid harmonic potential well due to Coulomb, polarization and repulsive interactions. In addition, relaxations of the hydride ion with respect to the surrounding lattice have been estimated.

The Raman spectrum of the Zn acceptor in GaP is examined under applied magnetic fields and uniaxial stress. The relative intensity and Zeeman splitting of the prominent B-line are not consistent with previous conjectures that it is the transition between the acceptor ground states associated with the P_3/2 and P_1/2 valence-band maxima. An alternative interpretation of this feature based on the coupling of the acceptor bound states of the two bands by the acceptor impurity potential is suggested.

A study is presented of the optical absorption edge of 2H-SnS₂ and 4H-SnS₂ at 77K and 273K and as a function of hydrostatic pressure. The results are interpreted to show that for both polytypes the lowest energy optical transition is an indirect one (E_g) and that the first direct transition (E_o) follows closely to higher energies. The results show that polytypism is significant in determining both the magnitude of the energy gaps and their pressure coefficients. This suggests that layer-layer interactions are very important for the electron states responsible for the band gaps.