Table of contents

The two-dimensional Born-Green-Yvon equation with anisotropic interactions is transformed into an integral equation. The isotropic case is treated explicitly as a special case.

It is shown that relaxation energies of core holes, calculated in the electron gas approximation are only weakly dependent on core state provided that intra-core relaxation is taken into account.

Elastic constant measurements for the LaAg_xIn_1-x system are presented. For large x(x approximately 0.9) the strong softening of the c₁₁-c₁₂ mode can be quantitatively interpreted with a strain-5d band deformation potential coupling. For smaller x(x<or approximately=0.8) strong strain-internal displacement coupling seem to be important.

An expression for the parameter ( delta ) is derived in terms of the Gruneisen parameter ( gamma ), the interatomic distance, the derivatives of the potential energy function ( psi r) and the derivatives of pi (r) (where phi (r) includes the repulsive energy, van der Waal's energy and zero-point energy). The expression is more general than Chang's expression (1967) and gives values in good agreement with experimental values.

The scattering, from powder and single crystal samples, appears only above the superionic transition temperature, 1000K. The integrated intensity is found to be strongly dependent on the direction and magnitude of the scattering vector, Q, (which suggests the scattering is coherent) but does not exhibit the periodicity of the reciprocal lattice. The energy width of 0.6-1.5 MeV (FWHM) is found to be only weakly dependent on Q, and almost independent of temperature, suggesting that the scattering is more complex than a direct result of the translational motion of the anions. A direct connection with the degree of disorder of the system and the intensity of the scattering is indicated.

In a previous neutron scattering experiment on ³He at T=15 mK (see Phys. Rev. Lett., vol.37, p.842, 1976) the present authors observed two peaks in the scattering function which were identified as the zero sound mode and the spin-fluctuation peak respectively. These results are different from those obtained by others at T=0.63K in which case no such structure was observed. In order to determine whether this discrepancy is due to the difference in the temperature of the two experiments, the authors have now made measurements at T=40 mK and at T=1.2K. These results show that the two-peak structure persists at the higher temperature although measurable broadening is observed in the spin-fluctuation part of the spectrum.

Magnitudes of crystal fields at the cation site in scheelites have been calculated numerically and are presented as functions of effective ion charges allowing comparisons between theoretical and experimental investigations.

Wilson's numerical technique has been used to show that a potential scattering term alters the fixed point of the Kondo problem. The implications of this result for the concept of universality are discussed.

The author gives a simple analysis of the flow of energy into and out of a dielectric under a charging or discharging voltage varying as a linear ramp in time with a variable rate. The limit of an infinitely steep rise gives the step function response, the other extreme corresponds to 'infinitely slow' response. It is shown that step function charging causes the loss of as much energy as is ultimately stored in equilibrium under constant field and analytical expressions are derived for the loss under conditions of a finite rate of rise and fall of the ramp. The results imply potentially very high dissipation of energy under transient conditions and a discussion is given of the practical implications of this.

Optical detection of EPR using recombination luminescence shows that the self-trapped exciton in AgCl is a (100) oriented complexes of the type (Cl-Ag-Cl)^-. In AgCl doped with AgBr, excitons are also trapped at bromine sites forming (100) oriented complexes of the type (Cl-Ag-Br)^-. These centres are produced either by X-ray or UV excitation. In very lightly doped crystals UV excitation is much more effective than X-rays in producing the bromine complex and the reasons for this are discussed.

It is suggested that there is a phase transition in nickel iodine boracite at approximately 120K, which is associated with no change of symmetry-an isomorphous transition. The cause of this transition is a condensation of O^I atoms from a gas-like state to a four-state disorder.

The concept of collective motion in classical liquids is discussed by using a general effective potential. In doing this a self-consistent memory function method is employed to analyse the density-density correlation function. The density response function is formulated rigorously in the form of a generalised random-phase approximation. The effective potential which appears therein is investigated by using successive approximations for a memory function. It is shown that the effective potential guaranteeing sum rules of the dynamical structure factor up to the fourth give a solid-like mean field in the high-frequency limit. From the lineshape of the dynamical structure factor corresponding to frequency-dependent potentials, effects of dynamical dampings are discussed.

The electronic energy loss of fast H₂⁺ ions in thin solids is investigated. The energy loss is influenced by the correlated propagation of the protons which act coherently on the target electrons through a pure Coulomb potential. This influence increases with increasing velocity and decreasing target thickness. The model proposed does not involve the so called 'wake potential'.

For pt.I see ibid., vol.11, p.2907, 1978. Experiments are described using a cylindrical sector energy analyser to measure the stopping power of 5-10 mu g cm^-2 carbon foils for atomic and molecular hydrogen ions. The incident energies of the ions varied between 100 and 300 keV amu^-1 and precautions were taken to avoid foil thickening due to contamination effects. A ratio of the stopping for H₂⁺ ions to that for protons of 1.06+or-0.03 was obtained rather than values up to 1.3 suggested in the literature. Stopping powers for protons at the lower energies were observed to be higher than those given in tabulated data.

The stopping power for 160 MeV alpha -particles in germanium has been measured in the (110) and (111) channelling directions and also for random impact. The respective values 0.716+or-0.032, 0.919+or-0.030 and 1.910+or-0.043 eV AA^-1 are discussed in relation to current theories of the channelled stopping power. The ratios of channelled to random stopping powers are lower than observed for silicon since for germanium the ten most tightly bound electrons play little part in the channelled stopping.

An excluded volume model is proposed for the cation-anion interaction in superionic conductors. The resulting cation-anion correlations are in excellent quantitative agreement with the pair distribution function derived from EXAFS measurements on AgI in both the normal beta and superionic alpha phases. Further, from the EXAFS data alone, the model predicts the substantial increase in ionic conductivity at the phase transition as well as the modest temperature dependence above it.

A new parametrisation scheme, applicable to the valence and conduction bands of solids is presented. The potential, expressed in separable form, is parametrised directly. The atomic phase shift, Green function and wavefunctions are obtained in closed form. The formalism is developed in detail and applied to the band structure of copper. It is found that the resonant d state is well approximated by a separable potential containing one free parameter, and that a two-parameter model gives a good qualitative account of s and p wave scattering from this system.

The metal-nonmetal transition in liquid alloys such as Cs_1-xAu_x and Li_1-xPb_x is explained as resulting from concentration dependent electron charge transfer causing short-range atomic order and a change from metallic to ionic bonds. Numerical results for the electronic density of states, the electron charge transfer, the free energy of mixing, the volume change, the electrical conductivity and the thermoelectric power are given.

Band structure calculations are presented for 4Hb-TaS₂. The alternating trigonal prismatic and quasi-octahedral sandwiches are shown to contribute their own approximately two-dimensional bands with some hybridisation where they overlap. The associated Fermi surfaces are presented and used to interpret the high-temperature incommensurate charge density wave in terms of nesting. A discussion is included of the charge transfer between neighbouring sandwiches; a range of possible values is considered.

Previous work has shown that the annealed bond Ising model provides a good approximation to the quenched bond Ising model when all the interactions have the same sign. A restricted form of annealing is developed which leads to even better agreement with the quenched model. By placing more and more restrictions on the annealing, successively better approximations to the quenched model can be developed. One of these schemes leads to an exact solution of the quenched problem on triangular cacti.

Methods are explored by which computer simulation can be used as a test of the extensive theoretical literature on the linear Heisenberg chain. The computed relaxation function is compared directly with theory where possible. For theories describing the spin-wave frequency and damping a new method is described allowing these to be deduced even when the frequency distribution contains a central diffusive peak and the lineshapes are asymmetric. The computed frequency distribution shows significant discrepancies with all published theories beyond the low-temperature regime.

A parametric equation of state for the Heisenberg system is given, which has the correct singularities due to Goldstone modes. It is shown to be an exact representation of the epsilon expansion to O( epsilon ) and of the leading order in 1/n expansion, and to be in reasonable agreement with Monte Carlo calculations and experimental data for CrBr₃.

The theory is based on the calculation of moments of response functions using dense random packing of spheres as the model for the lattice. The response functions themselves are obtained by fitting the moments to a continued fraction expansion. The method appears well suited to the study of amorphous materials, and results are obtained for both the density of states and the spectral density of a simple model system. A possible explanation for the so-called 'roton minimum' observed in some neutron scattering measurements is also indicated.

A coupling between the librations and internal vibrations is suggested to decrease the librational tunnelling frequency nu _t of the methyl group at low temperatures. The coupling originates from a low symmetry of the lattice potential at the site of the group. The experimentally observed nu _t at liquid helium temperatures is shown to be related to the ground and excited librational states. The initial temperature dependence of the coupling is determined by the methyl group distortion. A distortion becoming smaller when the temperature is raised is shown to make nu _t larger.

The authors report saturation effects at microwave frequencies of a paramagnetic two-level spin system by double-quantum transitions. The saturation factor and the power radiated at the second harmonic frequency are calculated by using the density matrix formalism. It is shown that the saturation factor (by double-quantum transitions) of Cr³⁺ in Al₂O₃ is field and frequency dependent and presents singularities at particular values of the field. The saturation factor can be orders of magnitude higher than in a Kramer's doublet.

Investigations of the low-lying energy levels of PrVO₄ using fluorescence, Raman and far infrared spectroscopy give the energies of the first excited singlet and doublet as about 35 cm^-1 and 84 cm^-1, respectively, above the ground singlet. Detailed measurements of the nuclear magnetic resonance spectrum of ¹⁴¹Pr show that the temperature variation up to 20K of the spin-Hamiltonian parameters gamma /sub ///, gamma _{perpendicular to} and P can be accurately fitted by thermal averages over these three levels. The positions of the remaining levels of the ³H₄ manifold are estimated by an extrapolation procedure based on properties of a crystal field Hamiltonian with tetragonal symmetry.

Transferred hyperfine interaction (THFI) parameters A_s and (A_p+A_d) have been measured by ENDOR for ¹⁷O^2- ligands of Yb³⁺ in CeO₂ in which about 10% of the oxygen has been replaced by ¹⁷O. A molecular orbital model is used to interpret the experimental parameters, and comparison is made with the analogous system CaF₂:Yb³⁺. It appears that the simple molecular orbital model gives an internally consistent explanation of the THFI, orbital reduction factors and crystal field splittings in both materials, in terms of empirically deduced covalency parameters. These parameters for the two materials are related to one another in the way that theory predicts. But the magnitudes of the covalency parameters required for this interpretation are more than three times larger than those calculated ab initio.

The mechanical and dielectric loss and the DC conductivity in iron-phosphate glasses were investigated. The similarity of activation energy suggests that the same mechanism associated with the hopping of the charge carriers between ions in different valency states is responsible for both loss processes as well as for the DC conductivity.

Evidence is presented for the existence of a tricritical point at the ferroelectric-paraelectric transition of PbZr_xTi_1-xO₃ close to a value of x=0.94. Details are given of measurements of dielectric constant, remanent polarisation and spontaneous strain. It is also observed that in single crystals of PbZr_xTi_1-xO₃ there is a peak in the transition temperature versus composition diagram coinciding with the tricritical composition. This is not seen in ceramics of the same composition.

Several zero-phonon lines are observed in the fluorescence spectrum at low temperatures and annealing experiments show that these are associated with aggregation of Eu²⁺-K⁺ vacancy pairs. Time-resolved spectroscopy measurements show that energy transfer takes place from ions in non-aggregated sites to those in aggregated centres. The time dependence is consistent with resonant electric dipole-dipole interaction with no back-transfer taking place. The vibronic sideband of the major zero-phonon line is analysed with a computer iteration process to determine one-phonon and multiphonon contributions. The thermal dependence of the width and position of the major zero-phonon line is shown to be consistent with strong coupling to only the low-frequency phonon modes. Non-radiative decay processes at room temperature are characterised by comparing fluorescence excitation and photoacoustic spectra.

Intrinsic electroluminescence has been observed in ammonium nitrate and ammonium sulphate at room temperature. The variation of light output with voltage and frequency is reported and the results have been discussed. Within experimental error, the voltage V and brightness B are related by B=B₀ exp(-C/V)¹2/, where B₀ and C are constants.

The variation with the photon energy of the photoelectron yield of Cu has been measured near the K edge. The fine structure of the yield agrees with the fine structure of absorption spectra (EXAFS). The radial structure function, the envelope function, and the scattering phase of a single Cu shell have been calculated from the yield spectrum. The results are similar to those extracted from the absorption coefficient. The yield increases with decreasing glancing angle of the incident photons. A variation of the height of the Cu edge and the amplitude of the EXAFS modulation with this angle is observed. This is explained by the escape mechanism of the electrons.