Table of contents

Calculated vibrational spectra are given for a range of disordered SiO₂-type networks, the interatomic force constants being chosen so that the spectra are dependent only on the topology of the networks and not their detailed geometry. It is found that the introduction of topological disorder leads initially to progressive smearing out of the main spectral peaks. In the limit of extreme topological disorder the vibrational spectrum, although occupying roughly the same frequency range as in the ordered case, becomes somewhat smooth and featureless in appearance.

The critical magnetic field for the cholesteric-nematic phase transition of the lyotropic liquid crystal poly- gamma -ethyl-L-glutamate in ethyl acetate has been determined by the use of light diffraction patterns. In spite of the variation of the zero-field helix pitch with concentration, the critical field is nearly constant at about 29 kOe.

Anderson localization of both electrons and holes in the inversion layers of n and p channel MNOSFET's has been investigated. The spatial extent of the localized state wavefunction has been obtained as a function of energy mod E mod from the mobility edge. The results are in agreement with theoretical predictions that a two-dimensional localized state wavefunction varies as exp(- alpha r) where alpha is proportional to mod E mod ^0.75.

Soft X-ray absorption spectra of alkali halides exhibit prominent peaks near the ionization threshold of alkali core orbitals. These experimental features, generally attributed to highly localized excitations, confined within the first layer of nearest neighbours, have been treated both from an atomic or molecular point of view and from a band structure point of view. The relationship between these two representations is discussed for the case of the Li⁺K spectrum of LiF, recently treated by Kunz et al. (see abstr. A69104 of 1973) using a Hartree-Fock band calculation.

An exact result for the long-wavelength dynamics of the singlet-triplet ferromagnet is constructed using arguments analogous to those for the nonrelativistic Goldstone theorem in the Heisenberg ferromagnet. It is shown that in the ordered phase the second moment of the transverse frequency is bounded by omega ^-(k)²<or=ck²/(J_z)². The consequences of this result are discussed.

A fully selfconsistent calculation in the correlated effective field approximation is reported for the paramagnetic phase of a singlet-triplet ferromagnet. Calculation of the frequency and strength of both the singlet-triplet and electronic triplet-triplet modes establishes that the approach to the Curie point is in general characterized by a diverging contribution to the scattering function from the long-wavelength zero-frequency electronic modes, while the frequency and strength of the corresponding singlet-triplet modes remain finite throughout.

A computer-based method is described for generating a homogeneous assembly of randomly close-packed spheres of packing density 0.606. The pair distribution function of this assembly is very similar to that of an unshaken pile of ballbearings and does not show the splitting of the second peak that is observed in shaken stacks of ballbearings. The average number of contacts of the spheres in the computed assembly is 6.0.

As the simplest binary system in which excitations as well as atomic transport can be studied, the theory of isotopic mixtures is developed in two directions: (i) Simple, though approximate, models are set up, to obtain a picture of the partial dynamical structure factors over a substantial region of the (q, omega ) plane. (ii) A careful study is made of the diffusion limit in the case of small mass differences.

The reorientation of impurity-vacancy complexes in KCl crystals doped with Mg²⁺, Ni²⁺, Mn²⁺, Ca²⁺, Sr²⁺, Pb²⁺ and Ba²⁺ was studied using the ITC method. These results are analysed together with those obtained by other authors by ITC, EPR and optical methods on Cd²⁺, Be²⁺, Eu²⁺, Sm²⁺ and Yb²⁺ ions. For the first series of ions and Cd²⁺, it was concluded that the most favoured vacancy position is the NNN site, and that it increasingly dominates the NN site as one proceeds from Mg²⁺ to Ba²⁺. The observed reorientation of complexes is mainly governed by the vacancy jump from the NNN to the NN position. In the case of Be²⁺ it was found that the majority of vacancies are in the NN position and the special behaviour of the Eu²⁺, Sm²⁺ and Yb²⁺ rare earth ions is discussed.

Colouration in KCl has been investigated by means of optical absorption and positron annihilation measurements. In additively coloured KCl crystals, positronium is formed in the F centres having a pick-off lifetime close to 1 ns. Angular correlation data obtained by magnetic quenching of orthopositronium indicate that a positronium-like system is formed with a hyperfine splitting close to the value in vacuo. During the measurements a considerable amount of F centres and other defects are produced by the positron source.

Neglecting the relative core-shell displacements at equilibrium the usual shell model formalism is applied to the rutile structure. A number of models of increasing complexity ranging from the simplest rigid-ion model to the full shell model (with both ions polarizable) are presented. When applying these various models to magnesium fluoride (MgF₂) it is found that a simplified shell model is already able to account for the experimental phonon spectrum. Such a comparison is based on optical data for the zone-centre frequencies, and on extensive inelastic neutron scattering measurements of dispersion curves in three symmetry directions and over a frequency range from 0 to 10 THz. Using best-fit values for the model parameters a number of dielectric properties for MgF₂ are calculated and compared with existing data.

The changes in work function with surface coverage of adsorbed alkali atoms on a transition metal substrate are studied. An analysis valid at low coverages incorporating the experimental results available in this region, is used to estimate values for the widths of the s and p levels (assumed equal), and their mean energy for different assumed values of the substrate-adatom distance d. A self-consistent numerical treatment is developed over a wide range of coverages. From this treatment a more complete determination of the parameters from comparison with experimental work function against coverage curves is achieved. The level width Delta for Cs on Re(0001) is estimated to lie in the range 0.7 eV<or= Delta <or=2 eV, with the Cs 6s valence level at about 1.8 eV above the Fermi level. An implication is that the absorbate levels are strongly hybridized. A much smaller value of Delta is estimated for Cs on a monolayer of oxygen on tungsten.

For pt. I see abstr. A32938 of 1974. The exact AC diffusivity formula derived in I reduces to D(0)=fd²/2 in the DC limit where f is the mean hopping frequency and d² is the mean square x displacement per hop in an infinite chain of hops. The mean hopping frequency is equal to a simple average of the total hopping rate out of each site. The predicted behaviour of f and d² contrasts strongly with that found in recent approximations to the stochastic formula in the DC limit because the approximations employed are equivalent to re-randomizing the site distribution after every hop.

A number of methods for predicting the effects of inhomogeneity have been published but none of them consider the most common situation in which isolated inclusions are present in an otherwise homogeneous material. The effects of inclusions are found by calculating the perturbation produced by a single inclusion and then summing the result to give the total change in properties. This is then used to find the transport properties of the inhomogeneous material. Expressions relating the electrical and thermal conductivities and the Seebeck and Hall coefficients of the inhomogeneous material to these values for the inclusions and the continuous phases are derived.

The dependence of critical temperature on hydrogen (deuterium) content in palladium hydride (deuteride) was measured on samples hydrogenized under equilibrium conditions with H(D) at high pressure. The superconductivity transitions can be observed for atomic ratios of hydrogen (deuterium) to palladium H(D)/Pd above 0.83 (0.77). The critical temperatures of deuteride are about 2.5K higher than those of hydride (at the same atomic ratios). The highest critical temperatures so far observed for the hydride and deuteride are 9.1K and 10.7K respectively. For both phases the effect of hydrostatic pressures is to decrease the critical temperature. The observed isotope effect does not support a metallic behaviour of hydrogen in palladium but is in reasonable agreement with Ganguly's model (see abstr. A6687 of 1974) for the superconductivity of palladium hydride or deuteride.

An analysis is undertaken for the case when the concentration p is less than the critical percolation concentration p_c. It is found that delta ²M/ delta H² undergoes a jump discontinuity at H=0. In discussing the critical behaviour of the dilute ferromagnet for p>p_c, it is suggested that separate attention should be paid to the contribution of finite clusters, and of the infinite cluster. The former should give rise to a weak set of singularities with critical temperature T_c(1), the latter to standard Ising-type singularities with critical temperature T_c(p). An explanation is thus suggested for the non-analyticity discovered by Griffiths (1969).

The ⁵⁷Fe Mossbauer effect and the magnetism of the ferrous compound Fe(4.7-(CH₃)₂-phen)₂(NCS)₂ have been studied in detail between 4.2 and 298K (phen=1.10-phenanthroline). At the temperature T_c=121.5K, the Mossbauer spectrum changes abruptly and the effective magnetic moment decreases. The observations are interpreted in terms of a high-spin (⁵T₂) to or from low-spin (¹A₁) transition associated with a phase change at T_c. The transition is incomplete, although only a slight contribution ( approximately 2% and approximately 4% respectively) of the complementary state remains at the highest and the lowest temperature studied. Debye-Waller factors were determined from the saturation corrected areas of the Mossbauer spectra and results provide evidence for different lattice dynamical properties at the sites of ⁵T₂ and ¹A₁ ground-state iron(II) molecules in the crystal.

The magnetic excitation spectrum in NdAl₂ has been measured by means of inelastic neutron scattering at 5.3K for momentum transfer in the (110) direction. Several excitation branches are observed and discussed in terms of a hamiltonian including an isotropic Heisenberg exchange interaction and a cubic crystalline electric field. The spectrum is interpreted as comprising of six excitation modes four of which are transverse spin-wave-like excitations (two acoustic and two optical). The remaining two excitations are longitudinal in character (acoustic and optical). The authors obtain values for both the exchange interaction and the crystal-field parameters.

The piezoelectric switching of antiferroelectric domains by the simultaneous application of an electric field E_i and mechanical stress T_jk is proposed as a direct method for examining whether some spinel compositions do have point symmetry 43m and concomitant antiferroelectric ordering as proposed by Grimes (see abstr. A30773 of 1973). For point symmetry 43m, various possible switching procedures have been described in detail. The smallest coercive product of electric field and stress (E_iT_jk)_c is required for switching when the electric field is chosen along cubic (001), and directional stress along (110) or (110). For particular directions, for example (111), the colinear application of electric field and stress also leads to switching.

Acoustic paramagnetic resonance and thermally detected, electric-field-induced electron paramagnetic resonance experiments have been carried out at liquid helium temperatures on the Ni³⁺ ion in Al₂O₃. Five features in the spectra are identified and it is shown that dependence on the angle between the magnetic field and the c axis can be explained by an effective Hamiltonian operating within the ²E ground state only but including the effects of the trigonal field and spin-orbit coupling by perturbation theory. The same hamiltonian also gives a complete explanation of the original electron paramagnetic resonance measurements of Geschwind and Remeika (1962) and of recent thermal-conductivity scattering experiments.

The pseudo-spin model of ferroelectric crystals like KDP is extended to include coupling between the soft spin-wave mode and fluctuations in the phonon numbers via spin-two phonon terms in the hamiltonian. It is shown that this provides an explanation of the temperature dependence of the width of the Raman scattering response and predicts a central peak in some circumstances. The magnitude of the coupling may be shown to be related to the splitting of the E modes below the transition temperature since this arises from the same terms. The frequency dependence of the elastic constants is also discussed.

The Raman spectra of the three phases of KMnF₃ have been measured and a detailed group theoretical analysis has been carried out in order to identify the observed peaks. The results for the intermediate temperature phase are consistent with a tetragonal structure of D_4h¹⁸ symmetry but the results for the low-temperature phase do not unambiguously determine the crystal structure. Particular attention was paid to the temperature-dependent modes. The soft modes associated with the upper phase transition were fitted to the response functions for a damped simple harmonic oscillator and for a model including damping by phonon density fluctuations. The results favour the latter model and suggest that non-classical critical fluctuations play an important role near the upper phase transition.

Photoemission studies in the energy range up to 11 eV of cleaved trigonal crystals and evaporated films of selenium and tellurium prepared in a variety of ways, are described. Photoelectric yields, thresholds and work functions have been established. The electronic band structures for selenium and tellurium determined by experiment are compared with calculations.