Table of contents

Nuclear-spin-dependent scattering of neutrons from ⁵⁵Mn was measured using the technique of polarization analysis. Magnetic contributions to the scattering were minimized by using beta -Mn and working at 4.2K where any oxides of manganese are magnetically ordered. The spin-incoherent scattering cross section was 0.11+or-0.075 b atom^-1; lower than previous estimates.

A theoretical expression is obtained for the small transverse NMR shift in the ³He B phase in terms of the parameters occurring in the Ginzburg-Landau expression for the free energy near T_c. It is suggested that measurements of this shift will provide useful information about these parameters.

Reports the results of a calculation of a high-temperature expansion for the non-linear relaxation time for a square Ising model with zero applied field. The author has also attempted to estimate the corresponding critical exponent.

A Monte Carlo method for calculating the anharmonic free energy of a solid is proposed. Application of the method to a model Lennard-Jones crystal shows good statistical resolution, for a small periodic system, except near the melting curve. The anharmonic free energy Delta F is seen to have the form Delta F/NkT=A₀-3/2AT for temperatures T above about one-half the melting temperature at all densities studied. The values for A are in better agreement with experimental rare-gas data than are previous estimates from conventional anharmonic perturbation theory. The value of Delta F/NkT as a function of the Lindemann ratio (root mean square displacement over nearest-neighbour distance) lie on a universal curve for all densities and temperatures studied.

The structure factor in liquid Sn has been measured, using the neutron diffraction technique, in the temperature range from the melting point up to 870 degrees C. A wide region of momentum transfer 0.5 AA^-1<Q<14 AA^-1, has also been covered. Diffraction patterns show the presence of a double structure up to 500 degrees C. Comparison is made with the theoretical structure factor obtained from the hard-sphere interaction potential. From the experimental structure factor, the atomic radial distribution functions are derived.

Anharmonic Debye-Waller factors and thermal smearing functions are derived for all temperatures in the one-particle-potential (OPP) model of vibrations by applying a perturbed density-matrix expansion about the harmonic case. Where possible, general results appropriate to sites of arbitrary symmetry are described, while the important case of sites possessing cubic point-group symmetry is investigated in detail. A useful finding is that the OPP model appears capable of describing the temperature dependence of anharmonic Debye-Waller factors at least down to Theta _M/4, while the previous classical approaches begin to fail, as expected, at about Theta _M, where Theta _M is the Debye characteristic temperature.

A model of KDP proposed by Kobayashi (1968), in which proton pseudo-spins are bilinearly coupled in the Hamiltonian to transverse optic phonon operators, is extended, following earlier authors, to include a similar coupling to acoustic modes plus a magnetostrictive dependence of pseudo-spin exchange on longitudinal acoustic vibrations associated with lattice parameter fluctuations. This provides a damping mechanism for propagating modes omega ₁(q) and omega ₃(q) having optic and acoustic character, respectively. Imaginary parts of the mode frequencies are extracted from the poles of the spin Green function, which is calculated by the equation-of-motion method, with random phase decoupling, for T>T_c.

Computed infrared absorption spectra, based on the point charge model are presented for vitreous SiO₂, GeO₂ and BeF. Wherever detailed comparison is available, the calculated relationship between frequency spectrum and absorption spectrum is found to agree well, in most respects, with that given by experiment. In particular, it is noted that the positions of peaks in the infrared spectrum need not coincide closely with those of peak in the frequency spectrum.

The change in the band gap of a semiconductor due to high doping is considered in the limit where the free carriers form a degenerate gas. In contrast to previous treatments, the effect of the change in the electron interaction on the whole band structure is considered within a self energy framework so that the difference between valence and conduction bands can be found more accurately. The results obtained are significantly different, in some aspects, from earlier work. This is discussed and the application to experiment reviewed.

Non-relativistic calculations on the band structures of the 2H polytype of NbSe₂, NbS₂, TaSe₂ and TaS₂ are reported. The slight qualitative differences which occur seem to be determined by the chalcogen rather than the metal. The Fermi surfaces are presented, together with a simple and accurate Fourier fit involving three parameters per sheet. The hole orbits are infinitely extended in the direction normal to the layers. A possible connection of these results to the observed charge-density waves is discussed. Some comments concerning the technique (layer method) are included. Special attention is devoted to the convergence of the method and the use of group theory.

The geometry of stationary or mobile metallic and semiconducting domain configurations and their formation during the switching of a VO₂ single crystal are discussed. It is shown that the motion of narrow semiconducting domains in the sense of the electric current is driven by the Peltier effect at the two oppositely biased interfaces. The Peltier heat may be partially or totally compensated for by thermal fluxes along temperature gradients in the supercooled metal and in the superheated semiconductor.

A new, general model explaining the electronic properties of dislocation structures in elemental semiconductors is presented. The model is inferred from phenomena observed in Ge and Si crystals with edge-type dislocations are analysed in terms if two types of states: local states resulting from the strain field in the distorted regions of dislocations, and states analogous to those in the amorphous semiconductors occurring in the highly disordered (amorphous) regions of dislocations. In both n- and p-type Ge the Fermi level in the amorphous regions is fixed about 0.45 eV below the conduction band edge. The experimental results of electron paramagnetic resonance, optical absorption, radiative recombination and photoconductivity in plastically deformed crystals are taken into consideration and explained in terms of the proposed model.

It is shown that a sufficient condition for any self-consistent approximation for response functions (configuration-averaged) in disordered systems to obey the macroscopic conservation laws is that the Ward identity holds for the approximate quantities. The diagrammatic and graphical techniques of generating the CCPA of two-particle Green functions are examined in this light.

The electrical resistivities of single crystals of LaB₆ and ReO₃ were measured in the temperature ranges of 4.2-360K and 4.2-300K, respectively. The residual resistance ratio of LaB₆ was 450 and that of ReO₃ was 90. The resistivity value of LaB₆ at 300K is 8.90*10^-6 Omega cm and for ReO₃, 10.5*10^-6 Omega cm. The experimental data were analysed from the scattering of electrons by polar optical phonons in addition to the scattering by acoustic phonons and impurities. The temperature dependences of the resistivities were explained well by this model for the temperature range measured, taking the Debye and the Einstein temperatures as 245 and 920K for LaB₆ and 330 and 800K for ReO₃, respectively.

Electron-hole and excitonic recombination processes in simple semiconductors are analysed for possible non-equilibrium phase transitions. A second-order type transition is found to be possible when impact ionization under an external electric field and radiative and Auger recombinations are taken into consideration. The order parameter of the transition is the electron concentration, and the control parameter is C_i-l where C_i is the impact ionization coefficient and l is an inverse carrier life-time for processes not considered explicitly. Using the Shockley model, some simple numerical estimates of the concentrations and electric field involved are made. It has also been shown that the quantum character of the carriers in semiconductors greatly limits the possibility of first-order-type phase transitions when one or two distinct types of fermions are involved with boson excitation.

Magnetophonon resonances have been observed in the warm-electron coefficient beta in n-GaAs, at temperatures between 55K and 77K. The resonances seen indicates a rapid change in importance of the energy relaxation processes involving LO phonons from impurity site capture at 60K to inter-Landau level transitions at 70K. The relative amplitudes of the different magnetophonon series was measured precisely by the use of Fourier analysis. The observed behaviour is interpreted in terms of a strong distortion of the electron distribution at energies one LO phonon distant from the donor ground state. At 77K it is found that high electric fields can change the sign of the extrema and also produce a small shift in the position of the extrema.

The thermal and electrical resistivity and Seebeck coefficient of heavily doped, pressure-sintered germanium-silicon alloy specimens have been measured in the temperature range 300-1200K. The thermal resistivity was considerably in excess of that for corresponding crystalline samples at temperatures below that of the thermal resistivity maximum; the temperature of this maximum was, however, about 75K below that found in crystalline samples. It proved possible to explain the data below the maximum by a Callaway-type thermal conductivity calculation including both free-carrier and boundary scattering of phonons. The boundary scattering mean free path was smaller than might have been expected from the grain size.

The problem of the phase transition from the paramagnetic to the ferromagnetic state in U₃P₄ compounds is discussed, based upon a previously proposed model which contains a three-axial single-ion anisotropy. In the framework of the molecular-field approximation, the thermodynamic perturbation theory is applied in the vicinity of the Curie point, and the Curie temperature and magnetic ordering are determined.

Low-temperature magnetic studies on Mn(ClO₄)₂6H₂O single crystals in the temperature range 80-300K are reported. From these studies the extreme low-temperature thermal and magnetic behaviour of the salt can be predicted. A thermally reversible magnetic anomaly associated with a change in crystalline symmetry from pseudohexagonal to monoclinic has been found to occur in this complex at T_c (about 120K), as in the isomorphous perchlorate reported elsewhere. Above this transition temperature T_c, where K/sub /// is found to be greater than K_{perpendicular to} , the ⁶S_5/2 ground term for the free Mn²⁺ ion is slightly split in the crystal due to the ligand field, with a spin doublet lying lowest and spin quartet above it with spin splitting parameter D=-0.0123 cm^-1. However, below T_c, K/sub ///<K_{perpendicular to} , and the spin pattern is inverted so that the quartet lies lowest and D=+0.0120 cm^-1. The change in sign and magnitude of D very satisfactorily explain the small observed deviation from Curie law, the magnetic anisotropy and the relative change of ordering of K/sub /// and K_{perpendicular to} above and below T_c.

Elastic neutron scattering from a single crystal of hexagonal ¹⁶⁰GdCl₃ was measured at liquid helium temperatures with neutron wavelengths of 1.45 and 2.36 AA. Large extinction was observed and analysed in terms of a recent theory developed by Becker and Coppens (1974-75). The strongest reflections were reduced by primary and secondary extinction by 2 and 80 per cent respectively. From magnetic Bragg intensities ((101) and (112)) in the temperature range 0.002<or=1-T/T_c identical to epsilon <or=0.07 the critical behaviour of the spontaneous magnetization was derived. The epsilon -dependence of the magnetization was explained by a power law with an exponent of 0.43+or-0.02 and an amplitude of 1.40+or-0.10, or with the same accuracy by a modified mean-field law predicted for the group of uniaxial dipolar ferromagnets.

The hyperfine magnetic field at ¹⁸¹Ta nuclei at the zirconium sites in ferromagnetic ZrZn₂ was measured by the method of time-differential perturbed angular correlations at 4.2K. The value obtained was -17.3+or-0.3 kOe. The field was also measured as a function of the concentration of hafnium and titanium substituting at the zirconium sites and compared with magnetization measurements on the same samples. The rate of change of hyperfine field with concentration is not as great as that of the magnetization.

DC magnetization measurements at 4.2K in applied fields up to 340 kG are reported. The smooth behaviour of the magnetization curves confirms that the anisotropy may be attributed to the octahedral site Mn ions.

Mossbauer and magnetization measurements have been carried out in Co₂-Z compound. The Mossbauer data have been correlated to measurements of magnetization and anisotropy constants. The temperature dependence of the second-order anisotropy constant K₂ turn out to be of single-ion origin and essentially determined by the magnetization of the sublattices containing cobalt ions. Making use of these data, the behaviour of the anisotropy constants K₁ and K₂ and of the spin reorientation transitions occurring in Co₂-Z have been analysed.

Structural magnetic and electrical measurements were carried out for the system Ni_xCu_1-xMn₂O₄ (0<or=x<or=1). Correlation of physical properties has been attempted in order to arrive at the ionic configuration for each composition of the system on the basis of existing theories. The cation valence distribution for NiMn₂O₄ has been obtained as Mn³⁺(Ni²⁺Mn_0.1²⁺Mn_0.90²⁺) O_3.95^2-.

An off-centre position of an impurity in a crystal is intelligible only when the polarization interactions are taken into consideration. The value of an off-centre displacement for an OH^- ion in alkali halide crystals as given by Pandey and Shukla (1971) has been used to calculate the polarization force parameter P. The parameter P has also been calculated directly and the values are in fair agreement with those obtained from the off-centre displacement data. Gradual variation of polarization in the different matrix cages has also been explained qualitatively.

Raman spectra of Fe³⁺ and Cr³⁺ doped MgO single crystals have been obtained, and are interpreted with the help of the projected densities of states of pure MgO. The spectra of the two crystals are different, presumably because the Cr³⁺ ions are at non-cubic sites.

The effects of uniaxial stress on the optical spectra of TmVO₄ and TmAsO₄ have been studied at liquid helium temperatures. For both these Jahn-Teller systems the predominant interactions have been found to be the coupling of the electronic system to bulk strain. In addition the behaviour of TmAsO₄ under simultaneous application of uniaxial stress and magnetic field in the plane perpendicular to the c-axis is found to be adequately described by a molecular-field model.

The K X-ray absorption spectra of some intermetallic compounds of cobalt have been recorded using a 40 cm Cauchois-type bent spectrograph. It is observed that the main K absorption discontinuity in the compounds shifts to the high-energy side with respect to that in the cobalt metal. The observed chemical shifts can be related to the effective charges on the cobalt ions in the different compounds, which are calculated on the basis of Suchet's theory. An attempt has been made to explain the observed correlation.

High-resolution, high-energy UV and monochromatized Al K alpha X-ray photoemission spectra of the valence bands of SnTe are presented and compared with the results of first-principles relativistic OPW and local and non-local EPM band calculations. Photoemission spectra and the results of band calculations for other FCC structure IV-VI compounds are also briefly discussed and compared to the authors' results. Relativistic OPW calculations are found to give the best overall agreement with experiment. Experimental Sn and Te d-band binding energies and spin-orbit splittings are also given. The Sn d levels are found to be considerably more tightly bound than predicted by OPW band theory, as are the outer d levels of the other IV-VI cations.