Table of contents

Room temperature electron irradiation is found to increase the conductivity of amorphous silicon evaporated in a partial pressure of oxygen. The conductivity changes by a factor of two for an electron dose of 10¹⁶ cm^-2; the original conductivity is restored by annealing at room temperature for one hour. No change in conductivity is observed due to electron irradiation in amorphous silicon evaporated in vacuum or in a partial pressure of hydrogen.

Disordered spin systems with the number of spin components m are studied in dimensions d=2+ epsilon for small epsilon . For dilute quenched random systems, it is found that the system has the same critical behaviour as for a pure ferromagnetic if m>2. For the spin-glass phase, new critical exponents are calculated for m>or=2. The x-y spin glass is found to have a phase, new critical exponents are calculated for m>or=2. The x-y spin glass is found to have a vanishing critical temperature as d approaches 2, in contrast to the ferromagnetic x-y model.

The spectrum of the longitudinal magnetisation in an isotropic model ferromagnet is calculated in 6- epsilon dimensions. The interaction with spin waves makes the transport coefficient diverge for small wavenumbers like k^(d-6)3/. The regular part of the transport coefficient is also determined at k=0. A self-consistent extension of the approximation to arbitrary dimensions is briefly discussed.

A heat capacity anomaly, observed around 172K in single crystal V₆O1₁, is associated with changes in magnetic and electrical characteristics. The corresponding enthalpy and entropy of transition were found to be 1.1 kcal/mol^-1 V₆O₁₁ and 6 cal deg^-1 K^-1 mol^-1 V₆O₁₁ respectively.

Heat capacity results on a-Fe_xNi_80-xP₁₄B₆ for x=0, 40, and 80 are reported between 0.4K and 10K and values of theta _D and gamma determined. For the alloys containing Fe, which are ferromagnetic a large spin-wave contribution is seen and values of D obtained. In a-Ni₈₀P₁₄B₆, which shows no bulk magnetism, the formation of magnetic clusters is suggested by the presence of a large, almost constant contribution to C_p.

The electrical resistivity at low temperature of metallic NiS_2-xSe_x is measured at a temperature between 4.2 and 300K under high pressure up to 15 kbar. The resistivity is found to obey an expression rho = rho ₀+ATⁿ and the coefficient of A has a maximum at the critical boundary of antiferromagnet. The index n decreases from 2.0 to 1.6 as one approaches the critical boundary from the paramagnetic phase. These results seem to be explained by recent theoretical works based on the spin fluctuations.

A method of correcting the displaced Maxwellian distribution in a case of velocity dependent scattering is suggested.

An experimental study of the EMF arising when semiconductors (germanium) in a magnetic field are bombarded with ionizing particles ( alpha -particles, protons)-the radiative electromagnetic effect (the REM effect)-has been carried out. It has been shown that the REM EMF depends linearly on the magnetic field and alpha -particle flux. The observed effect breaks out at the p-n junction which is created under the bombardment of germanium sample with alpha -particles. The even REM effect, which is quadratic with the magnetic field, has also been measured.

Numerous field-sustained surface states with energies greater than the work function are found to exist for a delta -well crystal in a positive applied electric field. Electrons in such states tend to be strongly localised outside the crystal.

A new-type spatial spin correlation function in the 'spin glass' phase of disordered magnets is studied.

Several recent treatments of the temperature dependence of the spin susceptibility chi (T) of nearly ferromagnetic Fermi systems suggest that there is a T²lnT term in chi (T). It is shown, in a simple microscopic model, that such a term arises from retaining only one class of contributions to xi (T), and is exactly cancelled by another class, present in a spin-conserving calculation.

It has been predicted theoretically that the tetragonal-orthorhombic phase transition of crystalline HCN is associated with a small change in equilibrium C-H...N distance. This change is too small to be observed directly by X-ray crystallography. It is proposed that the difference of C-H stretching frequencies in the two phases which has been observed in published infrared and Raman spectra of crystalline HCN can be assigned to this change of equilibrium C-H...N distance.

KCl(Sr) crystals have been irradiated at temperatures in the range 77K to 293K. At all temperatures in the range 140K to 293K two absorption bands in the neighbourhood of 220 nm and at 316 nm formed together with the F-band. The latter band is identified as the Hayes-Nichols (1960) (H-N)-band via its production, annealing and the interconversion behaviour between it and the band near 220 nm, the V₂^m-band. The irradiation at 77K produced a composite absorption peaking at 348 nm together with a very small V(240) band. The former absorption was decomposed into two bands, the H_Na-band and a small contribution from the 316 nm-band. It is concluded that H-centres released from the H_Na-centres are captured at IV dipoles to form the H-N and V₂^m-centres.

The equilibrium lattice configuration, electronic excitation energies and activation energies for hopping motion are calculated for a self-trapped hole in simple cubic CsCl, CsBr and CsI. The defect is regarded as a X₂^- molecular ion (X=Cl, Br, I) whose bond-length has been modified by the crystalline environment. Agreement with the experimental ultraviolet transition energies is good. Excitation energies deduced from measurement of g-shifts in CsBr and CsI are too low, a feature common to all alkali bromides and iodides, and attributed to the approximations involved in their deviation. The initial calculations predict lower activation energies of 90 degrees jumps than for 180 degrees jumps, in contrast with what is observed in CsI. An alternative model is presented, which reproduces the correct trend. Comparison of the actual numbers with experiment is hampered by the fact that the latter are done at low temperature (60-90K), the calculations being done in the high-temperature limit.

A crystal is divided into infinite strings of molecules assembled on a two-dimensional lattice, each string occupying a 'tunnel' formed by neighbouring strings. Molecular motion along the string direction only is considered. In a mean-field approximation, the positional correlation of a molecule with its neighbours within the string and those in different strings are separated. The resulting equations allow a smooth variation from a harmonic crystal to a 'tunnel' model of a liquid. Approximate solutions of these equations allow the melting temperature, if any, of this system to be predicted from intermolecular force constants of the crystal with no adjustable parameters. Melting occurs when the crystal becomes unstable to large-amplitude transverse shear waves (motions corresponding roughly to phonons of small longitudinal and large transverse wavevector). The theory has been tested by use of the elastic constants and melting temperatures of 51 crystals, metallic and molecular, of six crystal classes. The ratio of agreement between theory and observations is 0.98+or-0.22.

The relative stabilities of HCP, BCC and isotropic melt phases may be estimated from the temperatures at which the HCP phase 'melts' to either of two different anisotropic 'worm' fluids. The transition temperature HCP to BCC, T_c, and the melting point, T_m, in this model depend for any metal only on the elastic constants C₆₆, C₄₄, C₁₁ and C₃₃ and two empirical parameters which are ratios of entropy changes. More direct knowledge of possible pseudo-potentials is not required. An effective anisotropy is defined in the shear elastic constants of the HCP phase A' which is independent of the two empirical parameters. For A'<1 the author expects HCP crystals to melt without transforming. This is observed in the 19 HCP metals tested. For 1<A'<A'_max an HCP to BCC transition (A'_max approximately=2) is predicted and observed. For A'>A'_max the HCP phase is always expected to be unstable relative to BCC. The absolute values of T_c and T_c/T_m, are calculated with a standard deviation from the experimental values of 20%.

The static and dynamical properties of the molecular (two sites) polaron are investigated. The problem is reduced to that of a succession of electron hops in imaginary time, interacting with each other by phonon exchange. It can be solved analytically in the two regimes h(cross) omega ₀/kT>>1 (self trapped) and h(cross) omega ₀/kT<<1 (nearly free) where omega ₀ is the phonon frequency. There is a crossover between the two regimes which can be evaluated for a band of phonons where it is found to be smooth. The phonons have two effects on the motion of the electron: they reduce the hopping frequency and act also as a frequency modulation. The dynamical polarisability has a spectrum which is broadened by the frequency modulation. It is this fact which enables the molecular polaron (or any similar binary system) to be used at the emitting or receiving end of a communication channel.

The screened potential of an ion moving through a free-electron gas is calculated, and the resulting energy shift of bound electronic states of the ion is estimated. A decrease of the 2p-1s transition energy is found due to screening effects, it is compared with experimental results for high-velocity S projectiles in an Al target.

The low and high temperature DC and AC conductivities of As-Se-Te glasses have been studied with different Se:Te ratios. The transport behaviour has been discussed in terms of known transport mechanisms. The composition dependence of the conductivities are discussed in terms of known structures. The transport among extended states has been evaluated by subtracting hopping contributions and it has been shown that this part of the conductivity indicates the existence of a reference temperature for extended states transport.

Measurements of the electrical and thermal conductivity and the thermoelectric power have been made in the binary direction of arsenic between 2K and 300K. Below 30K the ideal electrical resistivity shows a T³ behaviour. The thermal conductivity has been tentatively separated into electronic and lattice contributions by means of the Wiedemann-Franz law. The lattice thermal conductivity shows a maximum at 30K and is attributed above this temperature to phonon-phonon Umklapp processes and below this temperature to phonon-electron interactions. The thermoelectric power, which also shows a maximum at 30K, evidences a marked phonon-drag effect in arsenic.

A theoretical study is made of the longitudinal magnetoresistance of photoexcited carriers in ultra-pure germanium at 2K in the presence of electron-hole droplet and acoustic deformation scattering for a magnetic field range B=0.19-0.5 T in the orientation j, B parallel to a (100) axis. Numerical calculations based on a relaxation matrix approach to the quantum transport theory reveal an oscillatory component to the magnetoresistance, periodic in B^1/2, with fundamental period ( pi /2a)(h(cross)m_c/2em_z)^1/2 where m_c and m_z are the cyclotron and kinetic masses and a is the radius of an electron-hole drop. The Fourier spectra show a sequence of peaks, each arising from a size resonance between the drop radius and the de Broglie wavelength for maximum momentum transfer in the elastic scattering processes.

A type of photoconductivity due to far-infrared light excitation is observed in n-channel inversion layers of Si MOS devices with high mobility. The photosignal is bipolar as a function of gate voltage and has little dependence upon the excitation light between the 337, 220 and 118 mu m laser lines. The photosignal is much stronger than that of inter-sub-band transitions. Shubnikov-de Haas oscillations are seen in the photosignals in a magnetic field and are found to be 180 degrees out of phase from the Shubnikov-de Haas oscillations in the conductivity. These characteristics can be understood in terms of heating of the inversion layer electrons. Photoconductivity due to cyclotron resonance is not observed in these samples.

The electronic structure of atomically clean cleaved (110) surfaces of InP has been investigated for a range of crystals of different doping. The Fermi level at the surface is not clamped by surface states. The chemical nature of clean and contaminated (110) surfaces have been investigated and interfaces between them and metal contacts examined by a range of techniques including a novel ultra-high-vacuum scratch test method. Whereas indium forms ohmic contacts to all faces studied, gold and silver form Schottky barriers which are nearly identical in magnitude. These barriers are of practically the same magnitude for intimate (clean) interfaces and for those with a thin oxide layer separating the metal and semiconductor. These results are considered in terms of existing theories. The adhesion of contacts is considered and the influence of annealing on contact behaviour briefly discussed. Au and Ag readily diffuse into InP but Al contacts are thermally more stable.

Renormalisation group methods are used to calculate to first order in epsilon =4-d the momentum dependent correlation function of an Ising-like model in the critical region for arbitrary momentum, temperature and external field. The results give an interpolation between various asymptotic limits.

The Niemeijer-van Leeuwen formulation of the renormalisation group has been used to discuss the quenched dilute Ising model and the Ising spin-glass in two dimensions. The authors chose a five-spin cell which, in the pure case, treats the Ising antiferromagnet equally as well as the ferromagnet. This choice of cell leads to the existence of a spin-glass ground state which was found to be stable for the whole composition range. The spin-glass transition was found to be sharp and the corresponding critical exponents were calculated. For the dilute Ising model the behaviour of the reduced critical temperature T_c(p)/T_c(1), the percolation concentration and the correlation length exponent were found to be in good agreement with the exact results and series calculations.

The anisotropy and magnetostriction of GdAl₂ has been studied by torque and strain-gauge measurements on single crystals. The temperature dependence of the constants are quite complicated but some evidence is presented which points to anisotropic s-f exchange as the principal origin of the effects.

The analysis of the EPR data of a first observed U⁵⁺(5f¹) ion in CaF₂ single crystals is reported. The trigonal EPR spectrum, observed at 77K, X-band, is characterised with very small g values, no superhyperfine splitting and a hyperfine structure with electric quadrupole splitting larger than the magnetic hyperfine interaction. The g values are used to justify a charge compensation mechanism implying six oxygens and two vacancies (along a (111) direction).

The local symmetry around the Cr²⁺ ions in Rb₂CrCl₄ was studied by means of nuclear magnetic resonance at 4.2K. From the hyperfine interaction of Cr²⁺ it was deduced that the chlorine octahedron is elongated with the tetragonal axis parallel alternately to the a and b axes whose bissectrix is the easy direction of magnetisation. The nuclear quadrupole moment Q of ⁵³Cr was also estimated.

The problem of inelastic scattering of Mossbauer radiation on the order-parameter fluctuations near phase transitions is considered by means of restricted scaling. Expressions for the scattering intensities that are valid for a wide class of objects with gapless dispersion of the fluctuation spectrum are obtained. In extreme cases (i.e. hydrodynamic and critical ones) the line shape of the resonance nuclear absorption is analysed depending on the temperature and the collimations of radiation beams.

It is shown that Lifshitz' theory of van der Waals interactions between dielectrics can be extended to arbitrary geometry. The van der Waals force density is expressed in terms of the electric Green function of Maxwell's equations. This force density can be shown to be identical to that derived by Dzyaloshinskii and Pitaevskii (1959) from quantum field theoretical considerations. For dissipationless dielectrics the force density agrees with that derived from the free energy as calculated by Casimir's method (1948) of summing over normal modes.

The magnetic circular dichroism (MCD) for all transitions within cubic centres in cubic crystals is shown to be independent of the orientation of the crystal at high temperatures. At low temperatures, where the C-type term is no longer linearly dependent on magnetic field strength, isotropy is maintained except in the case of a Gamma ₈ ground state. Isotropy and C-type term saturation characteristics are illustrated by calculations of the MCD associated with the ⁴A_2g(t³) to ⁴T_2g(t²e), ⁴A_2g(t³) to ²T_1g(t³) and ⁴A_2g(t³) to ²E_g(t³) transitions of a d³ ion at a site of O_h symmetry.

Optical absorption spectra have been taken at 300K for samples of amethyst quartz that show anomalous pleochroism instead of dichroism. The transition moment of the 945 nm band is parallel to a lattice C₂ axis, called the 'optical C₂ axis'. The strong visible band which produces the purple colour can be decomposed into three overlapping components B₀, B₁, B₂ at 527, 538.5 and 599.5 nm respectively. B₀ is polarised parallel to the optical C₂ axis whereas B₁, B₂ are polarised perpendicular to this axis at theta =+53 degrees , -57 degrees to C₃ respectively. The absorption coefficient in the visible was proportional to the concentration of a recently discovered C₂ symmetry S=2 centre through a series of annealing and re-irradiation experiments. This work establishes that the S=2 centre produces the optical absorption and that it is an Fe⁴⁺ ion on a silicon site, verifying Lehmann and Moore's model (1966) of the colour centre.

A comparison between reflectivity and photoemission (EDC) experimental results for GaSe and GaTe is given using a deconvolution of experimental epsilon ₂ omega ² curves. An interpretation of optical spectra is made in terms of interband transitions assuming suitable approximations.

A method for calculating directional photoemission from atoms adsorbed on surfaces is presented, which requires no translational symmetry parallel to the surface. The technique is applied to the oxygen on nickel system and the results compared with recent experiments.