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Low k, omega fluctuation spectra of the densities of the above two conserved quantities are evaluated using canonical ensemble averages. The effect of other conserved densities is determined for hard sphere potentials where the diffusivities can be expressed in terms of the self-diffusion constant.

The authors discuss the distinction between defect parameters (energies and entropies) defined at constant pressure and those defined at constant lattice parameter or constant volume and draw attention to the relations which exist between these quantities. These relations are important for the comparison of theoretical and experimental values, since the latter are almost invariably obtained from experiment at constant pressure while the basic calculations are most conveniently done at constant volume. They illustrate this importance with detailed calculations for AgCl and in particular show that the enthalpy of Frenkel defect formation at constant pressure, h_p, and the corresponding internal energy for defect formation at constant volume, u_v, depend in opposite ways upon the temperature. In addition the authors calculate the free volume of defect formation, also as a function of temperature. The agreement between these calculations and experiment is excellent.

Using a new and powerful real space renormalisation method recently introduced for phase transitions, the authors prove numerically that two dimensions are marginal for Anderson localisation. An associated transition is found, presenting close analogies with the Kosterlitz-Thouless transition of the XY model in two dimensions, while the behaviour in three dimensions is shown to be a standard one with a mobility edge. The accuracy of the method is better than previous approaches in two dimensions and is not reduced when the number of dimensions is raised to three.

The localisation of electrons moving in a random potential is studied in two dimensions using the real space renormalisation group method of Domany and Sarker. The effects of the cell size and of the second-order terms in the perturbation expansion are examined. While the method is not particularly sensitive to the cell size, its results depend crucially on the truncation of the perturbation series.

Gallium arsenide doubly doped with zinc and chromium has been subjected to 2 MeV electron irradiation in stages at room temperatures and examined by the EPR technique after each stage. When the free carrier absorption is first eliminated a signal attributed to Cr_s⁴⁺ is observed, but with further dose, spectra due to Cr in lower symmetries are observed. A predominant centre has C_3v symmetry and is tentatively ascribed to (Cr_s⁴⁺-Ga_i) nearest-neighbour pairs. The pairing results from the displacement and subsequent motion of intrinsic defects introduced by the irradiation.

Clebsch-Gordan coefficients are calculated for the irreducible representations in the marcasite structure.

Calorimetric, thermal expansion and optical measurements as well as optical observations confirm the sequence of structural phase transitions D_4h¹⁷(I) to D_2h¹⁸(II) to D_2h¹²(III) in the perovskite layer compound (C₂H₅NH₃)₂CuCl₄. For the I to II change the results show second-order character together with the appearance of ferroelasticity in the crystal whereas first-order character is found for the II to III change. A thermodynamic theory of the elastic and dielectric properties is developed. From the free-energy expansion compatible with both structural changes in the crystal the authors obtain the temperature dependence of the material constants which is in agreement with the experimental results in this work. The coupling of the order parameter as well as the origin of the spontaneous ferroelasticity are discussed.

A Raman study of single-crystal hexa-ammine nickel (II) chloride between 20 and 300K is presented. A static distortion of the lattice is observed at T_c=76+or-5K but the evidence suggests that proton orientational ordering begins before the lattice distortion occurs and continues down to T₀ with T_c-T₀ approximately 8K. A critical exponent of (0.48+or-0.03) is obtained for the growth of the light scattering intensity from internal modes associated with ammonia vibration as the proton groups order. The spectra obtained are consistent with a doubling of the primitive cell to a distorted D_3d space group below T_c.

ESR experiments performed on crystalline and amorphous WO₃-doped V₂O₅ indicate different behaviours of both spectra with temperature and microwave frequency. A line broadening is observed for the crystalline oxide as the temperature increases while the ESR spectrum of amorphous V₂O₅ remains well resolved up to 300K. This suggests that the electron hopping frequency between V⁴⁺ and V⁵⁺ ions is much smaller in the amorphous oxide. A strong frequency dependence of the ESR linewidth is observed for the amorphous sample, but not for the crystalline one. This has been attributed to a distribution of g-values among the different vanadium sites caused by a fluctuation of the local crystal field Delta . An analysis of the ESR linewidth as a function of the microwave frequency leads to a value delta Delta =0.24 eV of the crystal field fluctuations. Such a random distribution of Delta contributes to increase the activation energy of the hopping process and thus to the localisation of the charge carriers in the amorphous oxide.

The static and dynamic dielectric functions of pure zero-gap semiconductors with linear dispersion relations are calculated. They exhibit logarithmic singularities arising from the linear dispersion relations of Gamma ₈^c and Gamma ₆ bands. A finite Fermi level removes these divergences leaving, however, dielectric constants that are strongly concentration dependent.

The authors study the model of a quasi-one-dimensional metal in which the interaction of conduction electrons with intra-molecular modes and three-dimensional acoustic phonons generates two energy scales: the gap in the electron spectrum Delta and the temperature of the structural transition T_c. T_c proves to be the parameter determining the main properties of the system. An expansion in powers of the lattice deformation shows that the mean-field theory has no region of applicability in the system under study. Below the structural transition point there appears an optically active Frolich mode. In the case of compounds with two oppositely charged chains the frequency of this mode turns out to be of the order T_c. One further low-frequency optically active mode is found in the system under study. It corresponds to electron density excitations and has a gap approximately T_c. In the model considered, unlike the Peierls model, there is no adiabatic parameter and both modes result in the dielectric constant epsilon approximately omega ₀²/T_c², where omega ₀ is the plasma frequency.

Starting with the linearised master equation, the authors present a first-principles theory of conductivity for disordered systems. The theory is valid for all situations to which the master equation description applies. The path summation and configurational average are evaluated by generalising relations obtained from exactly soluble models. Both symmetric and asymmetric energy-dependent transition frequencies are considered. In the latter case they are able to define an energy-dependent conductivity from which it is possible to evaluate the thermopower. All electronic transport properties, including the frequency-dependent conductivity can be evaluated self-consistently, the only input parameter being the density of states. Numerical results for the DC conductivity and thermopower are presented using several model density-of-states functions. For random statistics, the results are in complete agreement with percolation theory for low densities (temperature). The theory is exact in the high-density (temperature) limit.

The diffusion of carriers in Si is shown to be drastically modified by band bending induced by gradients of carrier temperature and lattice temperature. The effect is studied at low and high temperatures, and consequences about the mechanisms of transfer of energy from the energy source (the laser) to the solid are discussed.

Electron drift, Hall mobility and thermoelectric power in InP_xAs_1-x, Ga_xIn_1-xAs and InAs_xSb_1-x have been calculated by an iterative solution of the Boltzmann equation. A Kane-type band structure corrected for the lattice disorder is assumed. All the relevant scattering mechanisms are included together with band non-parabolicity, wavefunction admixture, degeneracy of the distribution function and screening of the scattering probabilities due to free carriers. Only the ionised impurity concentration (N_i) and the alloy scattering potential ( Delta E_a) have been treated as adjustable parameters to be determined from a fit with the experimental data. Variations of the other different material parameters across the alloy system have been taken from different sources, or reasonable assumptions are made regarding such variations. A large number of experimental results on both mobility and thermoelectric power are considered and good agreement is obtained with Delta E_a larger than that given by the difference in electron affinity between the constituent compounds.

The critical properties of a random resistor network are investigated, using an infinitesimal Migdal-Kadanoff renormalisation group transformation, for cases where the non-zero conductors have a probability distribution h(g) which behaves as g^{- alpha} , 0<or= alpha <1 for small g. A new fixed point is found, leading to a new type of critical behaviour of the bulk effective conductivity near the percolation threshold. The critical conductivity exponent t is found to depend on alpha , but differs from the value found by Kogut and Straley (1979) for this case. The crossover between the two types of critical behaviour, characterised by the different fixed points, is also discussed.

An effective computer method for studying the spin Hamiltonian for 3dⁿ ions (except 3d⁵) within the lowest LS-term approximation is developed. The expressions derived enable one to compute spin Hamiltonian parameters from the knowledge of the crystal-field parameters for arbitrary symmetry and the spin-orbit and spin-spin coupling parameters. The method has been successfully applied by the author to the C₁ symmetry of case of Fe²⁺ ion in YIG:Si(Ge) in a separate paper (1979). The matrix elements given can be used for other 3d⁴ and 3d⁶ ions at arbitrary symmetry sites in crystals. The imaginary terms and the fourth-order terms (S=2) in spin Hamiltonian may now be studied from a microscopic point of view. Due to his method, model computer calculations of the zero-field spin Hamiltonian and the Zeeman parameters for 3dⁿ ions with an orbital singlet ground state at arbitrary symmetry can be easily carried out. It may be helpful in identifying the 3dⁿ ions with the various symmetry centres in crystals and in pressure studies of low-symmetry effects.

A new method of analysing critical behaviour of high-temperature series in a finite field is presented. In the study of magnetic critical behaviour, the critical point is ordinarily approached through either a zero field line or a critical isotherm. The authors present a method of approaching the critical point which is different from the conventional ones. Their method is especially useful for the analysis of high-temperature series in a finite field. It can be used to calculate critical indices beta , delta , gamma as well as to determine the critical point. However, these indices are not the same as the conventionally defined indices and may be regarded as a new set of critical indices. They are equal to the conventional ones if the scaling functions are assumed.

Magnons and phonons in the mixed crystal Fe_1-xMn_xCl₂ have been studied by Raman scattering techniques. The k=0 Fe²⁺ magnon-like excitations have been observed for T<T_N over the range x=0 to x=0.25. The temperature dependences of the Fe²⁺ one-magnon peak position, linewidth and intensity in FeCl₂ and in some mixed crystals have also been studied. In FeCl₂ the magnon frequency renormalises relatively slowly with increasing temperature (12.2.+or-0.5 cm^-1 at 19.6K compared with 17.1+or-0.3 cm^-1 at 5.8K). The magnon width increases sharply and the Raman intensity decreases as T_N=23.5K is approached. A theoretical interpretation of this behaviour is given, indicating the likely importance of quadratic magneto-optical coupling in the light scattering mechanism. The magnon at 17.1 cm^-1 in FeCl₂ is sensitive to dilution with Mn²⁺ ions. For small x it drops rapidly in energy with a linear concentration dependence, and it then appears to incurve for x>0.15. For the mixed crystals an Ising cluster model is proposed and discussed in terms of the observed concentration dependence of the magnon energy. The temperature and concentration dependences of the two Raman-active k=0 phonons have been analysed over the whole concentration range. The A_1g phonon frequency exhibits a linear concentration dependence in accordance with the predicted one-mode behaviour, whereas the E_g phonon behaviour is more complicated for <or approximately=0.6 because of electron-phonon coupling.

Optically detected magnetic resonance spectra of pure and doped crystals of AgBr have been obtained at liquid helium temperatures. The free-electron g-value was found to be 1.49. This resonance is eliminated by electron-trapping dopants and enhanced by Cd²⁺ and Pb²⁺. A resonance at g=2.08 has been assigned as the free-hole resonance. This resonance is enhanced by electron-trapping dopants and by Cd²⁺ and Pb²⁺. Two strong resonances were found at g-values of 1.81 and 1.76. These resonances are thought to be from carriers trapped at intrinsic sites.

Raman spectra of SrTiO₃ under a high DC electric field have been investigated for the ferroelectric soft mode at 77K. Frequency splitting was observed above 20 kV cm^-1 and the saturation of the integrated intensity occurred at 46 kV cm^-1. Two split modes correspond to A_2u and E_u modes in D_4h symmetry rather than the expected field-induced coupling with the soft A_1g mode for a 105K structural phase transition. The results suggest that field-induced anisotropy predominates over the intrinsic tetragonal anisotropy for E_ext>or approximately=20 kV cm^-1.

Schottky barrier tunnel junctions were prepared by cleaving degenerately doped silicon in a stream of evaporating indium. For heavily doped samples the bias dependence of the incremental resistance at helium temperatures showed agreement with many features of the theoretical predictions for a free electron model. For these samples striations, regular doping density variations reportedly present in all silicon crystals, played a minimal role. For lowly doped samples disagreement between theory and experiment was observed. The inapplicability to lowly doped silicon of the simple theory and the effect of striations are both interpreted to contribute to the deviations. Striations are essential for understanding the radiation compensations results; behaviour in a single junction appropriate to a parallel combination of junctions with different doping density was observed. For strongly compensated material the zero-bias conductance exhibited an exp(-C/T¹4/) behaviour characteristic of tunnelling via localised states.