Table of contents

The authors have studied the melting of commensurate monolayer krypton on graphite using very high-resolution synchrotron radiation X-ray diffraction techniques; a krypton particle size of 2300 AA is obtained. The melting transition appears to be second order at 112.9K with an order parameter exponent beta =0.065+or-0.015 and a broadening of T_c, sigma /<T_c> approximately 0.0006 which we attribute to finite particle size. Although the broadening is much less than that obtained in coarser resolution experiments, the exponent beta agrees with those previous results.

Low-energy electron diffraction (LEED) analyses of several structure models for the Ge(001) 2 × 1 surface confirm that the reconstruction involves at least 3 layers, and demonstrate that models with buckled asymmetric dimers are favoured over models with symmetric dimers. Three models with different amounts of buckling are given, which show equivalent moderate agreement with experiment.

The contact potential between clean Ti films and an Au reference surface have been determined by the Kelvin vibrating capacitor method. The majority of Ti films deposited on glass substrates had a high work function, which suggests that their surface was mainly (0001). Ti films on substrates of oxidised Ti had a much lower work function and probably had a surface of mainly (1011).

The behaviour of the neutron or X-ray scattering cross section near a continuous Ising-like phase transition is studied. It is established that the resolution-induced crossover from the pure (T_c-T)^{2 beta} power-law behaviour, characteristic of the regime below T_c where the superlattice scattering is dominant, to the (T-T_c)^{- gamma} behaviour of the fluctuation-scattering sufficiently far above T_c, is described by a scaling function with universal features. The crossover behaviour is studied numerically with the aid of a renormalisation-group representation of the response function. The results are used to assess the reliability with which scattering experiments can be used to determine the critical exponents beta and gamma .

The samarium monochalcogenides are treated here as ionic crystals with the samarium ion having anomalous compression characteristics arising from the easy delocalisation of a 4f electron. The compression curve of the samarium ion has been determined from experimental PV data in SmS while the compression curves of the chalcogen ions are already available. It is postulated that the compression properties of samarium are the same in all crystals. The theory successfully predicts the following: (i) the transition pressures in SmSe and SmTe; (ii) the occurrence of continuous and discontinuous transitions; (iii) the slope of the phase boundary in SmS; (iv) the properties of Sm_1-xGd_xS mixed crystals; (v) the optical band gaps.

The recursion method is applied analytically to the Anderson model in the strong disorder limit. The chain basis and chain parameters are determined exactly to lowest order in the hopping energy. It is shown that for all dimensionalities there is a dense spectrum of exponentially localised states with a characteristic length independent of energy and proportional to the ratio of hopping to disorder. This calculation extends the application of the recursion method to all degrees of disorder and answers some questions about denseness of the spectrum, correlation of parameters, and completeness of the chain basis.

The authors develop a fast algorithm for evaluating the Kubo formula for the conductivity of a linear chain, and use it to study the dependence of the conductivity as a function of imaginary frequency. The results for the Anderson model with different degrees of disorder and different energies can all be scaled onto the same curve, which is of the form expected from the theory of localised states. The universal curve obtained provides a simple connection between tight-binding model results and the conductivity which can be calculated for an electron in a white noise potential. Similar, but not identical, results are obtained for tight-binding chains with a Cauchy distribution of site energies.

Specific features of kinetic exchange at low charge transfer energies are analysed. It is shown that the exchange may be of ferromagnetic type. The examples of Cu²⁺-Mn²⁺ and Ni²⁺-Mn²⁺ doping pairs in perovskite-type crystals are considered. The theoretical predictions agree with the experimental data.

The Hamiltonian describing spins localised on lattice sites contains the bilinear and biquadratic pair interactions, as well as the three-atom interactions of the type (S_i.S_j)*(S_j.S_k). The molecular field approximation is applied to study the ground state properties and the interrelations between the three collinear phases taken into account: ferromagnetic (or antiferromagnetic), paramagnetic and quadrupolar. The following properties of the system are due to the three-atom interaction: (i) a nonsaturated ferromagnetic ground state; (ii) contraction or expansion (depending on the sign of the interaction) of the quadrupolar region on the phase diagram; (iii) up to three consecutive transitions (first or second order) on cooling; (iv) heat magnetisation and 'sluggish' magnetisation against temperature curves. In some respects the three-atom interaction acts like the single-ion anisotropy of the type (S^z_f)².

The behaviour of an ensemble of two-level systems, which is modulated by the optic phonon field coupled to the radiation field, is investigated. In the case of strong-field irradiation, side bands of the Raman spectrum are predicted. For low-field irradiation, the model explains the LO phonon replica of the first-order spectrum which has been observed in Ga_xIn_1-xP, GaAs_1-xP_x and Al_xGa_1-xAs.

Single crystals of KCl, KBr, KI and RbI doped with up to about 0.1% ReO₄^- have been grown and studied by infrared and Raman methods. The spectra indicate that contrary to the implications of Zaporozhets and Pasechnyi (1979), the isolated perrhenate ion possesses T_d symmetry in all cases.

Luminescence-excitation spectra of the GR defect have been recorded at high resolution for an irradiated natural semiconducting diamond. Luminescence produced in the GR1 region was monitored while the wavelength of the exciting radiation was scanned over the GR1 and GR2-8 absorption systems and part of the ultraviolet continuum. All the GR2-8 lines observed in absorption are faithfully reproduced in the excitation spectrum, showing that the centre relaxes via the first excited state (GR1). Luminescence measurements have shown that the relaxation to the first excited state is nonradiative. The lines are weaker in the excitation spectrum than in the absorption spectrum, and this is consistent with the fact that they may also be observed by photoconductivity measurements. In addition, the results show that there is absorption underlying what is normally regarded as the GR1 absorption band that does not give rise to GR1 luminescence.

Polycrystalline silicon films have been prepared using chemical transport in a low-pressure plasma in a temperature range 80-400 degrees C and at deposition rates up to approximately 3 AA s^-1. Their Raman spectra show several features which are correlated with X-ray diffraction measurements and attributed to the presence of crystalline and amorphous-like components. Optical absorption, infrared spectra, preliminary data on dark conductivity and some further properties are reported.

The simultaneous hydrogen and silicon atom densities in amorphous silicon, a-Si, films prepared by the glow discharge technique have been measured by 25 MeV alpha -particle elastic scattering. Integrated band intensities for the silicon-hydrogen stretching modes, omega ₁^s and omega ₂^s, in the region 1800 to 2200 cm^-1 were determined for the same freely supported films. A similar analysis has been carried out for the bands observed at 890, 840 and 640 cm^-1. Effective oscillator strengths for the omega ₁^s and omega ₂^s modes in a-Si films have been estimated and compared with the current theories on the effect of the silicon matrix on the infrared absorption characteristics.