Table of contents

X-ray diffraction measurements on two samples of BaMnF₄ with different defect concentrations show that the defects destroy the long-range order of the low-temperature incommensurate phase. There is however, a well defined structural phase transition, and the scattering has lorentzian-squared component in this d=3 system, with a continuous symmetry.

The author argues physically that a simple nonlinear integro-differential equation describes relaxation processes in many glass-forming materials. The properties of the solutions change discontinuously in the fashion of a second-order phase transition as a function of one parameter. The author argues that this is a transition between a supercooled liquid and an amorphous solid. The author examines the time dependence and shows how a stretched exponential time decay may be found. The author also examines the dependence of T of the viscosity and shows that the results are consistent with some experiments.

The binding energies of the excitons arising from the two highest hole subbands have been calculated including the coupling between heavy and light holes and the nonparabolicity in the conduction band. It is found to increase the binding energies by less than 1.8 MeV above those obtained for simple models. However, the subband mixing considerably increases the oscillator strength of the exciton with p-symmetry for the forbidden transition (HH2-E1) and allows the observation of this exciton in experiments. Its binding energy is therefore significantly smaller than the value normally used to estimate the subband minimum.

The vacuum gap-voltage oscillating characteristics in scanning tunnelling microscopy are calculated for a barrier between two plane electrodes. The transmittivity of the tunnel electrons, as well as the tunnel current, are calculated with and without the image potential. It is found that the image potential is important in determining not only the peak positions of the characteristics but also their shapes. Inelastic processes are also considered by using an optical potential. For zero applied voltage the oscillations in the transmittivity correspond precisely to the surface and image state resonances of the sample surface.

LEED, Auger electron spectroscopy, secondary-electron-emission crystal current measurements and angle-resolved photoelectron spectroscopy have been used to study the adsorption of thallium on silver (100). At room temperature and at coverages greater than 0.6 monolayers, LEED indicates the formation of two ordered overlayer structures, each with a (¹³₂₀) unit mesh; the second of these is ascribed to a dense hexagonal monolayer. At coverages below 0.6 monolayers cooling the sample to 70K produced a bright (¹⁰₀₂) pattern or another (¹³₂₀) pattern, depending on the coverage. Both patterns are ascribed to pseudo-one-dimensional systems. Measurements of binding energy shifts and widths of the thallium 5d core levels are consistent with the authors proposed overlayer structures. The thallium chains in the (¹⁰₀₂) overlayer show no evidence for the formation of a Peierls superstructure contrary to the findings for thallium chains on copper (100). Band mapping of the thallium 6p band shows no evidence for a gap at the Fermi level, confirming the LEED result that the Peierls distortion had been suppressed. The authors believe this is due to the commensurability with the substrate.

The authors show that the intensity of the 790 meV electronic absorption line and the C(3) vibrational line at 865 cm^-1 correlate in Czochralski (CZ) silicon regardless of carbon concentrations, radiation doses and stages of annealing. Three weak vibrational lines at 529, 550 and 742 cm^-1 are shown to correlate with the C(3) lines. Symmetry arguments, estimates of the concentrations of the centres and the local mode energies are consistent with all these transitions occurring at the same radiation-induced complex. A simple model of the radiation damage process in which mobile carbon interstitial atoms are trapped predominantly at carbon substitutional atoms or oxygen atoms is shown to be quantitatively consistent with these and earlier data. The strength of the C(3)/790 meV lines correlates with the carbon content in CZ silicon, providing a possible new means of measuring carbon in silicon.

Electroreflectance measurements have been performed on GaAs/Ga_1-xAl_xAs single and multiple quantum well samples for GaAs well widths of 100 AA. The electric field is applied perpendicular to the well using a PIN diode arrangement of a semi-transparent Schottky barrier. Sharp features are observed in spectra measured both at room and at helium temperatures (15K) at energies corresponding to transitions between heavy or light-hole quantum well levels associated with the valence band and electron levels associated with the conduction band. Comparison is made between the energies of optical transitions measured by electroreflectance and by complementary photoluminescence emission and excitation spectroscopy. The energies of optical transitions are observed to decrease with increasing DC electric field at the well and a resonance model has been developed to explain the observed shifts.

The variational method is employed to calculate the energy levels of a hydrogenlike system in a constant magnetic field of arbitrary strength. The general approach is similar to that of Aldrich and Greene (1979), but uses a different set of basis functions, enabling smaller matrices to be used. Results are obtained for energy levels up to n=4, and are of high accuracy judged by comparison with other published data.

Analytical resonance scattering and numerical quasi-band approaches in the theory of semiconductors doped by transition metal ions are compared and the qualitative equivalence of the main results of both theories is demonstrated. A simplified illustrative model is considered in which both resonance scattering from the atomic d-levels and single-site Koster-Slater potential scattering are included on an equal footing. The analytical solution of this model containing the dangling bond hybrids and crystal field resonances in the valence and forbidden energy bands is presented. The multiplet corrections to the mean-field one-electron levels and the applicability of the scaling parameters lambda in calculating the Racah coefficients are discussed. The connection between different possible theoretical schemes and real experimental situations as occurring in elemental Si and Ge on the one hand and III-V and II-VI compounds on the other hand is established.

The authors propose a two-band model for explaining Hall coefficient measurements in a weak magnetic field in SnTe polycrystalline films. The first band is a non-ellipsoidal nonparabolic Cohen type and the second band is parabolic. From the low temperature results the authors deduced m_d1 and the anisotropy coefficient K_al and some second band characteristics. The results for T>100K are used for testing values of m_d2. By means of this test and the comparison of electrical resistivity corresponding to one- and two-band models the authors deduce that the second band must be lighter than the first. The most convenient model corresponds to m_d1=0.1442 m₀, K_a1=0.253, m_d2<0.3 m₀ and K_a2<0.75.

The magnetisation of a single crystal of the two-dimensional (2D) easy-plane ferromagnet Rb₂CrCl₄ has been measured as a function of temperature (5-160K) and as a function of magnetic field (0-7T) applied within the easy (001) plane. Isothermal critical behaviour of the form M varies as H¹ delta / was observed at low temperatures with delta decreasing slowly with increasing temperature; this is characteristics of Kosterlitz-Thouless (KT) (2D) XY behaviour. An effective KT transition temperature, T_KT, can be defined as 45.5K, the temperature at which delta =15. Above T_C the susceptibility, found from the initial slope of the magnetisation with field, could be accounted for equally well by an exp(B'/(t_KT)¹2/) dependence, where t_KT=(T-T_kT)/T_KT, as predicted by KT theory, or by the usual power-law behaviour in t=(T-T_C)/T_C. The variation of the magnetic properties of Rb₂CrCl₄ with field and temperature was found to be qualitatively very similar to that of K₂CuF₄, whose properties have been described by the KT theory. The difficulties of isolating unambiguous evidence for KT behaviour in such experiments is discussed.

Proton and fluoride second moments and relaxation times have been measured as functions of temperature over the temperature range 131>T>7K. The results are dominated by the tunnelling of the CH₃ groups. The activation energy associated with the splitting between the ground torsional state and the first excited level was found to be 670 J mol^-1. An estimate of the temperature dependence of the average tunnelling frequency is obtained from the results. The fluorine spins relax through the tunnelling system of the protons. In the rotating frame the very high tunnelling frequency ( approximately=44 GHz) gives rise to a serious discrepancy between calculated and experimental values since a common spin temperature is not attained.

The vibrational absorption spectra of both lead and barium beta " aluminas have been measured in the region 5-100 cm^-1 at both 2 and 300K. The vibrations of lead and barium ions are discussed on the basis of a simple model. It is suggested that clustering of lead ions occurs in the lead compound and that a degree of preferential condensation of barium ions into one of two equivalent sublattices in the conduction slab occurs in the barium compound.