Table of contents

The authors report measurements of conductivity as a function of composition for a series of granular Au-SiO₂ films in the thickness range 100 AA to 11000 AA. They find a shift in both the critical metal volume fraction and the conductivity exponent (t) with film thickness, but no evidence of a dimensionality crossover in t as a function of film composition. They point out that inconsistencies in data in the literature raise doubts regarding previous reports of such a crossover in a random film system.

The authors have performed electrical resistivity measurements of the organic semiconductor (TMTSF)₂FSO₃ at low pressures and under controlled cooling conditions. At low temperatures applied electric field along the high-conducting axis can switch the material from a semiconducting to a low resistance metastable state, which can be either semimetallic or metallic in nature. This state is stable in time and is destroyed above 30K where the material displays a transition to high-resistance stable state.

Single-crystalline samples of CdIn₂S₄, ZnIn₂S₄, ZnGa₂Se₄ and CdGa₂S₄ grown by chemical vapour deposition were investigated. The authors have measured reflection coefficient at room temperature in the 1-8 eV energy range with unpolarised light. A discussion of obtained results and comparison with available literature data were undertaken.

Audiofrequency complex impedance measurements have been performed on pure and alkali-metal-doped lead fluoride at zero pressure over the temperature range 5.5 to 380K and at pressures up to 0.25 GPa over the temperature range 260 to 333K. The dielectric spectra show distinct alkali-metal-associated relaxations in each case. The activation enthalpies for the principal relaxations in sodium-, potassium- and rubidium-doped PbF₂ (0.221, 0.252 and 0.218 eV respectively) are close to that for the motion of free vacancies. Thus the peaks are attributable to the motion of bound fluorine vacancies around the substitutional impurity ions. The principal relaxation observed for the lithium-doped samples, lithium ions being the smallest of the alkali metals, has a higher activation energy (0.331 eV). This may be due to the reorientation of Li⁺-substitutional-Li⁺-interstitial pairs. The ionic conductivity at temperatures above those of the dielectric peaks is interpreted in terms of the thermal dissociation of the impurity-vacancy complexes. The activation energies and activation volumes were found to be very sensitive to ion size, both reaching a maximum for the potassium dopant. Finally, the ratio of activation volumes to activation energies was found to be the same for all dopants, in excellent agreement with theoretical predictions.

Methyl groups in solids are representative of systems which exhibit tunnelling at low temperatures and thermally activated hopping motion at high temperatures. They exhibit very clearly all the features of the transition between free and thermally driven quantum tunnelling, the latter being describable in terms of classical hopping concepts. A satisfactory theoretical description has been lacking because the usual approaches, which treat the dynamic part of the scalar hindering potential by time-dependent perturbation theory are, the author suggest, incomplete. It is shown that a better description is possible if a vector potential arising from the motion of the lattice is introduced in addition to the scalar potential. This leads to a new Hamiltonian and a representation in which the important terms, which lead to the experimentally observed spectroscopic changes with increasing temperature, are diagonal. In this new representation a simple model emerges which exhibits most, if not all, of the observed experimental features. It also resolves an old problem: how to reconcile the quantum mechanical requirement that the rotation angles phi =0 and phi =2 pi are identical, with the classical assumption that they are distinguishable, and thus provides a bridge between the quantum and classical descriptions of rotation.

The density dependence of the electrical conductivity of slightly ionised mercury vapour has been measured for the 1520 degrees C supercritical isotherm and the density range from 1.4 to 6 g cm^-3. The observed values are in excess of 10^-4 ( Omega cm)^-1 at the lower densities and exhibit a surprising density dependence. A theoretical model is proposed to interpret the observations; its central element is the thermal generation of a spectrum of molecular ions: Hg_l⁺. The large cohesion of these clusters plays a dominant role in the ionisation balance. Also, estimated limits on the spectrum of l-values can account for the observed density dependence.

By measuring the photoconductive edge and the magnetophonon effect in InP and GaAs as their band gap is increased by the application of hydrostatic pressure, it has been possible to obtain a direct experimental relationship between the electron effective mass, m*, and the direct band gap, E₀. Comparison with the k.p theory showed that in InP, E_p=16.7+or-0.2, E_p'=0 and C=0 while in GaAs E_p=25.0+or-0.5, E_p'=5+or-1 and C=0 where E_p, E_p' and C are measures of the k.p interaction of the conduction band with, respectively, the valence band, with the next ( Gamma ₅^c) conduction band and with the conduction bands lying even higher.

The systematic study of the high-frequency wavevector-dependent conductivity in the presence of static scatterers is presented for the multicomponent anisotropic plasma, coupled with optical phonons in the polar way. The kinetic equation is solved in the self-consistent-field approximation, expanding the one-particle density operator in powers of the screened static imperfection field and radiation electromagnetic field. The conductivity is determined by the current quadratic in the imperfection field and linear in radiation field, averaged over the statistical distribution of imperfections. The exact expression for the complex conductivity tensor, which properly accounts for collective effects, is obtained for the quantum plasma. In the zero-wavevector and isotropic limits with the neglect of coupling with phonons or making some assumptions about the impurity statistics, it agrees with the results of Ron and Tzoar (1963), or of McCumber (1966).

A systematic analysis of the high-frequency free-carrier absorption due to ionised-impurity-mediated processes is given for a highly polar doped degenerate semiconductor taking into account the plasma-phonon coupling and the anisotropy of the multivalley conduction band. In the approximation of isotropic valleys, the bump in the absorption due to higher-frequency plasmon-phonon excitations is shown to dominate strongly (by an order of magnitude for PbSe with n approximately=10¹⁸ cm^-3) the single-particle absorption calculated taking into account the dynamic screening. The absorption due to polariton excitations is found to be negligible. The absorption due to the long-wavelength plasmon-phonon excitations is then investigated in the case of an anisotropic multivalley conduction band. For PbSe and PbTe with n approximately=10¹⁸ cm^-3 the plasma anisotropy lowers the absorption by about 5 and 50% respectively, as compared with the isotropic approximation. A comparison with previous theories and the reflectivity data for n-PbSe is given. A possibility for a sharp dip in the reflectivity coefficient at the plasmon-phonon mode frequency is indicated.

The dynamic susceptibility _chi ( omega , T, H) is calculated for an Ising spin glass with infinite-range interactions for small frequencies, omega , and a large range of temperatures and fields. The observed sharp increase of the imaginary part Im _chi (T) near the freezing temperature T_f for small and vanishing fields is explained by the crossover from Im _chi varies as omega at high temperatures to Im _chi varies as omega ^1/2 at T_f. At low temperatures, Im _chi increases in proportion to T.

The temperature dependence of the spin-lattice relaxation time, T₁, for ³⁵Cl nuclei was measured in the two crystalline phases of p-chlorobenzenesulphonyl chloride. The observed behaviour of T₁ is ascribed to the triggering of thermally activated semi-external molecular motions. The results of the present study support the conclusion drawn from previous observations of the nuclear quadrupole resonance frequency, nu _Q(T)-namely that the triggering of wagging semi-external movements occurs in this compound at temperatures higher than 160K.

The spin-lattice relaxation time of Fe²⁺ in ZnCO₃ and MgCO₃ has been calculated in terms of the normal coordinates of the complex composed of the paramagnetic ion plus its nearest neighbours. The theoretical values are compared with the experimental ones obtained from Mossbauer spectra. It is found that the Orbach process can account for the temperature dependence of the relaxation time.

The antiferromagnetic resonance modes of alpha -oxygen have been measured below and above the spin-flop field. Spin wave theory is developed and is found to describe the experimental data if one disregards a constant frequency shift. The temperature dependence of the mode frequencies is discussed. The ratio of the intensities of the two magnons at low temperatures and zero field is derived, both theoretically and from experimental data.

The complex multiphonon fine structure of the d-d absorption bands ⁴E_g(D), ⁴E_g(G) and ⁴A_1g(G) in manganese dihalides is studied in detail by a fitting procedure. With this aim, an analytical expression for the absorption coefficient has been deduced, from the Fourier transform of the optical response function for weak electron-phonon interaction in the adiabatic and Condon approximations. The observed vibrational structure of these crystals is in general well accounted for by the superposition of one-phonon progressions, except for the intraconfigurational ⁶A_1g to ⁴E_g(D) band of MnCl₂ where the contribution of a two-phonon progression of E_g symmetry is essential to reproduce the shape of the optical profile. The long-wavelength phonon frequencies on Ma halides and Fe, Co and Ni iodides have also been measured by far-infrared transmission and Raman scattering techniques.

Optical absorption and reflection spectroscopy has been used to determine the pressure dependence of the 2 eV electronic transition of single crystals of toluene sulphonate diacetylene polymer up to 3 GPa. In an optical cell, which used argon as the pressure-transmitting medium up to 0.5 GPa, it was found that the peak in the absorption spectrum shifted to lower energy at a constant rate of 70+or-3 meV GPa^-1 over the whole range. Detailed study of the 0.03-0.09 GPa range gave no evidence for a structural phase transition previously reported. The higher-pressure range was investigated using an opposed anvil press with light introduced into the sample volume by means of optical fibres passed through the sealing gasket. The peaks in the absorption and reflection spectra continued to shift to lower energy but at an ever-decreasing rate. This departure from linearity arose primarily from pressure-induced changes of shape in the polymer backbone.

The results are reported of an experimental study of chromium ion X-ray K alpha ₁-line valency shift after gamma -irradiation of ruby and the theoretical calculations of chromium ion optical and X-ray spectra. A fraction of the Cr³⁺ ions are shown to be converted into Cr⁴⁺-state ions which possess 270 and 360 nm guided absorption bands.

A two-layer relaxation analysis of a very large LEED experimental database is performed on the Ni(110) surface. Among the important features used in this analysis the authors emphasise the metric distances between experimental and theoretical spectra required to reach the best model, and an energy-dependent optical potential that is similar to the local excited-state potential of the Hedin-Lundquist type recently used for a LEED study of Cu(111). The first interlayer spacing is contracted by 8.4+or-0.8% while the second is expanded by 3.1+or-1%. This oscillatory relaxation of the interlayer spacing found on the Ni(110) surface is very similar to what has been reported for Al(110) and Cu(110) and confirms this general trend of FCC(110) faces. The question remains open, however, for BCC(100) faces.