Table of contents

The authors have measured the inelastic scattering spectra of two samples of molten RbCl with different isotopic chlorine compositions using the IN6 time-of-flight spectrometer at the Institut Laue-Langevin in Grenoble. By a first-order subtraction they have substantially separated the contributions from the longitudinal mass-mass (acoustic-type mode) and charge-charge (optic-type mode) current-current correlation functions. Over the accessible Q range these have the form of single broad peaks, with the mass peaks being approximately twice as wide and half as high as the charge peaks. A quasidispersion curve has been derived from the peak positions and comparisons have been made with the low-temperature crystalline state.

Several InP samples have been irradiated with 2 MeV electrons. Six electron traps and two hole traps were detected by means of deep level transient spectroscopy (DLTS) and optical DLTS. One of these levels appears to have an unusually large lattice relaxation, having a small electron capture cross section, thermally activated with an energy of 0.33 eV. This is comparable to the thermal emission activation energy of 0.44 eV.

The path-integral formalism is developed to obtain the quantum spectrum of a kink in the classical continuous one-dimensional Heisenberg ferromagnet with easy plane, noting the analogy between its quantum spin ¹/₂ lattice version and the Heisenberg antiferromagnet of spin ¹/₂. From the equivalence of the spectra, the quantum kink (antikink) is identified with the antiferromagnetic magnon of spin ¹/₂, consistent with the recent argument by Faddeev and Takhtajan (1981). A picture of the doublet of this magnon in their claim is now given by the excitation of a pair of the kink and antikink which are stable against their mutual collision.

Direct recombination emission accompanying the 4d electron excited Auger spectrum of clean Ce metal is shown to be consistent with the observed resonant photoemission yield. Both results imply a systematic decrease in direct recombination probability with increasing resonance excitation energy.

Reports a study of the corrections to scaling in systems exhibiting crossover behaviour with specific reference to the Heisenberg-dipolar crossover and the anisotropic Heisenberg model exemplified, respectively, by EuO and SrTiO₃. The authors show that the correction-to-scaling amplitudes for these systems are uniquely prescribed by the shift in transition temperature induced by the symmetry-breaking perturbation promoting the crossover. They calculate these transition temperature shifts for EuO and SrTiO₃ and thus obtain direct estimates of the correction-to-scaling amplitudes for the susceptibility of EuO and the order parameter of SrTiO₃.

The authors have investigated the cubic to tetragonal phase transition (T_c=195K) in RbCaF₃ by monitoring the EPR lines of Gd³⁺-O^2- substituted to a Ca²⁺-F^- bond, from 100K to 380K. From the broadening and the shape change of the lines, they find evidence for critical static fluctuations in the tetragonal phase, precursor order clusters in the T_c-(T_c+2K) range and a disordered high-temperature phase. These experimental results are in substantial agreement with recent theoretical predictions and models.

Coupled equations for the time- and space-dependent occupancy correlation function of selfavoiding classical particles hopping in periodic lattices have recently been derived by Tahir-Kheli, Elliott and Kaski (1982). These equations are exact in the limit of low particle concentration. Here the authors present solutions for the correlation function in cubic Bravais lattices. Their results are applicable to a variety of physical systems, including the case of hydrogen diffusion in metal hydrides such as TiH_x and PdH_x.

The spin susceptibility of TTF-TCNQ and other organic metals increases with temperature by about a factor of two between 100 and 300K. This increase is attributed to the narrowing of the band due to strong electron-phonon interaction. The strong electron-phonon interaction leads to phonon dressing of the electrons. A theory is developed to calculate the electronic density of states using Holstein polaron theory. The effect is present even when the phonon frequency is somewhat lower than the electronic hopping integral.

In a previous paper a procedure was introduced for the calculation of probability distributions for the resistance of a 1D chain of atoms in the case that the site energies are randomly distributed: so-called diagonal disorder. In this paper the treatment is extended to treat the case where the hopping integrals are also disordered (off-diagonal disorder). Analytic solutions are possible in limiting cases and display some new features unique to the off-diagonal disorder situation.

A cluster theory, which contains off-diagonal randomness, configurational short-range order and diagonal randomness is applied to the study of electrons in damaged honeycomb lattices with correlated unpaired bonds. The densities of states, local densities of states and conditional local densities of states obtained by the theory clearly show the various effects of clusters. In relation to the band gap of the disordered systems, a repulsive effect between bands is discussed and a Saxon-Hutner-type theorem on the gap of the system with diagonal, off-diagonal and topological randomness is recalled to predict the correct band gap of the disordered systems.

The spin-valley splitting of the 1s(T₂) states of group V and VI donors in silicon is used to determine the degree of electron localisation at the impurity atom. From comparison with ESR results on 1s(A₁) states, it is obvious that the 1s(T₂) states are more than one order of magnitude less localised than the former.

The authors study the DC electrical conductivity of an itinerant electron system coupled to localised spins in a model intended to describe the rare earth metals at temperatures close to the magnetic transition point, with particular attention focused on the role of long-range order below the critical point. It is shown that, in the Born approximation, the critical indices for the specific heat (of the combined electron-localised-spin system) and the temperature derivative of the resistivity are the same both above and below the critical point. In particular, the singular temperature dependence of the resistivity immediately below T_c is not determined by the order parameter, neither directly nor indirectly through the associated deformation of the Fermi surface. Instead, the short distance spin pair correlations play the dominant role. These conclusions are not based on specific assumptions about the type of magnetic order or the shape of the Fermi surface.

Published specific heat data for nickel have been reanalysed to determine whether the parameters describing the critical temperature region are consistent with available theoretical estimates based on renormalisation group and field theoretical methods for continuous spin systems. The results for amplitude ratios are consistent with theoretical estimates based on the isotropic Heisenberg model, but the value deduced from the data for the specific heat exponent, alpha , is not in particularly good agreement with any theoretical predictions. Reasons for this are discussed.

The proton spin-lattice relaxation times of KCH₃COO, Ba(CH₃COO)₂, Cd(CH₃COO)₂ and Ca(CH₃COO)₂ have been measured over the temperature range 300-310K at 15, 30 and 52 MHz Larmor frequencies. The relaxation times have been found to be frequency-independent within the limits of the experimental errors and the results exhibit minima at 41, 40, 38 and 52K respectively. The low-temperature results have been explained by tunnelling rotation of the methyl groups and the tunnelling frequencies of the ground and the first excited states of the methyl group rotator levels of the compounds have been determined. Also the energy separations between the ground and the first excited states have been deduced. The experimental results at higher temperatures of T₁ minima display hindered rotation, and around room temperature nearly free rotation of the methyl groups.

Vanadium dihydride has been investigated by ⁵¹V and ¹H nuclear magnetic resonance measurements. The ⁵¹V Knight shift when combined with other related results indicate that the observed Knight shift K_v=-(0.016+or-0.005)% is a result of the strong competition between a negative core polarisation contribution and a positive orbital one. The magnetic susceptibility, in turn, is determined mainly by enhanced d-band spin susceptibility. The proton Knight shift value of K_H=-(0.023+or-0.005)% is interpreted in terms of direct contact interaction and assumed indirect polarisation of the hydrogen 1s band by d-like conduction electrons.

A full optical absorption spectrum for indium-doped silicon taken at different temperatures is presented which comprises the bound-to-free as well as the bound-to-bound transitions. The maximum absorption cross section for bound-to-free transitions is found to be sigma _max=4.5*10^-17 cm². The ionisation energy was determined by absorption and photoconductivity measurements. Both methods yield E_In=157+or-1 meV. The bound-to-bound transitions were also observed by photoconductivity measurements at intermediate temperatures. The relative oscillator strength for the absorption lines shows a linear dependence on the In concentration. Experimental data are compared with different models for bound-to-free and bound-to-bound transitions at impurities in Si.

Luminescence in semiconducting amorphous phosphorus is resolved into a band (HE) centred near 1.4 eV and a band (LE) centred near 1.15 eV by time-resolved luminescence measurements excited by 6 ns dye laser pulses. LE decays with a time constant around 150 ns. It is interpreted to be due to recombination of a singlet exciton state corresponding to the triplet found by ODMR. The singlet-triplet exchange splitting is 80 meV. HE shows fast initial decay within the first 800 ns followed by slow non-exponential decay, which is interpreted by recombination of pairs of separated carriers, recombining by radiative tunnelling with a distribution of lifetimes peaking around 3 ms. This interpretation is supported by the authors time-resolved photoconductivity measurements and the high quantum efficiency (0.85+or-0.3). HE shifts to lower energy with increasing delay time suggesting that charged defects are stable in a-P. The excitation spectra peak near 1.95 eV for both LE and HE, but light below 1.9 eV excites LE more efficiently than HE, as expected for excitons. Shape and Stokes shift of LE can be explained by strong electron-lattice coupling but the width of the HE bands is too large for this interpretation.

Near-band-gap bound and free exciton states in high purity InP are investigated using photoluminescence (PL) and photoconductivity (PC) techniques. Direct comparison is made between features observed in PL excitation and PL spectra. The effects of excitation intensity and applied electric field on the PL spectra are studied and comparison is made with corresponding phenomena in GaAs. Values for the shallow donor binding energy and for the binding energy of excitons at ionised donors are derived.

A predominant glow peak at 226+or-3K in calcium sulphide, reported here, is thought to be due to sulphur vacancies acting as electron traps. In all, there are four predominant glow peaks in pure and cerium-activated CaS, two of which have been previously reported by the authors. These exhibit a significant behaviour which has been analysed to reveal the presence of hole traps as well as deep electron traps due to intrinsic point defects in CaS. Cerium has been used primarily to enhance thermoluminescence emission of CaS. However, in view of the fact that the role of cerium in producing compensating defects in CaS is still highly controversial, its role in the present work has been examined.

The interaction of water with the atomically clean cleaved (110) indium phosphide surface has been investigated with LEED, Auger electron spectroscopy and angle-resolved photoelectron spectroscopy. At room temperature, it would appear that adsorption takes place in two stages: a rapid stage leading to an ordered overlayer, followed by a much slower disordered adsorption. The initial stage of adsorption is largely molecular in nature but at higher exposures some degree of dissociation is envisaged. The influence of adsorbed water on the Schottky barrier formation at the indium phosphide (110) surface upon subsequent metal deposition was also investigated. The transport properties of the metal-semiconductor contacts were observed to be drastically affected by the presence of the interfacial water layer. The formation of the Schottky barriers at these interfaces is considered with regard to the recent defect model.

Approximate analytic expressions are obtained for the scattering rates and momentum-relaxation rates of an electron in a quasi-two-dimensional quantum well interacting with acoustic, optical and intervalley modes via the deformation potential and with longitudinal optical modes via the polar interaction. These analytic expressions are obtained using a momentum-conservation approximation. The threshold for optical phonon emission, unlike the case in the bulk, is abrupt. All scattering rates are energy-independent and are inversely proportional to L, the thickness of the well. The momentum-relaxation rate associated with the absorption of polar optical phonons, on the other hand, proves to be proportional to L. These properties are shown to lead to a negative differential resistance for pure polar mode scattering, and to the existence of a runaway field for deformation-potential scattering. The self-energy associated with the emission of polar optical phonons at absolute zero is shown to be divergent unless the polar interaction is screened, and some consequences of this for laser and other optical processes are pointed out. The description of scattering by perturbation theory breaks down in very narrow wells.