Table of contents

The effect of pressure on Raman spectra of solid chlorine has been measured up to 45 GPa using a diamond anvil cell. The six Raman-active modes could be observed in first-order Stokes and anti-Stokes spectra at room temperature. The accidental degeneracy of the B_3g and A_g lattice modes is lifted under pressure.

For original paper see ibid., vol.16, p.1863 (1983). The authors point out that in the time regime of diffusion via the vacancy mechanism which is probed by the Mossbauer effect, the exact solution of L. Gunther and B.G. Gralla simplifies considerably, producing lorentzian spectra. Their solution thus corroborates the so-called 'encounter theory' approximation of M. Eisenstadt and A.G. Redfield (1963).

The quantised Hall resistance found in certain 2D systems may be an occurrence of a macroscopic quantum state. If so it has a simple flux-flow interpretation.

Thin amorphised silicon layers have been irradiated with ion beams of H or F, and, if desired, with doping elements. The results of DC and photoconductivity measurements show that after optimised annealing treatments the electronic transport properties of these films are similar to those obtained on glow discharge materials.

²³Na quadrupolar spin-lattice relaxation and dielectric dispersion data are reported for mixed crystals (K-Na)TaO₃. Below 150K, the Na ions are shown to associate a dipole moment of 0.04 AA e^-, which changes orientations among equivalent (100) directions. This hopping motion contributes to both dielectric and spin-lattice quadrupolar relaxation, thus allowing evaluation of the dipole reorientation rates. These appear to be widely distributed and, in particular at low temperature, their low-frequency tail is likely to prevent the thermodynamical equilibrium state being reached in experimental times. Some dynamics of resonant character is observed at 4K.

Soda-lime glasses have been irradiated with protons of 600 keV energy at different fluences. The surface temperature increments have been measured as a function of irradiation time by analysing the emission of infrared radiation. The experimental results have been compared with a theoretical model. The alkali migration processes in glasses during irradiation with low-mass particles, reported in previous papers, have been revised to take into consideration an enhanced diffusion coefficient.

A model is described which is used to compute the electronic band structure of transition metals and transition metal compounds. It uses a mixed basis set of plane waves and Bloch states constructed from d orbitals of the free ion. It was developed primarily to aid the modelling of metal oxide devices. The model is illustrated with applications to FCC copper, BCC tungsten and cuprous oxide. Good agreement is observed with previously published work.

A method of calculating the energy levels and Ham factors of a Jahn-Teller system interacting with many modes of vibration is described. The multimode effect in the ground states is dealt with by first doing a transformation to concentrate most of the Jahn-Teller interaction into an effective cluster Hamiltonian, and then allowing for the remainder by introducing one extra phonon mode at a time. Calculations are done using a numerical perturbation theory: energies are calculated by the Lanczos method of diagonalising large sparse matrices, and the small changes in energy resulting from the introduction of small interactions are used to deduce perturbation parameters. This method is applied to E*( Sigma _i epsilon _i), with the inclusion of a quadratic Jahn-Teller coupling or warping term. Experimental results on Ni³⁺:Al₂O₃ are discussed as an example.

Neutron inelastic scattering measurements have been performed on the intermetallic compound CeSn₃ with the use of high-incident-energy neutrons on a time-of-flight spectrometer. The magnetic response at low temperatures shows a broad inelastic hump centred around 40 meV, in addition to the usual quasielastic spectrum centred on zero energy. With increasing temperature the inelastic hump 'melts away', i.e. it spectral weight is transferred progressively into the quasielastic part such that around the temperature of the maximum in the susceptibility ( approximately 130K) no observable trace of the inelastic hump remains and the full magnetic response can be described by a single quasielastic spectrum. Several possible mechanisms, namely a modified form of crystal-field excitation, a transition across the hybridisation gap in the 4f band at low temperatures or a localised excitation of the 4f electron to the Fermi level could plausibly account for the observations.

The Shubnikov-de Haas effect has been studied in InSb to elucidate some remaining problems. A range of samples was produced by neutron transmutation doping to provide high-homogeneity impurity distributions. Double-differentiation techniques were used to improve the peak resolution. The Dingle temperature obtained from the low-field mobility was lower by a factor of three than that deduced from Shubnikov-de Haas measurements. This difference is attributed to a difference in the relaxation mechanisms as the quantum limit is approached. The small amplitude of the higher-field peaks of the spin-split longitudinal doublets is explained in terms of the predominance of small-angle scattering. It is concluded that the small 0^- longitudinal peak reported by most previous workers is real and not a transverse residue due to misalignment, g-values in good agreement with theory were obtained from spin-splitting measurements.

The conductivity and photoconductivity of a ferromagnetic semiconductor p-CdCr₂Se₄ (T_c=130K) was studied between 60 and 300K. The different variations of the modulated and the stationary photoconductivity with temperature given evidence for a significant role of magnetic-order-sensitive traps in the photocarrier recombination. The possibility of electron localisation in ferromagnetic clusters is discussed.

A theorem on the existence of a gap in the energy spectrum of quantum systems, the exact ground state of which is known explicitly, is proved. The theorem is applied to a three-dimensional Heisenberg spin-¹/₂ ferromagnet, with anisotropic nearest-neighbour interactions, and to an alternating Heisenberg antiferromagnet, with nearest- and next-nearest-neighbour interactions.

The ferromagnetically ordered states of Ising systems with short-range-correlated random fields are shown to be unstable with respect to domain formation for dimensionalities d<or=d_l=2. Domain wall roughening for 2<d<5 follows from the same mechanism as the Imry and Ma (1975) domain formation for d<or=2. No additional instabilities for d>or=2 are found. The results are extended to random fields with a correlation function obeying a power law approximately r^-a for large separation r. For a>d the same results as for short-range field correlations are found. For a<d the author finds domain formation for a<or=a_l=2 and wall roughening for a<or=4 (d>or=5) and a<d(d<5). The borderline value for a above which the mean-field theory is valid is found to be six. Experimental consequences of these results are indicated.

The authors discuss the domain structure in a ferromagnetic plate with strong uniaxial anisotropy in the perpendicular direction. The inclusion of dipolar interactions produces anisotropic magnetisation correlations above the ordering temperature T₀. They calculate the dependence of the width D of incipient domains on plate thickness L in the temperature region from slightly above to just below the transition temperature. They find that D approximately L^1/3 in this region, in contrast to the Kittel relation D approximately L^1/2 for stripe domains at temperatures well below T₀. At low temperatures the width of the walls is very small compared with the distance between them and so they do not interact with each other. They determine the range of validity of the Kittel theory and they use this to suggest substances upon which it might be possible to make appropriate observations.

EPR linewidth measurements on the one-dimensional Heisenberg antiferromagnet CsMnBr₃ are reported. The measurements were made at room temperature over a frequency range of 2-70 GHz using both conventional and variable-frequency EPR spectrometers. The results show the one-dimensional character of spin diffusion in CsMnBr₃, but a comparison with the near-ideal one-dimensional system TMMC reveals substantial differences in behaviour. These differences are ascribed to the larger interchain exchange and dipolar coupling in CsMnBr₃, as compared with TMMC.

The concentration dependence of the saturation hyperfine field as deduced from ¹⁵¹Eu Mossbauer spectroscopy in EuS_xSe_1-x allows one to conclude that there is a continuous change of the local magnetic structure along the series. It is ascertained that the spin glass phases consist of nearly two-dimensional ferromagnetic clusters whereas the ferromagnetic states close to the ferromagnetic-spin-glass boundary are 'frustrated ferromagnets'. Relaxation phenomena are observed in both the spin glass and 'frustrated ferromagnetic' phases.

The application of an external magnetic field to SrO:Ni²⁺ crystals that have been previously annealed in an oxygen atmosphere is shown to induce new optical absorption lines. This effect is interpreted in terms of a simple crystal-field model of a centre with C_2v symmetry weakly perturbed by a trigonal perturbation. The analysis supports the claim that the optically active component is a (110) off-centre Ni²⁺ ion associated with an interstitial oxygen ion along a (111) direction.

The Raman spectra of hexagonal RbCoCl₃ and RbNiCl₃ have been studied as functions of temperature. No light scattering from magnons was observed in RnBiCl₃ in contrast to the high intensities found in RbCoCl₃. The magnetic properties of RbCoCl₃ are similar to those of its structural isomorphs CsCoCl₃ and CsCoBr₃. The temperature dependence of the scattering revealed three magnetic phases: a quasi-1D phase for T<T_N^I approximately=28K; a partially ordered 3D phase for T_N^I<T<T_N^II approximately=11K; and a differently ordered 3D phase for T<T_N^II. From the magnon scattering the authors find the intrachain exchange J approximately=50 cm^-1, with epsilon approximately=0.1, and the effective interchain exchange J' approximately=1.5 cm^-1. The intensity of the exciton at 308 cm^-1 was found to be spin-dependent, involving the interchain exchange with J'=0.9+or-0.1 cm^-1. Major features in the electronic Raman spectrum of RbCoCl₃ were fairly well explained by an effective Hamiltonian model that included the intrachain exchange within a molecular-field approximation. However, other secondary features in the spectrum could not be accounted for. Magnetic scattering from Co²⁺ ions neighbouring divalent metal impurities in the crystal was observed at frequencies approximately J. The A_1g phonon in RbNiCl₃ and RbCoCl₃ was found to have an anomalous intensity in alpha _xy polarisation. Strong coupling was found to exist between the E_1g phonon and magnon in RbCoCl₃.

The Raman spectra of proustite and pyrargyrite have been recorded over the temperature range 4-540K. The results confirm the existence of structural phase transitions at 29 and 58K in proustite but do not give evidence for the suggested transitions at 10 and 140K (pyrargyrite) and 210K (proustite). The existence of Ag-ion disorder above 80K for both materials is inferred from the anomalous broadening of certain lines associated with S-Ag-S units and it is concluded that this disorder is not necessarily connected with the transitions in proustite. No evidence of any transition to a superionic phase resulting from the Ag-ion disorder was found. Some earlier mode assignments are shown to be incorrect and for proustite a number of TO-LO splittings are confirmed. The influence of sample imperfections on the spectrum of pyrargyrite is assessed.

Amorphous Si_1-xCr_x films with 9, ..., 19 at.% Cr were prepared by means of electron beam evaporation. The electrical conductivity was measured in the regions 2, ..., 4.2 and 14, ..., 20K. If the conductivity is smaller than sigma ₀=270+or-50 Omega ^-1 cm^-1, which corresponds to about 14 at.% Cr, its temperature dependence can be described by a scaling law, sigma (T)= sigma ₀ phi (T/T₀), where only T₀ depends on the Cr content and preparation conditions. The asymptotic behaviour for T to 0K is given by sigma (T)= sigma ₁ exp (-(T₀/T)^alpha ) with sigma ₁=110+or-20 Omega ^-1 cm^-1 and alpha =0.47+or-0.05. Hence, if the conductivity is smaller than sigma ₀, there should be only one conduction mechanism. This mechanism is thermally activated. Thus, sigma ₀ should be interpreted as the minimum metallic conductivity.

A realistic self-consistent calculation is presented of an abrupt metal-semiconductor junction by means of a tight-binding approach. A specific Si-Ag junction has been considered, and the charge neutrality level as well as the barrier height have been determined in good agreement with experiments. For a general junction the authors show that the interface properties depend essentially on the characteristics of the first metal layer and its interaction with the semiconductor.