Table of contents

The authors have carried out high-momentum-resolution studies in diffuse X-ray scattering of barium and lead B" aluminas in the temperature range 20-700 degrees C. They have also measured the vibrational spectra of these compounds between 2K and 300K in the energy range 10-100 cm^-1. The results are discussed in terms of preferential occupation of a cation sublattice in the conduction slab of B" alumina and of cation clustering.

The density-gradient expansion, one of the approximation schemes of the density functional theory, is used to investigate the influence of finite size on the surface properties of the classical one-component plasma. Variational calculations are performed within the square gradient approximation for the surface properties of a spherical system. It is found that the calculated ion density profile and surface energy for a small sphere are quite different from those for the planar surface in the weak-coupling regime and in good agreement with the recent Monte Carlo simulations. These results provide further support for the previous argument that the Monte Carlo results for a spherical system could be explained, at least in the weak-coupling regime, by taking into account the finiteness of the system as well as the presence of a hard wall introduced for achieving an appropriate equilibrium ion configuration. It is also confirmed that, in the strong-coupling regime, surface properties are not much influenced by the finiteness of a system and by the presence of a hard wall.

A model on super-change two-dimensional magnetic system with frustrations is proposed. The free energy of the system in an external magnetic field is calculated exactly. By considering the low- and high-temperature expansions, it was found that the system behaves more or less paramagnetically at high temperatures, but its residual entropy variation with the applied field is rather peculiar.

A Boltzmann equation treatment of the dependence of thermopower on Fermi energy at low temperature gives a positive peak when the Fermi energy moves into a new sub-band. Numerical estimates suggest that this structure remains observable at temperatures below 10K when level broadening is taken into account provided that k_Fl>or approximately=3.

The ohmic mobility due to acoustic and polar optical phonon scattering in a GaAs-GaAlAs heterojunction is calculated by the use of a formula in which temperature, wave vector and frequency-dependent screening are built in. It is shown that under RPA the enhancement of the polar optical phonon-induced resistivity due to dynamical effects almost compensates the reduction by static screening over the entire temperature region.

Recently the Hall coefficient temperature dependences in PbTe:CrTe samples have been interpreted on the basis of an arbitrary separation of the conduction band carriers into two types. In this work, the interpretation has been extended to the two other types of PbTe:Cr samples. The calculations support again the finding that Cr atoms in PbTe act as electron trapping centres.

Oscillatory stuctures recently reported by Hickmott et al. (1984) in the I-V characteristics of n⁺GaAs/(AlGa)As/n^-GaAs/n⁺GaAs tunnelling devices are observed at zero magnetic field and temperatures up to 50K, demonstrating that neutralisation of donors by magnetic freeze-out in the n^-GaAs layer is not required to observe the structure.

The upper critical field H_c2 of the series Pb_xLa_1-xMo₆Se₈ was measured in high magnetic fields up to 35 T. The temperature dependence of H_c2 was found to be explained by the term of orbital critical field.

The three-state Potts model in three dimensions in the presence of a random field has been investigated by Monte Carlo simulation. For a range of values of the random field strengths, results for the internal energy and order parameter are consistent with the existence of a first-order phase transition to a ferromagnetic state.

The influence dependence of P_In anti-site centres in electron-irradiated InP:Zn has been investigated by MCD-ODMR, where the extra sensitivity of the method has revealed a threshold at 3*10¹⁶ cm^-2 and a saturation of the concentration at 6*10¹⁶ cm^-3. Anti-site resonances with small differences in parameters are observed for InP:Sn and InP:Zn.

The annealing of IBr^- and I₂^-V_k-type centres in mixed KBr-KI crystals with varying iodine concentration has been investigated by the fractional glow technique. It is shown that for an I^- concentration exceeding 1-5 mol.% the hole delocalisation from iodine monomers and dimers is characterised by both a reduced activation energy and a reduced annealing temperature. The kinetics of the corresponding thermoactivated recombination process reveals clearly a diffusion-controlled behaviour. A new mechanism of hole migration through chains of spatially separated iodine impurities is proposed.

From a discussion of the experimental results concerning defect creation (versus the energy of irradiation, temperature, doping concentration, nature of the material, crystalline orientation) and annealing (versus doping concentration, carrier injection), it is concluded that the defects produced in n- and p-type GaAs by electron irradiation consist in a distribution of vacancy-interstitial pairs in the As sublattice. The vacancy-interstitial pairs in the Ga sublattice recombine immediately after their creation. The mobility of the As interstitial, which can be induced by hole injection in the case of irradiation with high fluxes, explains the formation of antisites As_Ga by exchange with donor impurities on Ga sites and of few complexes following irradiations at high fluences. The existence of several types of pairs explains the occurrence of the numerous electron and hole traps observed. The electrical and optical characteristics of the electron traps are consistent with the existence of a strong-electron-lattice coupling which can be evaluated semiquantitatively.

In LaF₃:Nd³⁺, R₁-level quenching is found to be more efficient than that of R₂ and significantly depends on the overlap of ion-pair transitions involved in energy transfer. The authors also have evidence that the energy shifts of R₁ and W₁ levels are in opposite directions at high concentrations of Nd³⁺ in LaF₃. S levels fluoresce only when their population is sustained due to thermalisation from the lower R levels. Two ions in D and Y levels undergo energy transfer interaction causing up-converted fluorescence from E levels.

In Tl⁺-doped alkali halides laser-active centres can be produced by ionising irradiation. This treatment inevitably produces other Tl-associated centres, which might degrade the laser performance. One of these centres that absorbs in the wavelength region where the laser centre is pumped was identified in the hosts KCl, RbCl and KBr by measuring its magnetic circular dichroism (MCD) tagged by electron spin resonance (ESR). The positions of the lowest-energy absorption and emission bands as well as sign and magnitude of the MCD resemble closely those of the laser centre. From the analysis of the optically detected ESR the structure of this new centre was determined. It consists of two neighbouring Tl⁺ cations adjacent to an anion vacancy, which share an electron. The ESR spectrum is explained by a fast hopping of the unpaired electron between the two Tl⁺ positions.

A compilation of data that concern the point defect parameters in LiF is presented. It is shown that the entropy (s) scales with the enthalpy (h) for the following processes: Schottky formation, free cation migration and (re) orientation of the 'Mg²⁺-cation vacancy' dipole. The ratio s/h is governed only by well known macroscopic properties of the bulk solid. Furthermore, the ratio of the defect volume and the defect energy is discussed.

A theoretical model for amorphous silicon-fluorine alloys proposed earlier by one of the authors in the study of vibrational excitations using a cluster-Bethe lattice method has been investigated in detail. The vibrational frequencies have now been obtained for isolated SiF_n (n=1,3) configurations for the two adjacent interacting monofluorides (SiF)₂ and the polymer-like (SiF₂)_n chain configurations in a rotationally invariant valence force-field model (VFFM). Extra structure in the bulk phonon density originates from the two interacting T₂⁰ defects lying on the nearest-neighbour sites. The bond-stretching vibrational frequencies associated with the Si-F bonds in SiF_n complexes in isolation are retained when embedded in the silicon matrix. A very successful assignment of the peaks observed in the infrared data, both within and outside the bulk phonon region, has been made in terms of the vibrational mode frequencies of the various simple units and the interacting ones for the first time. In particular, the location and the relative strengths of the three peaks at 212, 300 and 515 observed in the bulk phonon region of the infrared data are very well explained by the presence of the simple SiF and SiF₂/SiF₃ units in a-Si:F alloys. The other high-frequency peaks observed at 650, 827, 870, 920 and 965 cm^-1 originate from the interaction of the two adjacent SiF₂ units, a result also supported by the analysis of the photoemission spectra of these alloys made elsewhere.

Domain-wall arguments are used to study the stability of long-period commensurate phases in the presence of quenched impurities. It is found that in three dimensions phases with periodicity larger than a critical value l_c approximately 1/c, where c is the impurity concentration, are unstable. This results in disordered 'spin glass'-like phases. The effect can be studied experimentally by varying the impurity concentration in modulated systems.

The role of anharmonicity on the commensurate (C) phase of Kr and Xe monolayers adsorbed on graphite is analysed. If the usual interatomic interaction parameters are accepted, Kr monolayers at low temperature should be in an incommensurate (I) phase. Thermal expansion stabilises the commensurate phase at temperatures higher than T_IC, the temperature at which the energy of creating a wall in the commensurate phase becomes positive. A rough estimate yields T_IC approximately=34K. Anharmonicity also explains the positive slope of the experimental chemical potential versus temperature curve at the C-I transition.

For pt.I see ibid., vol.18, p.3919 (1985). The authors calculate wall and wall crossing energies numerically as a function of V_G, the modulation of the substrate potential, at T=0. The wall energy is negative if V_G<or=11K. The wall profile does not agree with that predicted by the analytic theory, because of anharmonicity in the Kr-Kr interaction. Wall crossing energies have function of V_G as wall energies.

The thermal conductivity, lambda , of polycrystalline KCl, KBr, KI, RbCl, RbBr and RbI has been measured at temperatures (T) from 100-400K at pressures (P) up to about 2.6 GPa, using the transient hot-wire method. Extensive results were obtained for lambda (T,P) of both the low-pressure (NaCl-type) and high-pressure (CsCl-type) phases. There was generally good agreement with previous results for lambda (P) at room temperature, the decrease of lambda through the phase transition and lambda (T) at atmospheric pressure. The heat capacity per unit volume was also measured simultaneously but for technical reasons data could be obtained only near room temperature. Corresponding values of specific heat capacity were in good agreement with previous work over the whole pressure range.

For pt.I see ibid., vol.18, p.3943 (1985). The results in the preceding paper concerning the variation of the thermal conductivity, lambda , of six potassium and rubidium halides with temperature and pressure are analysed using a Leibfried-Schlomann formulation for the phonon-phonon scattering. The concept of a reference thermal conductivity is introduced which demonstrates how lambda varies with both the mass ratio and the structure change through the B1 and B2 phase transition. The pressure variation of lambda is dependent on the first and second Gruneisen parameters. The large decrease in lambda at the phase transition is seen to be primarily due to the sudden increase in the first Gruneisen parameter as the coordinate number changes from 6 to 8.

The authors investigate non-interacting electrons moving in a two-dimensional, disordered potential under the influence of a perpendicular magnetic field. The random potential is assumed to have a minimum length scale for fluctuations. If the magnetic field is sufficiently strong the ratio of the cyclotron length to the correlation length of the potential is a small parameter. They develop a systematic perturbation theory in this quantity for the disorder-averaged Greens functions from which physical properties are calculated. The method yields useful results for single particle quantities, including the density of states, but not for the low-frequency transport coefficients.

The electron gas in a periodic potential field is studied. By the use of the kinetic theory of fluctuations and the polarisation approximation, the kinetic equation for the non-equilibrium electron gas, including generation and recombination effects, is derived. Furthermore, the equation for the electron density in the conduction band in the state far from equilibrium is obtained. Expressions for the coefficients of the Auger generation and recombination are obtained.

Selection rules for electric-dipole-excited impurity spin-flip transitions induced by inversion asymmetry in InSb-type semiconductors are obtained. Magnetic fields are considered in (001) and (110) planes. It is demonstrated that inversion asymmetry is the dominant mechanism allowing for pure impurity spin-flip transitions (impurity electric-dipole-excited spin resonance: EDSR), i.e. Delta n=0, Delta M=0, Delta beta =0.

Accurate theoretical results are reported for the pair correlation function of the classical two-dimensional electron liquid with r^-1 interactions at strong coupling. The approach involves an evaluation of the bridge diagram corrections to the hypernetted-chain approximation, the role of low dimensionality being evident, relative to the case of the three-dimensional classical plasma, in an enhanced sensitivity to long-range correlations. The liquid structure results are utilised in a density-wave theory of first-order freezing into the triangular lattice, the calculated coupling strength at freezing being in reasonable agreement with computer simulation results and with data on electron films on a liquid He surface. The stability of the triangular electron lattice against deformation into a body-centred rectangular lattice is also discussed.

Measurements are reported of the temperature dependence of the resistivity, Hall effect and cyclotron resonance, in n-type InP samples which show activated conduction. After an important correction for Fermi level variations was made, accurate values of threshold energies are deduced. It is shown that the threshold for DC conduction is higher than for both the Hall effect and the cyclotron resonance effect. An explanation is given in terms of the long-range impurity potential fluctuations, and is based both on an effective medium model, and on the manner in which carrier localisation explains the Hall and resistivity results in 2D systems.

The specific heat of KCN_xCl_1-x mixed crystals was measured for four chlorine concentrations between x=0.90 and x=1.00. The entropy change Delta S and critical temperature T_c were obtained and the results are discussed in terms of the orientational motion of the CN^- molecular ions.

For pt.I see ibid., vol.16, p.4719 (1983). Experimental and theoretical studies are presented on the secondary emission (SE) due to recombination of excitons near the indirect band bottom in BiI₃. The authors have made systematic measurements to observe the excitation energy, temperature and sample dependences of SE spectra, and obtain a consistent interpretation on these experimental results applying the theory developed in preceding work. The observed SE spectra consist of the Raman scattering (RS) lines, involving resonant higher-order Raman processes, and the hot luminescence (HL) bands in the scheme. Under the excitation near the indirect absorption edge, the spectral structure of SE reflects the relaxation processes of excitons which are governed by the intraband scattering by acoustic phonons and a non-radiative decay due to crystal imperfections. It is shown that the two parameters characterising these two processes are uniquely determined by fitting the calculated intensities of RS lines to the observed ones, and the computed spectral lineshape of HL using these parameter values reproduces experimental results satisfactorily. The relationship between RS and HL intensities is also discussed by comparing the observed and calculated intensity ratios.

The chemisorption of sulphur at the (110) face of nickel induces drastic changes in the multilayer relaxation: the oscillatory relaxation of the first and second interlayer distances at the clean surface (-8.4+or-0.8% and +3+or-1%, respectively) changes in sign and increases slightly (+10+or-2% and -3.5+or-1.5%) upon sulphur chemisorption. The sulphur atoms are in the hollow adsorption site at 0.84+or-0.03 AA above the top nickel layer. This position is in good agreement with previous results obtained from angular-resolved Auger emission of sulphur or from Rutherford backscattering (RBS) with medium-energy ions. The authors also agree with the sign reversal of the relaxation of the top layer reported from RBS experiments, but they find that it is larger (+10% instead of +6%) and discover, in addition, a substantial contraction of the second interlayer spacing.

A LEED study of the (110) surface of palladium using a Data-Quire video-LEED system has determined that the laterally unreconstructed layers of Pd(110) have an oscillatory relaxation with respect to the bulk interlayer spacing. The first and second interlayer spacings were found to be -5.7+or-2% and +0.5+or-2%, respectively, with respect to the bulk spacing. These results agree reasonably well with recent embedded-atom theory results which predict a first-layer contraction of about 11%. The possible role of surface disorder in surface relaxation is discussed.