Table of contents

The vacancy and divacancy (V₂) in silicon are assumed to be strongly coupled to the lattice via a linear Jahn-Teller process. Consequently, the lattice coordinate of the defect should strongly reflect the defect charge state. The authors report experimental and theoretical evidence which shows that the thermal broadening of the V₂⁺ to V₂⁰+h optical transition is too small to be compatible with the above model. It would appear that the Jahn-Teller effect is relatively weak, and that the lattice coordinate of the defect is independent of charge state.

The authors use a Migdal-Kadanoff renormalisation group to study the behaviour of the Blume-Emery-Griffiths model when quenched, random-bond dilution is introduced. Phase diagrams are calculated for varying dilution. The behaviour of the multicritical points of the pure system as dilution is introduced is of particular interest. They find that the tricritical temperature tends to zero as the percolation threshold is approached but that the tricritical field remains finite in agreement with experimental results on the dilute metamagnet Fe_pMg_(1-p)Cl₂.

The authors present the results of an investigation of the rate of electron-electron scattering in the two-dimensional electron gas of the Si inversion layer. The electron-electron scattering rate was extracted by analysis of the negative magnetoresistance in the regime of weak localisation when k_Fl>1, where k_F is the Fermi wavevector and l is the mean free path for elastic scattering. The results showed that over a wide range of k_Fl, carrier concentration and electron mobility the scattering rate was given by two processes which were additive. The first was the normal T² (Landau-Baber) law and the second was the T law expected when the effects of disorder are considered. There was no evidence for the recently proposed T ln T law.

The authors present results on the Hall effect in the Si inversion layer in the regime where the temperature dependence of the conductivity is due to corrections arising from the electron-electron interaction. They find that the temperature dependence of the Hall coefficient is independent of the magnitude of the magnetic field which does, however, have a pronounced effect on the temperature dependence of the conductivity. This is not explained by present theories, and suggests that whereas the effects of screening on the conductivity are well understood this is not the case with the Hall effect.

The resistivity, thermopower and thermal conductivity of a two-dimensional electron gas (inversion layer) in high magnetic field has been calculated using the Kubo formula. It has been found that all transport coefficients are universal functions of the reduced temperature k_BT/h(cross) omega _c and the reduced chemical potential mu /h(cross) omega _c, provided that the thermal broadening k_BT is substantially greater than the width of the Landau levels.

The authors argue that standard treatments of dynamics are incorrect for non-ergodic systems such as the Sherrington-Kirkpatrick model of a spin glass. Since the system never loses the memory of its initial state it is necessary to average over initial states with a Boltzmann weight to obtain time-dependent correlation functions as conventionally defined. It is now necessary to introduce replicas. The 'dynamical' order parameter, q_EA, is equal to q(x=1) in Parisi's scheme whereas the statistical mechanics order parameter, q, is given by integral ₀^Iq(x) dx. They also propose an interpretation of the function q(x).

Unexpectedly small velocities of demagnetisation processes have been observed in Y₂Ca₁Fe₃₉Co_0.1Ge₁O₁₂ films. These phenomena have been investigated in the experimental set-up based on the Faraday effect. The investigated after-effects have been numerically analysed in terms of continuous distribution of relaxation times tau . Strong dependences of tau on temperature and applied magnetic field have been found. The activation character of relaxation processes has been determined.

The ¹⁸¹Ta NMR in 1T-TaSe₂ and 1T-TaS₂ has been studied at 4.2K using single crystals as well as powders in order to obtain detailed information of the superlattice structure in the commensurate charge density wave state. The results are discussed on the basis of the star-shaped cluster model and an effect due to the three-dimensional (3D) order of clusters is pointed out.

Laser-induced spectral hole burning in the presence of applied electric and magnetic fields has been used to determine the symmetry and electronic character of the colour centre in X-irradiated NaF whose zero-phonon line is at 5770 AA. The centre is found to have C_s symmetry, perturbed only slightly from trigonal. The authors speculate that it, and other centres in this special region, may arise from F₃⁺ centres perturbed by structural or chemical defects.

A new method to determine the volume distribution of structural domains is presented. The scattering intensities of fundamental Bragg reflections are analysed. The theory applies to all phase transitions which exhibit structural domains. Thus, there is a subgroup relation between the low- and high-temperature symmetry. The knowledge of both structures and the temperature-dependent order parameters are required. The method is applied to the structural phase transition of the perovskite KMnF₃.

The Fourier series method for calculating the inverse of the metric matrix of crystals (needed in the orthogonalisation of one-electron orbitals) is presented for the general case. In this method the matrix is first partially diagonalised by a unitary transformation (the Fourier series). After inverting the small diagonal blocks, the inverse transformation (summation/integration in the Brillouin zone) is performed to obtain the elements of the inverse metric matrix. Crystals with full orthorhombic, hexagonal or cubic site symmetries are considered in detail. Numerical applications to NaF, LiF and MgO crystals with NaCl structure have been performed and the results are compared with those obtained by the cluster method. For ionic crystals the present procedure is better than the cluster one as regards the accuracy and stability of the results even though the computing times become larger in some cases. The computing procedure in this form is found to be unsuitable for metals and several improvements are found necessary.

The electronic contribution to the cohesive energy is calculated in narrow band systems where the splitting of bands due to intra-atomic correlation is larger than the band width by solving the effective Hamiltonian for the lower Hubbard subband in the non-degenerate band approximation on the basis of different decouplings of the quasielectron Green function. Using a rectangular form of the density of states for non-interacting electrons analytical expressions for the ground state energies for different magnetic phases are obtained. The importance of the kinetic exchange on the stability of the magnetic phases is indicated for different fillings of the band. The significance of these results for the understanding of the cohesive energies of strongly correlated systems is discussed.

Using the Lawaetz method the authors have estimated the k.p band parameters and determined the band structure near the valence-band edge of the Ge-Si alloy system. They have also obtained the coupling constants between holes and phonons by Wiley's method. Using these two kinds of parameters they have calculated the time for relaxation of holes due to the lattice scattering, where they have taken into account the nonpolar optical phonon, the impurity scattering due to ionised and neutral centres and also the alloy scattering. The intervalence-band interactions are shown to produce the complicated temperature dependence of the hole mobility.

A method to compute the electronic and structural properties of semiconductors self-consistently is applied to analyse the electron-phonon interaction in these crystals. The authors have calculated the optical-phonon deformation potentials along the symmetry directions in Si, Ge and GaAs. Their results confirm the previous ones obtained in more simplified approaches. Quantitative discrepancies with experimental data suggest what must be the next step to be taken in the theoretical effort.

A non-equilibrium delta-function distribution of carriers can move up the potential energy gradient associated with an electric field as a consequence of the energy dependence of the quasi-one-dimensional density of states, characteristic of magnetic quantisation.

A theoretical calculation of the quantum efficiency of the internal photoelectric effect in InSb is made. In addition to electron and light-hole impact ionisation and intraband thermalising processes (deformation potential, optical polar mode phonons and particle-particle collisions) there are also included the near-horizontal transitions between the Gamma and L conduction bands and between the light-hole and heavy-hole bands. There is good agreement with experiment, with the suggestion that either the L band is lower in energy than previously thought or that there is also the participation of a localised level associated with the L band.

The complete solution of the non-degenerate Anderson model describing the formation of the localised moment in a metal is constructed on the basis of the Bethe ansatz method. Explicit expressions for the magnetic susceptibility and specific heat are derived at zero temperature but with arbitrary parameters U, Gamma and epsilon _d. In the symmetric case the dependence of the impurity magnetisation on the magnetic field is obtained at zero temperature. The asymmetric Anderson model is studied in detail. Explicit expressions for the dependence of the susceptibility and occupation number on the effective resonance level position are derived.

For pt.I see ibid., vol.16, p.2281 (1983). On the basis of the exact solution which was considered in the previous paper the thermodynamic properties of the Anderson model are studied.

The authors have measured magnetic diffuse elastic cross sections for the Mn_1-nZn_nF₂ system at 4.2K and for one composition at 25K. Neutron polarisation analysis with a time-of-flight technique was used to separate the magnetic elastic cross section. Most magnetic disturbance appears on next-nearest-neighbour manganese atoms to the Zn impurity and is greater than would be expected from Lovesey and Marshall's model of very dilute alloys (1966). The range of the disturbance increases at higher temperatures.

For pt.I see ibid., vol.11, p.4187 (1978). Effects of tunnelling rotation of ND₄⁺ ions on ²H-NMR lines are analysed. Detailed lineshape calculations are supplied. All terms in the quadrupolar Hamiltonian and secular dipolar interaction between deuterons are taken into account. Asymmetry and level crossing are characteristic features observed in the lineshapes. The latter effect makes measurement of the tunnelling frequency possible. Sets of theoretical lineshapes are presented showing their evolution with tunnelling splitting and with magnetic field. The main conclusion is that complete interpretation of a given ammonium compound (ground torsional level structure and tunnelling splitting value) requires measurement of the field dependence of the ²H-NMR lineshape.

For pt.II see ibid., vol.16, p.2351 (1983). Deuteron NMR lines were measured in ND₄ClO₄ at 4.2K and at various magnetic fields from 0.63 to 1.48T. A remarkable number of lines were found to exhibit evolution in shape and width. This effect arises from level-crossing between Zeeman and tunnelling systems appearing around 0.85 T and around 1.45T. The theory of ²H NMR spectra, from previous work, has been applied to the interpretation. Tunnelling splittings were estimated by fitting theoretical lineshapes and were found, for ND₄⁺ in ammonium perchlorate, to be reduced by a factor of 304 with respect to those of NH₄. Rotational tunnelling effects in proton and dueteron NMR spectra are compared.

The effect of quenching from 650 degrees C to room temperature as well as that of heavy predoses at room temperature on the thermoluminescence induced by X-irradiation at temperatures between 80 and 300K in KCl:Sr (80 ppm) samples have been investigated. These heavy predoses strongly reduce the glow peaks in the 170-300K range which are clearly related to interstitial centres involving impurity-vacancy dipoles. This result gives additional evidence to support the idea that such dipoles are also involved in the thermoluminescent processes operative above room temperature. The study of quenched samples has revealed the existence of a dipole aggregation process which is induced by irradiation. It is suggested that the energy transfer mechanism is the recombination of the radiation-induced interstitials and vacancies close to the dipoles, either during irradiation or in the thermoluminescence run.

The amplitude-weighted vibrational density of states of hydrogen adsorbed on nickel (100) and (111) surfaces is computed and compared with inelastic neutron scattering data on nickel catalysts. Local modes at 113 meV and 140 meV are interpreted as being due to vibrations of hydrogen parallel and perpendicular to the surface at a three-coordinated site respectively. A peak at 80 meV is interpreted as arising from vibrations of hydrogen perpendicular to the surface at a four-coordinated site. The relative intensities of the local modes in the data suggest that the three-coordinated site is the preferred site for adsorption on nickel catalysts. However the measured density of states below 300 cm^-1 indicates some occupancy of four-coordinated sites in agreement with the existence of a peak at 80 meV.

Previous studies of Schottky barrier formation between metals and clean cleaved (110) surfaces of n-type InP have shown that chemically reactive metals (Ni, Fe, Al) pin the Fermi level relatively close to the conduction band whereas unreactive metals (Cu, Ag, Au) lead to pinning closer to mid-gap. The authors have selected Ni and Cu, as representative of each group, and studied the detailed interactions between these and the InP surface using a range of experimental techniques. These include LEED, Auger and photoemission using a laboratory source (He I radiation) as well as a synchrotron source. Nickel was found to interact very strongly with the surface resulting in the formation of nickel phosphides at the interface and the metallic indium diffusing out into the Ni contact. Copper interacts much more weakly with the surface, the core level and valence band photoemission spectra contrast markedly to the behaviour observed for Ni. The formation of Schottky barriers has also been probed by monitoring appropriate shifts in the photoemission spectra. The importance of the interactions at the interface on Schottky barrier formation is considered. Interface widths are also discussed.

In spite of the very large number of experiments (LEED, AES, UPS, MeV He⁺ scattering, work function, FIM) carried out on W (100), no structural model consistent with all the data has been proposed yet: in particular, the question of the reconstruction thermally induced when the sample is cooled below room temperature remains a puzzling problem. Furthermore, from a theoretical point of view, no definitive answer has been given. Actually, either the mechanism invoked for the reconstruction is too weak, or some contributions are omitted or calculated without sufficient care. The authors compute here the surface energy of W (100) taking into account the band term treated in the tight binding approximation, a pairwise repulsive potential of the Born-Mayer type and the electronic correlation contribution obtained from a perturbation treatment of the Hubbard model in the band limit. This energy is then fully minimised with respect to all coordinates of surface atoms, keeping all atoms neutral for any displacement. They find that the unreconstructed surface is the most stable at T=0K and discuss this unexpected result. A similar calculation for Ta (100) leads to opposite conclusions.

In a forward-biased Schottky diode, the charge of the interface states can be electrically modulated. This results in the appearance of additional capacitance whose variations with frequency, bias and temperature can be used to determine the characteristic parameters of the states (energy position, density, capture cross sections). A simple model is described, taking into account any possible charge exchanges between the interface states and the three 'reservoirs' surrounding them (conduction and valence bands of the semiconductor and conduction band of the metal). Limit cases where one or two of these exchange paths are dominant are detailed and a few typical experimental examples are given.

In InSe electron accumulation layers are formed in the vicinity of stacking faults with charged impurities adsorbed to the defect plane. In this two-dimensional system the dependence of the cyclotron resonance linewidth on magnetic field is significantly different from silicon n-channels and from inversion layers at heterojunction interfaces. While in the latter the linewidth is dominated by short-range scattering and thus follows a B^1/2 dependence, the CR linewidth in InSe does strongly decrease with increasing magnetic field for intermediate fields and saturates for higher fields. This behaviour can be quantitatively described as due to neutral impurity scattering via a gaussian potential with a range comparable with or larger than the cyclotron radius. At high magnetic fields, screened Coulomb scattering and deformation potential scattering also contribute.