Table of contents

The critical behaviour near the commensurate-incommensurate transition of a two-dimensional elastic lattice on a one-dimensionally modulated substrate is studied by a novel renormalisation group approach which includes the mismatch between lattice structure and substrate as a renormalisable parameter. The critical exponent nu is found to depend continuously on the model parameters. For typical experimental values of the latter, nu is close to ¹/₃, which is in good agreement with experiments on adsorbed monolayers.

The shear elastic modulus C₄₄ of graphite has been determined by Brillouin scattering from surface waves. The author's value agrees with that obtained from neutron scattering experiments but differs from those determined by ultrasonic technique.

When one determines the critical de Almeida-Thouless line on a spin glass system by studying the equilibrium magnetisation as a function of magnetic field and temperature, the result depends on critical exponents of the spin glass transition. This offers a self-consistent check relating the low-field critical line to the critical scaling exponents on spin glass systems.

Rectangular bars of silicon containing oxygen impurity have been cut with the long axis parallel to a (111) crystal direction. The bars were stressed at 450 degrees C and measurements made of the resulting dichroism of the 9 mu m oxygen band following: (a) isothermal anneals at 350, 370 and 390 degrees C; and (b) successive doses of electron irradiation up to a total dose of 1.1*10¹⁹ cm^-2 with the samples at room temperature. The authors' diffusion data agree with other current work and they observe radiation-enhanced diffusion corresponding to D=10^-21 cm² s^-1 at 300K.

It is pointed out that the spin-correlated crystal field operator is diagonal in the total spin of open-shell systems, thus simplifying the evaluation of its matrix elements.

The temperature dependence of the ²³Na central lineshape of Na₂CO₃ has been observed by NMR. At the phase transitions at T_cl=763K and T_c2=633K there is no change of the distorted octahedral oxygen environment of the sodium ions observable. But changes of this oxygen environment far below T_c2 and above T_c1 were observed to occur in contrast with structural investigated. The temperature dependence of the quadrupole frequency nu _Q in beta -Na₂CO₃ is explained by stretch vibrations of the CO²/₃^- groups along the C-O bond.

The extended fine structure at the K absorption edge of metallic cadmium (26.7 keV) has been measured with synchrotron radiation to test possibilities and restriction of the EXAFS technique at this photon energy. The analysis of the signal has been performed to extract the distance of the nearest neighbours: the value R_E=2.965+or-0.025 AA obtained is in good agreement with the crystallographic value R_c=2.979 AA. The experimental phaseshift of the photoelectronic wave has been extracted and compared with theoretical values.

The authors consider the problem of Auger recombination of electrons and holes in quantum well heterostructure. An expression for the Auger recombination rate is derived for a particular process which is the quantum well equivalent of the so-called CCCH process with the carriers involved remaining in the lowest electron and hole sub-bands. The recombining carriers are assumed to obey Boltzmann statistics. This restricted calculation is regarded as a useful preliminary to the study of the complete range of Auger processes in the quantum well, and the use of a Fermi-Dirac statistics.

The authors have investigated the bonding in cubic and rhombohedral GeTe by studying the charge density of each individual valence band, using the Baldereschi method. The calculated charge densities can be interpreted in terms of atomic orbitals. The orbitals that participate in the bonding are s Ge, p Te and p Ge. In cubic GeTe there are p orbitals along the three orthogonal axes forming polar sigma -bonds between the Ge and Te atoms. There is a pronounced admixture of s Ge states with p states in the second and the last valence band. This admixture is responsible for the rise of the last valence band, which makes the GeTe a narrow-gap semiconductor. For rhombohedral GeTe the bonding can be described in terms of one p orbital along the (111) direction and the three p orbitals orthogonal to the (111) direction. These orbitals interact and form an admixture of polar sigma - and pi -bonds. The bonding charge is greater for rhombohedral GeTe, resulting in an increase of the bonding and a lowering of the electronic energy of rhombohedral GeTe with respect to that of cubic GeTe. By calculation they find the dipole moment of the rhombohedral phase to be 1.4 D along the (111) direction.

The full infrared and Raman spectra of K₂TaF₇ are reported and it is shown that although an unambiguous assignment of all the bands is not possible the results at ambient cannot be interpreted in terms of a rigid, distorted monocapped trigonal prism. At lower temperatures the vibrational spectra are characteristics of a rigid structure of low symmetry. These results are supported by NMR measurements showing that the anions are stationary on the NMR timescale below approximately 150K but that they execute reorientations which can only be visualised in terms of a nonrigid molecule at higher temperatures.

The authors report new Raman and neutron scattering measurements made on the cubic perovskite SrTiO₃. By comparing the soft-mode lineshapes in the low-temperature phase they have been able to observe the breakdown of the one-phonon approximation conventionally used in the analysis of Raman scattering due to the scattering from critical two-phonon and interference scattering processes. They have determined the limiting behaviour of the soft-mode frequencies and widths on the approaching the transition temperature, T_c. Below T_c the soft mode has components of E_g and A_2g symmetry. On approaching T_c from below, the E_g mode frequency saturates at a value of 0.16 THz. On approaching T_c from above, the widths of the phonons are about 0.20 THz. They have found that there is appreciable scattering at low energies, which cannot be accounted for by a simple classically damped phonon model. This extra scattering may be interpreted as a broad, critical central peak with a width comparable to that of the soft phonons.

X-ray diffraction measurements on various (ranging in thickness from approximately 30 to approximately 330 mu m) platelet-shaped crystals of alpha -MnS (bounded by (111) planes) and temperature-dependent neutron diffraction measurements on a polycrystalline sample are reported. It is found that a previously reported sign reversal of the rhombohedral lattice distortion Delta ( Delta = alpha - pi /2 with alpha the corner angle of a pseudo-cubic rhombohedron) at T approximately=130K observed for thick crystals is not substantiated. The observed anomalies for these crystals are related to a spontaneous domain rearrangement from a multiple T domain structure in the region 130K<T<or=152K into nearly exclusively single T₁ domains for T<or=130K. Thin crystals show a continuous temperature dependence of Delta ; Delta is positive for all temperatures below T_N. The formation of T₁ domains in these iodine-grown crystals of alpha -MnS is presumably related to the presence of a monolayer of iodine on top of a (111) plane of divalent positive manganese ions. Temperature-dependent neutron diffraction experiments show that for all temperatures below T_N the magnetic ordering is best described by a collinear in-plane FCC type-II antiferromagnetic spin structure.

The authors re-examine the technique of perturbed molecular dynamics as a method for calculating the autocorrelation function of the microscopic heat current and hence the thermal conductivity of a liquid. They show how the problem of taking the zero-wavevector limit, encountered by Ciccotti et al. (1979) may be overcome. They present calculations on the Lennard-Jones liquid which show that the perturbation technique gives an effective practical method of computing thermal conductivities.

For pt.I see ibid., vol.15, p.4807 (1982). Starting from the virial and Budd-Vannimenus theorems for electrons bounded by a jellium background of constant density within an arbitrary volume, the special case of a spherical jellium shell is considered. Two limits are treated: a plane slab and a spherical void. In both cases kinetic and potential energy- and via these quantities also momentum and pair distribution-are related to each other and to the total charge distribution. From the case of a plane slab a further limit, the semi-infinite jellium, is obtained. Surface theorems, connecting the surface energy with certain semi-moments of the total charge density or of the electrostatic potential inside the jellium, are derived. With the help of the generalised Budd-Vannimenus theorem, expressions for the work function of a sample with arbitrary geometry and especially of a plane slab are derived.

The authors report results of a relativistic ab initio band-structure calculation via the KKR method for the SmS compound with the NaCl structure in the low-pressure phase. The exchange potential was taken in the free-electron Slater approximation. They have solved the full Dirac equation in the interior of the 'muffin tin' spheres. The energies were evaluated at several symmetry points and other general points in the 1/48 wedge of the FCC Brillouin zone. The energy band obtained indicates a gap throughout the whole BZ. The most excited valence bands are f-like and energetically located immediately below the conduction band. The electronic structure is discussed in terms of the proposed models for the understanding of the unusual properties of this compound.

The problem of localisation is considered for the Anderson model with diagonal disorder and where energy values of neighbouring sites are correlated. The percolational criterion of Thouless is used to determine the delocalisation condition. Estimates for the critical disorder are obtained by straightforward calculations for the Cayley tree and with the help of continuous percolation theory for common lattices. It is shown that the condition for the Anderson transition is mainly determined by short-range order in these systems.

An expression for the pair distribution function, g^+or-(r) of electrons around a finite-mass positively charged point defect in a one-component classical rare plasma, is derived in the linear response theory. The expression for g^+or-(r) is quite involved and different from that corresponding to the infinite-mass point defect. The dependence of g^+or-(r) on the temperature of the system and the mass of the point defect has been studied and reported for a given system.

A recently proposed theory for the behaviour of a zero-temperature electron gas in an environment allowing for random scattering and random tunnelling is extended by treating the conductor phase with an electron-phonon coupling taken into account. The conductivity is expressed in terms of two frequency- and temperature-dependent kernels for which self-consistency equations are derived. Solution of these equations in the DC limit shows that phonon-controlled tunnelling processes dominate the conductivity if the electronic mean free path approaches the De Broglie wavelength. The resulting competition between scattering and tunnelling gives rises to anomalies of the temperature-dependent resistivity which are frequently observed in high-resistivity conductors as, e.g. liquid and amorphous alloys or A15 compounds.

Carrier concentration under crossed electric and magnetic fields is analysed for the first time in the case where the magnitudes of both fields can be large. Carrier distribution, surface concentration and average concentration are studied for intrinsic semiconductors; similar or contrasting behaviour of the effect with increasing magnetic or electric fields are underlined. Influences on the Hall voltage and magnetoresistance measurements are then considered; it is shown that, quite generally, the influence of a (high) magnetic field on the transverse diffusion currents cancels the usual magnetoresistance effect. It follows that the apparent conductivity is only related to the mean carrier concentration. Simple analytical expressions are derived that allow the authors to propose simplified methods for the determination of bulk and surface recombination parameters, and to obtain the optimum parameters (field intensities, sample thickness...) for sensor applications. Experimental data for Ge and InSb with fields in the ranges 0-12 T and 0-10⁴ V m^-1 agree well with theoretical results.

The authors have studied the internal field distribution P(H) in the infinite-range Ising spin glass within the framework of the replica method. Their results show that breaking of replica symmetry in the spin-glass phase yields a non-gaussian internal field distribution. Using Parisi's scheme for symmetry breaking they derive a set of Fokker-Planck-like differential equations that are obeyed by both P(H) and the multiple correlation functions in replica space. For t=1-T/T_c<<1 the equations can be integrated perturbatively and the field distribution thus obtained deviates from a guassian form, exhibiting a second maximum which increases and moves toward the origin as t increases. The asymmetry factor for P(H) is shown to be negative and of the order of t⁵.

Recently a number of experiments have been performed to study the spin dynamics of CsCoCl₃ which behaves like a spin-¹/₂ one-dimensional Ising-like antiferromagnet. Ishimura and Shiba (1980) have performed first-order perturbation calculations to explain various features of the spin dynamics for this system. Their first-order theory, however, does not give an adequate description of the experimental spin-wave dispersion and observed lineshape. In this work, the Ishimura-Shiba formalism is extended to the second order. Reasonable agreement is obtained with the experimental spin-wave dispersion. The lineshape is, however, modified only slightly from the lineshape obtained from the first-order theory.

Resonant two-phonon Raman scattering involving the indirect triplet exciton as an intermediate state is studied in AgBr at T=2K and magnetic fields up to 20T. The symmetry of the hole state is found to be L_4,5^-. From a quantitative analysis of the scattered 2TO(L) intensity as function of magnetic field and at different excitation photon energies the authors were able to determine the exchange splitting Delta =0.17 meV and effective g values of g_c=1.46 and g_v^h=2.61 for electrons and holes, respectively. The observed diamagnetic shift leads to a value of 32 meV for the indirect exciton binding energy.

Cathodoluminescence in the 1.40 eV and 2.56 eV systems is observed from most nitrogen-containing synthetic diamonds. The authors show that there is a tendency for the 2.56 eV luminescence to be strong when the 1.4 eV luminescence is strong, and that both are strongest for diamonds grown in the presence of nickel. Absorption due to the 1.4 eV centre has been observed for the first time, using diamonds grown with a nickel catalyst-solvent. Temperature-dependence measurements confirm that the 1.4 eV zero-phonon doublet is due to a split ground state, and show that the 2.56 eV system has a complex excited state structure with a group of four well defined levels at 2.5634, 2.5618, 2.5602 and 2.5587 eV and a diffuse level consisting of one excited state, or two or more overlapping states at approximately 2.588 eV. No absorption has been detected in the 2.56 eV system, and in most diamonds the 1.4 eV absorption is too weak to detect. This indicates that the centres are present in low concentrations, and the circumstantial evidence suggests that they both involve nickel complexes. The latter conclusion is strongly supported by recently reported studies on diamonds implanted with nickel.

A study of the interaction of sulphur dioxide with cleaned Zn(0001) surfaces using the techniques of RHEED, LEED and AES is reported. Experiments performed at 165K, 295K and 420K showed both oxygen and sulphur to absorb in proportions varying with temperature and degree of electron beam exposure. A single adsorbed oxygen species was observed but, under certain experimental conditions, more than one adsorbed sulphur species occurred. At 295K and 420K the adsorbate had the structure of epitaxial ZnO but at 165K no new structure was visible.

Concepts borrowed from the effective mass theory of impurity states in semiconductors are shown to be useful in the theory of surface states. Explicit application is made to surface phonons. For a bcc (100) surface, in particular, the authors compare their approximate surface eigenvalues and eigenvectors with essentially exact results from brute force slab calculations, and find generally good agreement. Several approaches to the solution of the effective mass equation are considered, and their relative accuracies are discussed.

Expressions are derived for the far-infrared transmission at non-normal incidence of thin films of anisotropic single crystals. This sellotape stripping technique is used to produce suitable films of CdCl₂ type crystals with the crystal c axis perpendicular to the plane of the film. With these films the frequencies of the E_u(TO) and A_2u(LO) infrared-active modes have been measured for CdCl₂, CdBr₂, CdI₂, MgCl₂ MnCl₂, FeCl₂, CoCl₂, NiCl₂, CoBr₂ and NiBr₂ at low temperatures. Although CdI₂ has a slightly different structure from the CdCl₂ type structure it has a similar spectrum. Powdered samples are shown to yield misleading results due to the finite size of the platelets forming the powder and the preferred orientation of the platelets.