Table of contents

New very long-energy-electron reflection data on W(110) are presented. Spectra measured at several angles of incidence in the 0-30 eV range are compared with calculated profiles. These are obtained using a surface barrier model in which possible variation of the inner potential at the top layer is taken into account. An accurate barrier shape is required to reproduce fine structure oscillations in the vicinity of beam emergencies. However, best overall adjustment to experimental data is obtained when saturation of the inner potential at the surface is introduced. The analysis suggests that the absolute value of the inner potential in the topmost layer is reduced by the order of 1 eV on W(110).

The authors construct a two-parameter scaling function due to power-law localisation in two dimensions. The scaling function is in good agreement with the experimental beta function of Davies, Pepper and Kaveh (see ibid., vol.16, p.L285, 1983). It is shown that although a diffusive-hopping transition is expected above the mobility edge, this has little effect on the form of the two-parameter scaling function. The authors also show that transport mechanisms associated with exponentially localised states lead to a beta function which contradicts the data. They conclude that the experimental scaling function of DPK strongly supports the existence of a mobility edge with power-law state above it.

The authors have investigated the current-voltage characteristics of Al:Si and Ag:Si point contacts in the region of energy corresponding to plasmon excitation. They present results showing resonant structure in d²I/dV² close to the Al surface plasmon energy and bistable switching close to the Al and Ag surface plasmon energies.

Report ODENDOR spectra obtained by monitoring the ODMR signals from shallow donor centres in ZnSe and in CdS. ODENDOR signals corresponding to ³³S, ⁷⁷Se, ⁶⁷Zn, ¹¹¹Cd and ¹¹³Cd nuclei have been observed. The signals are very strong, in some cases representing 1% increase in emission intensity, and provide further conformation of the validity of the model used to describe the ODMR of donor-acceptor pairs. The ⁷⁷Se and ³³S ODENDOR signals provide evidence for the existence of significant chalcogen contributions to the donor wavefunction.

Photoquenching experiments of the magnetic circular dichroism and optically detected ESR and ENDOR signals of the As-antisite in various semi-insulating GaAs materials show that the isolated As_Ga defects cannot be identified with the EL2 defect in either charge state. Investigations of the spin-lattice relaxation time T₁ in conventional ESR and ODESR experiments show that the As_Ga defects seen in conventional ESR are not isolated As_Ga centres but most probably complexes of As_Ga defects. Therefore the correlations established previously between EL2 and As_Ga defects from conventional ESR experiments must be reinterpreted to be correlations between EL2 defects and complexes of As_Ga defects.

The authors present a unified approach to repulsion in ionic and van der Waals solids based on a compressible-ion/atom model. Earlier studies have shown that repulsion in ionic crystals can be viewed as arising from the compression energy of ions, described by two parameters per ion. Here they obtain the compression parameters of the rare-gas atoms Ne, Ar, Kr and Xe by interpolation using the known parameters of related equi-electronic ions (e.g. Ar from S^2-, Cl^-, K⁺ and Ca^2-). These parameters fit the experimental zero-temperature interatomic distances and compressibilities of the rare-gas crystals satisfactorily. At high-temperature equation of state based on an Einstein model of thermal motions is used to calculate the thermal expansivities, compressibilities and their temperature derivatives for Ar, Kr and Xe. It is argued that an instability at higher temperatures represents the limit to which the solid can be superheated, beyond which sublimation must occur.

The off-centre instability (OCI) of a Li ion in a KCl crystal (the stable position for the Li nucleus is slightly displaced from the normal lattice site) has been investigated by means of an ab initio calculation of the total energy of a small ion cluster, and a Mulliken population analysis. The adiabatic potential for the Li nucleus and its lattice parameter dependence have been calculated. The distance and depth of (111)-off-centre position turned out to be 0.526* square root 3 AA and 51.570 meV respectively at the lattice parameter of 3.147 AA. The total dipole moment of the cluster has also been calculated. It has been shown that the totally symmetry charge transfer from the 3sp atomic orbitals of the Cl ions at nearest-neighbour sites to the 2sp atomic orbitals of the Li ion, which is brought about by the substitution of the Li ion for the K ion at the O_h-symmetrical configuration, can be the origin of the OCI in this system. This totally symmetric charge transfer (about 0.3e) has been considered to play two roles on the off-centre instability; one is to increase the electrostatic interaction to shift the Li nucleus to the off-centre position and another is to weaken the repulsive interaction between the bare Li⁺ ion (Li core) and the surrounding Cl ions.

A real-space renormalisation group transformation for anisotropic directed bond percolation is defined in arbitrary dimensions. For the symmetric case ( phi =0), the percolation threshold p_c, obtained is very good in two dimensions, asymptotically exact for large dimensions and quite good in between. The flow diagram is correct for all anisotropies. For the non-symmetric case ( phi =0), the transformation can be generalised and the flow diagram obtained is compatible with the Domany-Kinzel hypothesis (1981).

The ground state of a one-dimensional model with two sublattices which is a variation of the piecewise parabola Frenkel-Kontorova model, is explicitly calculated. This model involves an additional parameter (the electric field) which breaks a (non-symmorphic) symmetry element when it is non-zero. At a fixed commensurability ratio zeta , it is shown that the polarisation curve as a function of the electric field is the sum of a linear part and a staircase. This staircase is either a harmless staircase ( zeta rational) with true first-order transitions or a Devil's staircase ( zeta irrational). The plateaus of these staircases are obtained when an unusual condition (called subcommensurability condition) which involves the relative phase shift between the two sublattices is fulfilled. The phase diagram of this model as a function of the chemical potential and the electric field is explicitly calculated. At fixed electric field, the commensurability ratio zeta varies as a function of the chemical potential as a Devil's staircase with plateaus at rotational values of zeta but these ones have widths submitted to selection rules related to the model symmetry. The corresponding curve for the variation of the polarisation is a new 'devilish' function called a Manhattan profile. This curve exhibits infinitely many plateaus but unlike a Devil's staircase is alternatively increasing and decreasing infinitely many times by discontinuities. The results obtained on this model are favorably compared with experimental results in thiourea.

A high-pressure X-ray scattering study has revealed that the compressible molecular crystal SnI₄ gradually transforms from the crystalline to the amorphous state, this process being accompanied by a remarkable decrease in electrical resistivity induced by the pressure. Taken together with the Raman scattering study described in the following paper, this indicates that the amorphous structure attained above about 20 GPa is formed by dimerised SnI₄ tetrahedral molecules packing randomly and densely.

For pt.I see ibid., vol.18, p.789 (1985). The lattice vibrations in the molecular crystal SnI₄ have been investigated by high-pressure and high-temperature Raman spectroscopy. This material shows successive phase transitions at about 1.5, 10 and 20 GPa in a pressure-increasing process. Above 20 GPa an amorphous state is realised. On decreasing the pressure this amorphous state persists to about 2 GPa. Below this pressure the structure changes to another amorphous state and then a new crystal phase, at very close to atmospheric pressure. In the high-pressure amorphous state a strong A₁-symmetric peak arising from the breathing mode of the SnI₄ tetrahedral molecules splits into a doublet. This is interpreted in terms of pressure-induced dimerisation. The spectra for the liquid are also measured, for comparison with those for the amorphous state.

High-resolution neutron diffraction studies of NbC_0.83 powders are described. In a first set of experiments the order-disorder transition was studied by measuring the integrated Bragg intensities. A first-order transition is observed around 1025+or-5 degrees C on heating, which shows a hysteresis. In a second set of experiments, a profile refinement method was used to get information on the crystal structure of a well ordered Nb₆C₅ sample. The V₆C₅-type trigonal superstructure previously proposed was confirmed (rather than the V₆C₅ monoclinic form). A shift of the niobium atoms ( approximately=0.05 AA) away from their first-nearest-neighbour vacant site, towards a first-nearest-neighbour occupied site (trans to the former) is found.

The electronic band structure of black phosphorus has been studied by angle-resolved ultraviolet photoelectron (ARUPS). The experimental valence band structure has been successfully obtained by the use of the He I and Ne I resonance lines. The binding energies of some special points with almost no interlayer energy dispersion have been experimentally determined and compared with the result of the self-consistent pseudopotential (SCP) band calculation on black phosphorus. The comparison indicates that the SCP calculation overestimates the 3p band width by about 1.0 eV and that the displacement is not necessarily uniform. The conduction band structure has also been studied by the ARUPS with the Ar I resonance line, which is largely modulated by the conduction band structure.

The specific heat of a two-dimensional inhomogeneous electron gas has been calculated using Bardeen's formula in the exchange approximation. High- and low-temperature limits are obtained The specific heat vanishes inversely with temperature in the high-temperature limit and the expected T ln T behaviour is obtained in the low-temperature limit. A universal curve is obtained which gives the specific heat in the exchange approximation for any density and temperature.

Photoconductivity measurements of MgO crystals that have been implanted with alkali ions and annealed are presented and compared with optical absorption spectra and the results are discussed. It is suggested that the absorption peak, which appears after annealing at high temperature and does not correspond to a photoconductivity maximum, comes from aggregates which are not in electric contact with the MgO lattice. It appears that the photoconductivity is increased by surface plasma oscillations of spherical sodium particles.

The authors use domain arguments to study the behaviour of a sequence of long-period commensurate phases in the presence of quenched randomness. The phases are found to be stable for dimensions d>or=3. For 2<d<3, however, phases of sufficiently long wavelength are destroyed by the disorder.

The authors calculate the magnetic susceptibility of the intermediate valence phenomena. They have used the periodic Anderson Hamiltonian (PAH) as the theoretical model, and the calculations were made using the two-particle Green function formalism. The PAH is decoupled using the alloy analogue approximation (AAA). The average one-particle Green functions are calculated using the coherent potential approximation (CPA), and with these functions they calculate the density of states. These results are compared with a pseudo-virtual-crystal approximation model. The magnetic susceptibility is expressed in terms of a two-particle Green function. This function, with the above model, can be reduced to two one-particle Green functions, and these can be averaged using the two-particle extension of the CPA formalism. The AAA breaks the spherical symmetry of the Hamiltonian, giving different results for the longitudinal and transverse terms. From symmetry considerations the authors find that the total susceptibility is the weighted average of the two terms. The susceptibility can be interpreted in terms of the Pauli, Van-Vleck-and Curie-Weiss-like terms. An extra term arises from the random character of the model. The results are compared with the experimental results.

Analytic expressions are derived for the longitudinal sixth frequency moment of the spin correlation function for a dilute anisotropic Heisenberg paramagnet in the high-temperature limit. The results are valid for arbitrary spin and arbitrary range of the exchange interactions. Both site and exchange-bond dilution are treated. The latter is treated using two models introduced by Kawasaki and Tahir-Kheli (1978). In one the existence of longitudinal and transverse bonds depends only on a single random variable, whilst in the other they can fluctuate independently. As an application of the use of the moments, the extent to which pseudo-dipolar interactions can affect the high-temperature spin dynamics of a Heisenberg magnet is examined.

For pt.I see ibid., vol.17, p.893 (1984). Thermodynamic equations are derived, for a model of a one-dimensional metal containing magnetic impurities, using the exact Bethe ansatz solution. Explicit expressions for the magnetisation, susceptibility, and specific heat are given in three low-temperature regions of the H, T plane.

Dielectric measurements in the temperature range 33-300K with simultaneous optical observations on LiKSO₄ slabs perpendicular to the c hexagonal axis have been performed. A few dielectric anomalies and domain textures are obtained according to the different thermal treatments. A specific thermal treatment has been found allowing reproducible results. Previous contradictory results in the literature can therefore be explained.

Dielectric susceptibility measurements and Raman scattering observations under electric field in the ferroelectric (F) and incommensurate (INC) phases of normal thiourea are reported. In the F phase, the dielectric susceptibility and the Raman spectrum of the LO soft mode are very sensitive to an applied electric field. In addition, a pseudo-LO mode persists in the INC phase, with electric-field-independent frequency and intensity, in contrast with the behaviour in the F phase. These results are described in connection with the expected domain structure in F and INC phases of thiourea.

The reduction of computer time and memory in quasidynamical LEED calculations compared with full dynamical ones allows their implementation on a minicomputer. This is done on an LSI 11/2 which is also used to control the measurement of LEED intensities through a video system. Thus the complete structure determination can be performed with one single piece of equipment as is shown in the case of Cu(100) in the energy range 40-600 eV. Only the first to second layer contraction was determined, leading to the same value as a full dynamical analysis, i.e. Delta d/d=-1.5% within the usual errors.

The electronic structure of Ni overlayers on Si (111) has been calculated by the tight-binding method for various possible bonding geometries. Surface EXAFS suggests nickel exists at six- or sevenfold-coordinated, Ni₆ or Ni₇, sites buried between the topmost layers of Si. The authors results favour the Ni₆ geometry for which E_F lies in the lower gap, as observed, while bonding at the Ni₇ configuration appears to be less stable. A general bonding model for other transition-metal overlayers is developed. The Co₅ geometry at CoSi₂:Si(111) interfaces is rationalised and an interstitial threefold coordination is suggested for Co monolayers to account for the photoemission data.