Table of contents

A two-dimensional Wigner crystal of electrons can re-evaporate into a gas in the limit of very low densities when correlation with the positive background becomes important. The densities required are very much below current experimental values. However, if the plane of motion of the electrons and that of the positive background are close enough, crystallisation is prevented at all densities.

The 1.945 eV optical centre is produced in type Ib diamond by radiation damage and annealing at approximately 800 degrees C. The author reports that H3 (2.463 eV) centres, characteristic of irradiated and annealed type Ia diamond, have been created by heating an irradiated type Ib diamond to temperatures where nitrogen aggregation occurs. This observation strongly supports the models recently proposed for the H3 and 1.945 eV centres. The rate of nitrogen aggregation appears to be considerably faster than that reported previously for unirradiated samples.

Spin glasses in the strong magnetic field are considered. The spin excitations in such systems appear to be localised. Their dispersion law and the density of states is obtained. Owing to the damping of the excitations the density of states is not equal to zero at zero energy and causes the linear temperature dependence of the specific heat at low temperature.

An alternative formulation of the cumulant expansion method of Frank and Mitran (see ibid., vol.10, p.2641 (1977) and Solid State Commun., vol.24, p.343 (1977)) is developed for the self-consistent calculation of the two-spin correlation function of the three-dimensional Ising model. The present technique is much simpler and more accurate since the cumulant series is summed exactly and certain other approximations made by Frank and Mitran are avoided. In addition to obtaining critical temperatures for the zero-field ferromagnet which are within 1% of the exact values, the entire phase boundary of the antiferromagnet in an external field is accurately given by the present method. Approximate critical exponents are also obtained. The present method requires considerably less numerical effort than real space renormalisation group methods and gives better results.

Recent examinations of KH₂PO₄ via inelastically scattered light have revealed a central component in the ferroelectric phase which was attributed to an inhomogeneous thermal diffusion mode. A strikingly similar effect has been observed by the authors utilising radiofrequency dielectric constant measurements. They suggest here that the same mechanism is responsible for both effects and is, instead, a homogeneous polarisation mode.

The zero-phonon line of the transition ⁴A_2g to ⁴T_2g in MgO:V²⁺ has been investigated in a magnetic field in order to clarify the origin of its splitting. A strong magnetic circular dichroism effect is observed which can be understood considering the spin-orbit interaction to be active in the ⁴T_2g state. Two of the models so far proposed have been checked against the experimental data and qualitative agreement is found in both cases.

Discusses critically the existing mean-field theories of the metastable supercooled state of a classical liquid. The authors' criticism is based on a wealth of experimental and computer simulation data which seem to be in disagreement with the mean-field theories. They propose an essentially non-mean-field model, which links the properties of the supercooled liquid to those of D_s is also discussed in view of its relation to the shear viscosity and the rate of nucleation of the new crystalline phase. In all the considerations the static characteristics of the liquid-for example, the radial correlation function-are perfectly regular and do not exhibit any singularity near the limit of stability.

A new compound FeMnGaO₄ has been synthesised and its crystal structure determined by the powder method. It is found that this compound crystallises in the cubic spinel structure with a=8.32+or-0.02 AA. The oxidation state of manganese and iron have been determined by X-ray spectroscopy to be two and three respectively. The ionic configuration of this spinel is found to be Mn²⁺(Ga³⁺Fe³⁺)IO₄^-2. The electrical resistivity-temperature behaviour obeys the relation rho = rho ₀exp( Delta E/kT). The electrical resistivity of 40 degrees C is 6*10⁸ ohm cm and the value of activation energy ( Delta E) for conduction is found to be 0.36 eV.

The adsorption and reaction of chlorine with clean cleaved (110) faces of indium phosphide crystals has been investigated by a range of electron spectroscopic techniques. For low exposures the chlorine is relatively weakly bound but for high exposure reaction with the surface leads to corrosion, the removal of some phosphorus from the surface layers, and the creation of a disordered surface. Work functions and electron affinities have been established and the behaviour of Schottky barriers formed at metal-indium phosphide interfaces are discussed in the light of these findings.

The electron momentum density is obtained, in the coherent potential approximation, for a substitutionally disordered binary alloy. The Hamiltonian of the single-band alloy is assumed to have, in the tight-binding representation, no off-diagonal disorder. The occupancies of the Bloch states, of the reference ordered Hamiltonian, are numerically evaluated for a BCC lattice with the off-diagonal part of the Hamiltonian having only nearest-neighbour-overlap terms. The results are presented for k along (100) and (111) directions.

Reports experiments to detect transient infrared absorption in the spectral range 0.1 to 2 eV following electron pulse excitation of NaCl. Specifically, a transition of the triplet self-trapped exciton predicted by theory to occur in the 0.7-1.1 eV range was sought. The experiment demonstrates the absence of any such transition, with sensitivity limits corresponding to 2% of the predicted oscillator strength. Assignment of features in the visible absorption spectra of self-trapped excitons is discussed in light of this result.

For pt.I see ibid., vol.10, p.987 (1977). A previous calculation of the exchange and correlation energy in silicon is extended to the conduction band. The authors include the self-energy operator in the electron Hamiltonian to recalculate the band structure and compare this new band structure with that from a purely local pseudopotential. A simplified method of calculating the self-energy operator is suggested.

Expressions are given for the configuration interaction contributions to the correlation crystal field in lanthanide ions. Contributions to the parameters B₀^K(LL) are calculated for the system Pr³⁺-Cl^-. Wavefunctions derived from a modified Hartree-Fock potential are used, with excited states up to n=18 and l=5. The results obtained show that configuration interaction gives reductions in the Slater parameters as well as a significant term dependence of the quadrupole crystal-field parameters, mainly due to an L-dependent radial expansion of the Pr³⁺ 4f wavefunction.

The model of Miller and Abrahams (1960) describes the effective conductivity in a system of localised electronic states as a thermally stimulated hopping conductivity between the sites. Using Liouville's theorem and applying the method of Argyres and Sigel (1974) to the physical processes involved, the author has derived a system of Boltzmann-type equations for the average occupation of the electronic states. Until now, the treatment of this transport problem with a Boltzmann formalism was only justified phenomenologically. The electrical equivalence scheme of hopping conduction already proposed by Pollak (1974) is confirmed and an improved expression for the conductance of the pairs used in the Miller and Abraham's network is given.

The electric field variation of the diffusivity in n-type silicon at 300K is studied theoretically for different impurity densities (N_i) in the range 10¹⁴ to 10¹⁸ cm^-3. Increasing N_i makes the parallel diffusion constant weakly field-dependent for high values (10¹⁷-10¹⁸ cm^-3) of N_i.

The free electron contribution to the workfunction of metals is estimated using the linearised hydrodynamic model. It is shown to be inversely proportional to square root r_s. Numerical values obtained for various metals show reasonable agreement with experimental results, as also with previous theoretical predictions based on the density functional approach, despite the simplicity of the model.

It is shown that the surface barrier potential can confine electrons in surface states, which because of the coulombic tail on the potential form a Rydberg series. This previously known result is made more rigorous by a discussion of lifetime broadening of the states. Observability of the states via LEED experiments is also investigated. The Rydberg series should give rise to structure in LEED curves which can in principle be resolved for all members of the series as n to infinity .

The AC and DC conduction properties of very thin Langmuir films are reported. Films consisting of 1 to 20 monolayers (2.5 nm to 50 nm) of fatty acid salts were deposited on to semiconducting InP as well as the more conventional metal electrodes. The authors' results indicate that the first monolayer may well have different substrate dependent properties from the rest of a multilayer assembly. An unusual log J versus V^1/4 relation has been observed in single monolayer films.

In a new theory for the spin one-half Ising ferromagnet, the fourth-order equation is used to provide expressions for the higher-order site-independent correlation functions M_2n(n=3,4,5,...) in terms of the two- and four-spin correlation functions M₂ and M₄, at the critical point. As an example, the expression for M₆ is M₄²/M₂-9M₂M₄-6M₂³. These expressions (for up to M₆₀) are applied to the calculation of T_c for the SC, BCC, and FCC lattices. The results converge to, at most, 0.755, 0.799, and 0.819 respectively, which are within ²/₃% of the accepted results 0.752, 0.794, and 0.816 of Sykes et al. (1972).

The rational values beta =⁵/₁₆, delta =5, alpha =¹/₈, nu =⁵/₈, and eta =0 of the critical exponents for the three-dimensional spin ¹/₂ Ising ferromagnet are shown to be consistent with a recently-derived linearised equation for the Ising two-spin correlation function. This equation arises from an exact integral representation which previously, on application of a straightforward approximation for higher-order cumulant averages, was shown to yield transition temperatures T_c to within 2% of their correct values for the cubic lattices. In the search for beta , the requirement is imposed that the correction terms for the magnetisation and for M₂, a quantity closely related to the system internal energy, be analytic in some power of T_c-T (which turns out to be ¹/₈) when the external magnetic field h is zero.

The Monte Carlo simulation approach is applied to the classical (S= infinity ) Ising model in a transverse field. The results are presented for the case of a 3D simple cubic lattice. Thermodynamic spin averages, correlation functions and the internal energy are considered above as well as below the transition temperature, with an emphasis on the critical regime. The comparison with predictions of various theoretical approaches is performed. In the region where the transverse field does not exceed its critical value, the longitudinal quantities are shown to exhibit an Ising-type behaviour. On the other hand, the transverse functions reveal some new qualitative features, which so far have not been well represented theoretically.

The real space block spin renormalisation method is applied to an Ising model spin glass with various symmetric distributions of nearest-neighbour interactions. For different blocks in two and three dimensions, the first-order cumulant approximation yields a phase transition with a critical exponent alpha <or approximately=-2. However, two and three block-spin cluster approximations do not yield a phase transition. The authors conclude that previous results obtained by them and by other authors on this level of approximation are inconclusive.

The authors deal with the analysis of the equivalent magnetic structures of type-II antiferromagnets (a FCC lattice with k=(¹/₂, ¹/₂, ¹/₂)). This analysis is essentially based on symmetry considerations taking into account the symmetry of the group G_(k) corresponding to a particular set of k-vectors and the symmetry group G_m of the ordered phase, which results from the direction of the Fourier component associated with each k-vector. Only a few magnetic structures are possible if two physical restrictions are imposed: the angle between k_i and m_ki, is the same for each k_i and each magnetic moment has the same value. It is concluded that MnO, NiO and FeO have a collinear magnetic structure whereas in CoO the collinear structure and the multiaxis Van Laar model are consistent with previous experiments. From neutron diffraction experiments on single crystals of CoO, under uniaxial stresses applied out of a tetragonal axis one can conclude that CoO is also a collinear antiferromagnet.

Using pulsed nuclear magnetic resonance methods the central component magnetisation recovery curves have been measured for ⁵⁹Co in a single crystal of K₃Co(CN)₆ at 295K. By using two different sets of initial conditions it has been possible to measure the parameters W₁ and W₂ which are related to the transition rates for Delta m=+or-1 and Delta m=+or-2 transitions. Previous relaxation measurements have suggested that an optical mode of the Co(CN)₆ octahedron is responsible for spin-lattice relaxation in the temperature interval 100-300K. The present value of 0.2+or-0.06 obtained for the ratio W₂/W₁, together with theoretical calculations, suggests that a band of several overlapping modes, of wavenumber approximately 412 cm^-1, is involved in relaxation.

Raman spectra of FeCl₂ and FeBr₂ have been measured above and below their Neel temperature within an energy range of 0 to 900 cm^-1. Spectral features due to scattering from phonons and electronic excitations are analysed and assigned to symmetry species. A dynamic Jahn-Teller calculation using a full-lattice model has been extended, for the first time, to describe all levels within the ⁵T_2g(⁵D) manifold and gives the best, and an accurate description of the observed electronic transitions. Justification for the use of this model comes from the good agreement between calculated and experimentally determined ground-state g values and its successful extension through a molecular-field calculation to predict the energies of transitions observed in the antiferromagnetically ordered crystal. The results of this work indicate that even though the dynamic Jahn-Teller effect is weak, it nevertheless plays an important role in FeCl₂ and FeBr₂. A weak temperature-dependent band at 127.7 cm^-1 in the spectrum of FeBr₂ is assigned to second-order scattering from a magnon-phonon combination mode.