Table of contents

Recently there has been proposed a modification of the spectral density method (SDM) to take into account damping effects for Fermi systems. There are in any case some difficulties concerning Bose and classical systems. The authors suggest a simple way of avoiding such difficulties and preserving the earliest idea for discussing damping effects. A classical ferromagnetic linear chain in external magnetic field is studied.

The s-state Potts model with a gaussian distribution of pair couplings is found to possess, in the infinite-range limit, a replica symmetric (RS) solution that exhibits a spin glass transition which appears to be second order for s<6 and first order above. The solution is not stable in the spin glass phase, and is only stable throughout the paramagnetic (PM) phase for s<or=2. For s>2, the stability of the PM phase breaks down above the transition temperature indicated by the RS solution suggesting a first-order transition which, however, cannot be located by the RS solution.

The authors present results of a neutron scattering study of the fast oxide ion conductor delta Bi₂O₃; both pure and Y³⁺-doped materials are studied. The authors' results reveal extensive disorder with oxygen distributed over lattice and interstitial sites.

The author reports a new material constant in amorphous semiconductors. This quantity is the sum of the optical band gap and the disorder energy, E₀=E_og+1/ beta . The band-gap fluctuation model of amorphous semiconductors predicts that E₀, so defined, should be a constant; the author finds that experimental results in the literature yield a constant E₀ in a-Si and in a-Se it is very nearly constant.

Magnetophonon resonance results are reported for two-dimensional electron gases confined in the GaInAs layers of GaInAs-InP superlattices. Two series of oscillations are observed: one due to scattering by the 'GaAs-like' LO phonon mode of GaInAs, and the second due to interaction with InP LO phonons. The strength of the latter series increases relative to the former as the GaInAs layer thickness is reduced. This is evidence for a long-range phonon interaction, with the InP phonon field extending into the GaInAs to couple significantly with the electrons bound in the quantum wells. No evidence of interface phonons is seen.

For pt.I see ibid., vol.16, p.L573 (1983). Reports magnetophonon oscillations in the two-dimensional electron gas formed in GaInAs at GaInAs-AlInAs heterojunctions. One series of oscillations is seen, corresponding to the energy of the degenerate 'InAs-like' LO phonon modes in GaInAs and AlInAs. There is also an interface phonon mode at the same energy. In bulk GaAInAs scattering is dominated by the 'GaAs-like' mode, and so as long-range phonon interaction between the electrons in the GaInAs and the interface and/or AlInAs 'InAs-like modes must be invoked to explain the dominance of scattering by these modes. Comparing the results with those for GaInAs-InP superlattices, it can be seen that changing the electrically inactive component of the heterostructure produces a radical change in the electron-phonon interactions.

Some critical comments on a recent paper by Nakamura and Sasada (see ibid., vol.15, p.L915, 1982) are presented. Gauge equivalence between anisotropic and isotropic continuous Heisenberg spin chains (CHSE C) is established, which allows one to find the correct equivalence of easy plane CHSC. Repulsive-type NLSE is shown to be gauge equivalent to SU(1,1) isotropic CHSE or its equivalent corresponding anisotropic model.

The authors study the thermodynamics of a 2D electron system in perpendicular magnetic field H. It is shown that, for a wide class of interaction and external potentials (including irregular ones), the grand partition function takes the form of a classical partition function as H to infinity . Some consequences of this theorem are also discussed.

Molecular dynamics calculations have been carried out for the orientationally disordered phases of solid potassium cyanide. It has been found that the properties of the simulated crystal are very sensitive to the assumed charge distribution of the cyanide ion and that there is a strong correlation between the static and dynamic consequences of assumptions about the form of the interionic potential. Results are reported for the orientational distribution of the cyanide ions and for time correlation functions relevant to polarised Raman and inelastic neutron scattering experiments. The anomalous dispersion characteristic of transverse acoustic phonons in phase I is shown to be reproducible, but no single model has been found that will account for all the existing experimental data.

Malononitrile undergoes a series of 're-entrant' phase transitions from a low-temperature monoclinic phase to an intermediate triclinic phase and, finally, to a high-temperature monoclinic phase whose structure is nearly identical to that of the low-temperature phase. A model is suggested for this system which is based on the interaction between the order parameter associated with the transitions and strains in the unit cell caused by thermal expansion. The energy is calculated as a function of temperature on the basis of this model, using atom-atom '6-exp' potentials along with long-range dipole-dipole forces. The model produces values of the coupling coefficients that agree satisfactorily with values derived from the experimental data and predicts re-entrant behaviour quite similar to that observed experimentally.

The incommensurate phases of K₂SeO₄-type crystals show thermal hysteresis of dielectric and structural parameters, which has been ascribed to pinning of the modulation. From the temperature dependence of the permittivity, discontinuous processes of the moving lattice of discommensurations are derived and explained by the creation of annihilation of single discommensurations. An analysis of the dielectric measurements demonstrates that the modulation lattice is clearly pinned within the range of the thermal hysteresis and that the coherence length of the modulation parallel to its wavevector must be very small compared with its value in directions perpendicular to it. Furthermore the patterns of discommensurations are stabilised by a defect-induced after-effect and also nominally pure crystals of Rb₂ZnCl₄ show memory effects in the incommensurate state.

The influence of the external electric field on ferroelectrics exhibiting the incommensurate-commensurate phase transition is investigated within the Landau-type theory and the constant-amplitude approximation. The temperature variation of the modulation period, the polarisation and the dielectric susceptibility are calculated. It is shown that finite fields preserve the character of the transition, but lead to a shift of the transition temperature. A comparison with experiments on Rb₂ZnCl is presented. The possibility of a field-induced Lifshitz point in incommensurate ferroelectrics is discussed.

It is argued that an earlier calculation by Hille (see ibid., vol.14, p.4027, 1981) for the force between a vortex and the roton gas in He II was incomplete in that the emission of rotons by the vortex core was ignored. It turns out that the values previously deduced for the force parameters C and C' were in fact values of D and D'. The corrected calculation is in reasonable agreement with the mutual friction data for T<or=2.0K but deteriorates nearer T_lambda . The agreement is better when the emission of rotons is ignored.

The authors employ a functional integral approach to the Coqblin-Schreiffer Hamiltonian, similar to that of Yoshimori and Sakurai (YS-1970) for the Kondo Hamiltonian, except that a field lambda enforcing the constraint n_f=1 is introduced. By a gauge transformation they show that the phase of the complex sigma field introduced by YS may be absorbed into lambda , leading to a new two-field formulation in terms of lambda and s= mod sigma mod . The static approximation leads to a simple Friedel resonance on the impurity of width approximately T_K/N, where N=2J+1 is the number of channels, and whose position is determined by the Friedel sum rule, as in the 'local Fermi liquid' theory. The authors show that the charge susceptibility is zero, is required. Gaussian fluctuation corrections to the static approximation are determined' taking account of fermion propagator renormalisation they then find that the Wilson chi / gamma ratio R=N/(N-1) is correct to order 1/N². The value of T_K is that of a lowest-order renormalisation group treatment, but may be corrected by fluctuation effects.

The many-body effect on the interband optical absorption strength of alkali metals is re-examined on the basis of the Hopefield formula. An expression for the dielectric function of an electron liquid which the authors have obtained in recent work is used in the formula. The calculated absorption strength for sodium includes two effects important in the metallic density region, short-range correlation and a coupling between one-pair and multi-pair excitations. It is found that the enhancement of the optical absorption strength due to electron-electron interactions is much larger than the previous estimate. A deviation from the spectral shape given by the Wilson-Butcher formula is discussed.

For pt.I see ibid., vol.15, p.1663 (1982). The Green function formalism of Haley and Erdos and of Bak is used to compute the excitation curves epsilon (q) versus q for the Van Vleck enhanced nuclear antiferromagnet HoVO₄ in the ordered state. Calculations are presented and discussed for both the linear and puckered antiferromagnetic spin structures. In particular, it is shown that HoVO₄ is characterised by a vanishing energy gap, below the Neel temperature(4.8 mK), in accord with the Goldstone theorem for a continuously broken symmetry. Other features such as (i) magnon/single-ion excited-state interactions and (ii) the longitudinal-transverse splitting at q=0 associated with the dipole-dipole interactions are identified and discussed. Particular attention is paid to the symmetry properties of the enhanced dipole-dipole interaction F_ij and its Fourier transform f(q) for the HoVO₄ lattice. These symmetries are used not only to reduce the calculations to a minimum but also to identify crossing and anti-crossing behaviour between the various excitation branches. Finally some comments are made concerning the possibility of NMR and neutron scattering experiments in the nuclear ordered phase.

The dispersion of photoelastic and elasto-optical coefficients of GaAs in the infrared region of the spectrum is calculated. The basic physical parameters entering the calculation correspond to the strain derivatives of the transverse phonon effective charge and of the transverse and longitudinal optical phonon frequencies. They are all deduced from existing piezospectroscopic data. The calculation extends to other physical parameters such as the pressure derivatives of the effective charge and index of refraction. The dispersion of the integrated intensity for ionic Brillouin scattering is also computed. Both the linewidth of the transverse optical phonon and its strain dependence are taken into account explicitly. They are shown to have a substantial effect on the profile of the dispersions.

Phase transitions have been observed in mercury (II) cyanide near 2.5, 89, 19 and 60 kbar. The I/II transition appears to be of second order with the structure of phase II closely related to that of the parent molecular type. Using Landau theory, three possible structures have been identified for the new phase II, of which S²₄ is marginally better in accord with the evidence. A major reconstruction of the lattice occurs at the II/III transition; the coordination number of mercury appears to increase to four, and the structure becomes covalently bonded, rather than molecular. Phase IV has a spectrum consistent with it having the cubic anti-cuprite structure adopted at STP by Zn(CN)₂ and Cd(CN)₂. Above 60 kbar, brown polymeric Hg(CN)₂V is formed irreversibly. It has a single IR absorption <4000 cm^-1 at 1273 cm^-1, attributed to C=N bonds.

The authors study the effects of a thermal annealing at 500 degrees C on the spectroscopic properties of the Er³⁺ and Eu³⁺ ions in CdF₂, in order to progress in the way of an accurate microscopic interpretation of the conversion mechanism of CdF₂ to the semiconducting state. Whereas the annealing mainly quenches the Er³⁺ fluorescence by energy transfer from the 4f states to the donor level, such a quenching does not occur with Eu³⁺. An analysis of the Eu³⁺ redistribution among the different types of site, and peculiarly the minor (Eu³⁺-F_i^-) centre, allows the authors to specify their structure and the source of fluoride interstitials available for the conversion process.

A simple analytic expression for the ionisation coefficient for impact ionisation is derived on the basis of a new approach which exploits the difference between momentum- and energy-relaxation rates for hot electrons. The basic mechanism whereby an electron gains sufficient energy to ionise is lucky to drift in which the electrons relax momentum but not energy. The electrons gain energy by drift and not by ballistic motion, and a few lucky ones reach the threshold. Those which thermalise may also contribute through the lucky-drift mechanism, starting from the average hot-electron energy. Good agreement with Baraff's theory is obtained. It is shown that neither the Schockley lucky electron nor the Wolff thermalised electron contribute significantly, in agreement with Baraff. However, the concept of Schockley's lucky electron is an essential part of the lucky-drift mechanism. The theory is simply extended to accommodate electrons injected at energies above zero, and some calculations are presented on this topic. A discussion is given of the effect of real band structure and it is concluded that the theory based on parabolic bands remains good provided the mean free path is taken as an average quantity over the relevant energy range. It is argued that the theory has wide application to semiconductors with moderate-to-large energy gaps because of the predominance of nonpolar scattering at high energies. A specific model of a nonparabolic band structure is discussed in which the electron distribution function has a Gaussian form, rather than the Maxwellian form associated with parabolic bands, and a weak negative differential resistance is exhibited.

The authors calculate the binding energy, the equilibrium bond length, the surface diffusion activation energy at 0K and the stretch vibration frequency of atoms with a partially filled p valence shell (N, O, F) adsorbed on BCC transition-metal surfaces. The binding energy is split into three contributions: a band term which is treated in the tight-binding approximation, and electronic correlation part obtained from a perturbation theory using the Hubbard model and a pairwise repulsive potential of the Born-Mayer type. The binding energy is minimised with respect to the position of the adsorbate keeping neutral all atoms involved in the bonding, although charge-transfer effects are considered. In agreement with experiments, the authors find that the binding energy decreases with the substrate band filling as a result of the progressive filling of the antibonding state of the 'surface molecule', formed by the adatom and its substrate nearest neighbours. This trend is also followed by the surface diffusion activation energy at 0K and the stretch vibration frequency. Consequences on heterogeneous catalysis are discussed.