Table of contents

A classical model that qualitatively describes the pressure-induced off-centre to on-centre transition of an impurity ion such as Ag⁺ in RbCl is considered. The range of pressures over which the transition takes place in the model is comparable to that observed experimentally. Possible extensions of this model to glasses are noted.

Four bound-state resonances have been observed using He scattering from a Pd (110) surface. The bound-state energies can be fitted with good accuracy to a Morse potential with depth D=-8.05 meV and range parameter sigma =0.85 AA^-1. The shape of the potential shows that the repulsive part is appreciably 'softer' for metals than for alkali halides and graphite. The effective corrugation amplitude perpendicular to the close-packed metal rows z(01)=0.22-0.24 AA is similar to Ag (110) and appreciably larger than for Ni (110) and Cu (110).

It is pointed out that a recent theory by Chattopadhyay and Ghosal (1983) which concludes that intrinsic NDR in quantum wells does not occur is logically indefensible.

When InSb is illuminated by CO₂ laser light the absorption of two or more photons in causing transitions from the light-hole to the heavy-hole bands can create light-holes with sufficient kinetic energy to ionise by impact. The feasibility of this as a method of producing heavy-hole multiplication is investigated. It is found that at 77K the two-phonon process will give multiplication when the light intensity reaches approximately 2.3 MW cm^-2 which is the value observed experimentally for the onset of opacity.

On the basis of the Bethe ansatz solution of the degenerate Anderson model the magnetic susceptibility and orbital occupation number of cerium or ytterbium impurities in a metal are calculated in an explicit form.

It is shown that the low-temperature phase of the three-dimensional (3D) random-anisotropy planar magnet is of the spin glass type. This phase is characterised by the finite correlation length (large for small random fields) and finite spin stiffness which coincides with that of the ferromagnetic phase of the pure magnet. The critical region near the phase transition point is investigated (except the closed vicinity) and the critical exponents of the static susceptibility in zero external field is found for different types of anisotropy.

Numerical finite-cell methods are used to estimate the ground state energy and the out-of-plane spin fluctuation at T=0 and a finite T, for a one-dimensional S=1 ferromagnet with easy-plane anisotropy. For parameters appropriate to CsNiF₃ the magnitude of the spin-fluctuation is such as to cast doubts on the validity of the sine-Gordon approximation for this system.

It is shown by using a field-theory model for an Ising spin glass that the mean-field equation for the de Almeida-Thouless line, h² approximately (T_c(0)-T_c(h))³, breaks down below eight dimensions. It is argued that h² approximately (T_c(0)-T_c(h))^d2-1/, for 6<d<8.

By Monte Carlo simulations the phase transition is investigated for bond- and site-disordered Ising ferromagnets with special emphasis to the temperature dependence of the paramagnetic susceptibility chi ₀(T) outside the real critical temperature range. For site-disordered systems a large temperature range of deviations from the Curie-Weiss law and a non-monotonic temperature dependence of the effective exponent gamma (T)=(T-T_c) chi ₀(d_{chi 0}^-1/dT) was found, in close analogy with the experimentally observed behaviour of amorphous Heisenberg ferromagnets. Bond disorder affects the critical temperature T_c but has little influence on the temperature dependence of chi ₀(T).

The authors review the use of the ideas of 'ionicity' and 'covalency' in quantitative studies of the solid state. Such ideas have taken several distinct forms: a description of charge distributions of bonding, a spectroscopic classification of perfect solids, and a framework for quantitative modelling of defects in polar solids. The limits on the usefulness of ionic pictures clearly depend on which form of idea and which application is involved. Nevertheless, many misunderstandings persist through failing to recognise that unique, universal, charges cannot be assigned except in trivial cases. Further confusion arises from the temptation to ascribe almost any unexpected features of ionic crystals to covalence. They discuss in particular the use of ionicity in the quantitative modelling of polar solids. Here, it appears, the ionic picture provides a framework for defect models and theories which, properly used, covers both the traditional 'ionic' crystals and ones, like silicates, where appreciable covalency is acknowledged.

The break-collapse method recently introduced for the q-state Potts model is adapted for resistor networks. This method greatly simplifies the calculation of the conductance of an arbitrary two-terminal d-dimensional array of conductances, obviating the use of either Kirchhoff's laws or the star-triangle transformation. In addition, a real-space renormalisation group based on a new type of averaging gives excellent results for the conductivity of the random-resistor network on the square lattice.

For pt.I see ibid., vol.15, p.6991 (1982). The authors have derived the phonon relaxation rate for anisotropic mass cases in heavily doped many-valley semiconductors by solving the equations of motion for the single-particle density matrices within the self-consistent field and the relaxation time approximations. Explicit expressions have been given for longitudinal and transverse waves propagating along the principal directions in Ge and Si. Numerical calculations have been performed for n-type Ge at T=0K. The following have been found. The effect of the mass anisotropy is remarkable in the high-frequency region, while in the low-frequency region it becomes negligibly small. There exist two cut-off wavenumbers in the electron-phonon interaction for the (110) and (111) longitudinal mode is qualitatively the same as in the isotropic mass case. A discussion of the recent work on phonon absorption due to the electron-hole liquid (EHL) in Ge by Dietsche et al. (1982) is also given.

A systematic study of the possible phase symmetries derived from a prototype phase with the space group Fm3m (O_h⁵) has been performed. These structural phase transitions involve the onset of an order parameter transforming according to one irreducible representation of the prototype space group. The investigation is performed on the basis of the Landau theory for continuous transitions. Accordingly, the irreducible representations of Fm3m are examined at the Gamma ,X,L and W high-symmetry points of the cubic face-centered Brillouin zone. The order parameters relevant to all the arms of a given star are considered. The results are summarised in several tables. These results are discussed and compared with the available experimental data relative to Fm3m subgroups.

Investigates the propagation of quasiparticles in inhomogeneous samples of ³He-A, at temperatures low enough that the quasiparticle mean free path is long compared with the scale of textural variation. Under such conditions a quasiparticle may be reflected from a texture by conversion into a quasihole, a phenomenon analogous to the Andreev reflection well known in superconductors. The authors discuss various experiments which should illustrate this effect, and show that at least one of them should be feasible with current cryogenics.

A one-dimensional system with strong on-site electronic interactions and a local coupling to intracell vibrations is considered. Ground-state properties and the zero-temperature phase diagram are discussed. It is shown that localised interactions favour the coexistence of a static CDW and attractive pairing effects, especially at high phonon frequencies.

Expressions for the elements G_pq of the spin-strain coupling tensor have been derived for ruby (Al₂O₃:Cr³⁺) in terms of the superposition-model parameters describing the axially symmetric field of each ligand. Four intrinsic parameters, A₂, A₄ and their radial derivatives, provide a reasonable fit to the ten G_pq but the zero-field splitting D cannot be fitted and must be largely due to other mechanisms. The parameters obtained are used to estimate the elements of the G tensor in MgO:Cr³⁺ and these are compared with measured values.

Experimental evidence is presented for a ferromagnetic ordering with domains of the proton spins in Ca(OH)₂. This ordering was produced by dynamic nuclear polarisation (DNP) and adiabatic demagnetisation in the rotating frame (ADRF) and occurred at negative spin temperature. The transverse and longitudinal susceptibilities were measured and were in agreement with theoretical predictions for a ferromagnetic ordering with domain structure. Further evidence for such a type of ordering is contained in the relaxation behaviour of the dipole energy after ADRF. The observed relaxation behaviour was found to be due to spin diffusion between the ferromagnetic domains. From these relaxation data the size of the domains could be calculated.

The use of a compound Weibull distribution in both time and electric field for the description of dielectric breakdown data is examined. It is shown that it is insufficient to measure breakdown experimentally using only a single variable such as a constant electric field, since the evaluation of the minimum number of parameters required to characterise the simplest system requires four independent pieces of information. Applicable statistical relationships are derived and applied to a wide range of experimental data. It is shown that, in general, the information that has been published on dielectric breakdown is limited and does not allow full assessment of the breakdown mechanism. Such information as is available is discussed and some broad conclusions about breakdown mechanisms are proposed.

The broadening of quasi-atomic lines in the CVV and CC'V Auger spectra of solids is discussed and a new, resonant mechanism is proposed. Explicit expressions are derived that allow this resonant contribution to be computed directly from XPS and Auger spectra. The results are compared with outer contributions arising from lifetime, dispersion and phonon broadening, with special reference to a series of transition metals. When certain conditions are met, as in the case of silver, the resonant broadening makes up a substantial contribution to the experimental linewidths.

The authors present a study on the conductivity, sigma , and thermopower S, obtained at low temperature for degenerate silicon-on-sapphire (SOS) samples. They show that the anomalous variations of both sigma and S are well interpreted by assuming a mobility edge in the band structure of the degenerate samples. They analyse theoretically the influence of the mobility edge on the conductivity and thermopower variations versus temperature. Two conduction mechanisms are present relating to carriers above and below the mobility edge. They deduce from the analysis and the experimental data for SOS the contribution to sigma and S from conduction mechanism and the relation between the Fermi level E_F and the temperature T. They also obtain the approximate form of the density of states, N(E), around the mobility edge.

The authors consider a plasma of point ions in the presence of a non-uniform neutralising background. This background, the source of an external field, may have some of its parameters (density, form of surface profile, etc.) modified, as long as the total charge is maintained. By considering such modifications in the context of the density-functional formalism for the ions, they prove sum rules giving the first and second moments of the ion density rho (z) in terms of other properties (bulk pressure and temperature derivative of surface tension). The Poisson-Boltzmann functional is considered in detail. They show that the first and second moment conditions on rho (z) are verified. They calculate rho (z) exactly for this system, and also perform variational calculations: comparison shows the importance of respecting the asymptotic behavior of rho (z). Variational calculations have been performed, using the density-functional formalism in the square-gradient approximation for systems with plasma parameter Gamma from 1 to 10. For Gamma >3, important oscillations appear in the profile, as shown by recent Monte Carlo calculations. The profiles calculated variationally also show increasing oscillations, but are not in good agreement with the Monte Carlo results. The surface energies are poor even for Gamma =1 showing the inadequacy of the square-gradient expansion for this system.

A microscopic model for Kr physisorbed on graphite is studied by the molecular dynamics technique. The model contains a Lennard-Jones potential describing the interaction among the Kr atoms and an external potential with sixfold symmetry describing the interaction between the Kr atoms and the graphite surface. The number of Kr atoms and the area of the surface are chosen to correspond to a five per cent overfilling of the perfect Kr monolayer. The Kr atoms are confined to move in a plane implying that second-layer promotion is inhibited in this study. Most simulations are performed with rectangular lattices of dimensions up to 170.4 AA*147.6 AA with periodic boundary conditions. A few simulations have also been performed with a circular lattice with reflecting walls in order to study the effects of the boundary conditions on the domain structures. The structure factors indicate two phases for the model: a low-temperature incommensurate phase and a high-temperature fluid phase. The transition between the two phases proceeds via a first-order transition with coexistence of the two phases in a temperature interval. No commensurate phase is seen in the simulation, in contrast to the experiments.