Table of contents

A numerical calculation for a binary system similar to solid electrolyte (or superionic conductor) is performed. The validity of the theoretical treatment by Sakuma (1978) of the coherent scattering intensity for the binary system is confirmed.

The authors have succeeded in preparing flexible poly(p-phenylene) film by means of an improved method for electrochemical polymerisation of benzene. Evidence for electrochemical n-type doping of this polymer is obtained from simultaneous measurements of a cyclic voltammogram and absorption spectra. Although the polaron and/or bi-polaron levels in the gap states appear on n-type doping, the levels demonstrate an anomalous shift to the band edge. The electronic band scheme of poly(p-phenylene) film is discussed in comparison with those of other conducting polymers.

The authors report cyclotron resonance measurements on GaInAs-AlInAs heterojunctions in which two electric subbands are occupied in magnetic fields with components both perpendicular and parallel to the interface. The parallel field component increases the energy separation of the two populated subbands and hence shifts the two sets of Landau levels due to the perpendicular field relative to one another. When the two sets of levels are coincident, a splitting is seen in the cyclotron resonance due to coupling of the degenerate states by the parallel magnetic field. The magnitude of the splitting is in agreement with a simple perturbation theory calculation.

The clean and oxidised beta -Sn(101) surface has been studied over the temperature range 135-473K by AES, ELS and LEED. Unlike the Sn(001) surface, where a terrace structure was normally present, the clean surface structure was that of an ideal (101) plane of body-centred tetragonal beta -tin above and below the alpha - beta transition temperature. The oxidised surface was amorphous up to 388+or-5K, above which an Sn(101)pl*2-O oxide structure was present at sub-monolayer exposures, and an Sn(101)c⁷/₈*⁴/₅-O structure formed with rotational disorder at post-monolayer exposures. The ELS spectra from the oxidised surfaces resembled those for bulk SnO. No chemical shifts were measured in the Auger spectra from the oxidised surfaces.

Thermodynamic functions and pair correlation functions obtained from numerical solutions of the hypernetted chain (HNC) integral equations are presented for two lattice gas models in which the interaction energies are chosen as far as possible appropriately for cation vacancies and divalent cation impurities in AgCl. For the primitive model, where the interaction energies are coulombic at all separations, results have been obtained to very high defect concentrations. For this model the configurational energy, defect activity coefficient, and lattice gas compressibility are qualitatively very similar to those predicted by the analytic method of Walker and Gillan, and both thermodynamic functions and pair correlation functions are qualitatively similar in character to those of the restricted primitive model of electrolyte solutions. The results for a modified model, which incorporates more realistic estimates of the defect interaction energies at first- to third-neighbour separations, differ substantially from those for the primitive model. Results for both models suggest the existence of a spinodal boundary which it was not possible to locate in detail. The solid solutions also become metastable with respect to ordered Suzuki phase structures at high concentrations.

The authors have studied the zero temperature properties of the intermediate state of an Ising metamagnet in a simple geometry (infinite slab of thickness D). For D small they present an analytical expression for the periodicity of the stripe structure as a function of the applied field H; this allows them to obtain the asymptotic behaviour of the periodicity as H approaches the critical fields. Because of dipolar interactions, branching occurs in the domain structure above a critical thickness D_c(H). The basic asymmetry between the para- and antiferromagnetic phases yields a rich phase diagram in the (D, H) plane; they analyse the nature and boundaries of the first few branched patterns in that plane; these agree with their previous treatment of Ising dipolar ferromagnets. Finite temperature effects are briefly discussed.

The specific heats of Tb₂O₃ and Tb₄O₇ have been measured from 0.5 to 22K. Maxima corresponding to reported antiferromagnetic transitions are found at 2.42K and 7.85K respectively. Thermodynamic analysis of the results for Tb₂O₃ shows that the crystal-field ground state of the terbium ions on C₃₁ sites is a singlet, and that on C₂ sites is a 'doublet' with (J_z) approximately=+or-6.

A continuum theory is employed for investigating the longitudinal optical (LO) modes in polar semiconductor heterostructures. Particular emphasis is laid on the symmetric double heterostructure (DHS) such as occurs in a semiconductor quantum well. The existence of two-dimensional (2D) bulk-type double-interface-type and guided-type LO modes is examined for this case and their characteristic dispersion relations derived. It is shown with reference to a typical GaAs quantum well that the presence of at most two double-interface modes and a finite number of guided LO modes depends on the difference between the squares of the limiting bulk LO frequencies of the two materials. The implications of the results for light scattering experiments and for the properties of electrons confined in quantum wells are pointed out and discussed.

A matrix reduction method is developed for calculating the normal mode frequencies of the finite-layered structures formed by an arbitrary number of thin films of different atoms. The simple cubic lattice with the nearest-neighbour central and shear force interactions is used for explicit formulation. The usefulness of the resulting expressions is demonstrated for bicrystal and tricrystal structures.

The normal-mode frequencies of a finite simple-cubic Rosenstock-Newell bicrystal with a coherent planar interface are evaluated and analysed as functions of the relative thickness of the component subcrystals using three different general boundary conditions. The calculation was performed using the matrix reduction formalism developed for finite-layered structures. The result obtained includes details of earlier work based on a different method as a special case. It also provides some additional insights into the characteristics of vibrational modes in a finite bicrystal, such as the existence condition for a 'localised-localised' (LL) interface mode.

The authors describe an extended recursion method that allows direct calculation of the average density of states and that possesses the numerical stability of the recursion scheme. They show the relation between the extended recursion and the moments methods.

A decimation approach to the electronic properties of three-dimensional systems with both strong electron-electron interactions and randomness is developed and applied to the Hubbard model with disorder. To probe the nature of many-body eigenstates, the authors have calculated the energy gap, localisation of states, magnitude of local spin moments as well as low-lying excitation spectra from which the phase diagram is discussed.

Calculations are presented for the collective plasmon modes of a superlattice consisting of two-dimensional electron gas layers separated by media of alternating thickness and/or dielectric constant. The theory is developed for bulk plasmon modes in an infinite superlattice, and for bulk and surface modes in a semi-infinite superlattice. Applications are made to layered electron gases in semiconductor heterojunctions (such as GaAs/Al_xGa_1-xAs), and in particular to light scattering from the plasmons.

Analytical, three-dimensional Wannier functions have been constructed from Bloch states formed from a lattice of gaussians. By considering a natural extension of the three-dimensional Brillouin zone to a six-dimensional complex space, the authors are able to investigate the localisation of the functions. The localisation and asymptotic behaviour are determined by examining the zeros of the Fourier transform of the atomic overlap matrix. They discuss in detail two systems, based on the simple-cubic and body-centred-cubic lattices respectively. They find both cases exhibit faster than exponential localisation. It is shown that the localisation of the three-dimensional functions depends on the angle of the asymptotic direction relative to the host lattice as well as on an exponential of the radial distance. A simple interpretation is developed: three-dimensional Wannier functions are least localised in the direction of the nearest perturbing neighbour atoms. Wannier functions from a one-dimensional lattice of gaussians have been introduced earlier by Wannier. The authors show analytically they decay asymptotically like mod x mod ^-1/2 exp(- pi phi mod x mod /4), where phi is the gaussian localisation parameter, and also that they oscillate in phase with the host lattice but are phase-shifted away from the atomic positions except for the zeroth site. They show that the behaviour in three dimensions is similar.

A study of the electronic transport in MOS transistor inversion layers is presented. The use of the Kubo-Greenwood formalism allows the possibility of explaining the global behaviour of the transport coefficients (conductivity, mobility, etc.) from the weak inversion regime (non-degenerate) to the strong inversion regime (degenerate) over a wide range of temperature. In particular, it is shown that physical phenomena arising from miniaturisation and/or low-temperature operation such as quantisation of the inversion layer or carrier localisation are of primary importance for the determination of the transport coefficients of MOS transistor inversion layers. The incorporation of such phenomena into the modelling of the operation of MOS devices, is thus highly desirable, even at temperatures close to room temperature.

An expression is derived for the transfer Hamiltonian matrix element that takes account of the image potential and allows the tunnelling system to be separated into two coupled subsystems, despite the long-range image potential in the tunnel barrier. It is shown that Bardeen's expression for the transfer Hamiltonian matrix element is not correct if an image potential is present in the tunnel barrier and that his matrix element does not conserve current if the Hamiltonians used to calculate the eigenstates of the two electrode-barrier subsystems include the image potential. The matrix element derived in this paper is applied to a simple one-dimensional barrier to show that the matrix element is current conserving and that it correctly describes the increase in tunnel current which results from the image potential lowering of the tunnel barrier.

Room temperature EPR spectra of the Cr(CN)₆^3- ion substituting for KCl₆^5- in single crystals of KCl show evidence for two different orthorhombic centres. The first of these shows an unexpected reduction in the number of spectral lines. This is due to the simultaneous occurrence of a crystallographic equivalence of the principal y and z axes of the spin Hamiltonian along the (110) directions, and the unusual value for the ratio E/D (where D and E represent axial and rhombic terms of the crystal field) of exactly ¹/₃. The second centre has E/D approximately=¹/₃ and a similar arrangement of magnetic axes, except that the y and z axes are at 45 degrees to (100) in the (110) planes. Models for the two centres are discussed in terms of different configurations of two charge compensating K⁺ vacancies.

The room-temperature paramagnetic and 4.2K Zeeman ⁵⁷Fe Mossbauer spectra are reported and analysed for both crystalline (c-) and amorphous (a-) NaFeF₄. The results point strongly to the presence, in a-NaFeF₄, of distorted FeF₆ octahedra which are locally coordinated to (predominantly) four other octahedra via a linkage in which the two non-bridged, or terminal, fluorine bonds are in a trans-configuration ( approximately=180 degrees apart). This locally planar magnetic structure is a distorted form of that which occurs in c-KFeF₄ and c-RbFeF₄, but is different from that in c-NaFeF₄ for which the smaller sized alkali-metal ions induce a puckering of the quadratic layer iron sublattice via a cis-terminal ( approximately=90 degrees ) coordination configuration of the FeF₄ octahedra. It is concluded that the crystalline constraint which produces this unusual cis-coordination in c-NaFeF₄ is sufficiently relaxed in the amorphous state that the more common trans-coordination is restored. Amorphous NaFeF₄ should therefore be pictured as a continuous random network of predominantly four-coordinated trans-terminal FeF₆ octahedra with sodium ions filling the holes. Since 4.2K is more than one third of the spin-freezing temperature ( approximately=12K) of a-NaFeF₄, significant effects relating to thermally induced spin fluctuations are observed in the amorphous spectrum at liquid helium temperature, and are interpreted self-consistently within the framework of the basic model.

The authors introduce a method to investigate experimentally the relative populations of spin-orbit-split states in the ground state of a material. The method uses multiplet structure in X-ray absorption spectra and selection rules limit the subset of terms that can be reached in a given final-state configuration. This subset is dependent on the precise details of the ground state. A test of this method on Ce materials is presented. The results are consistent with an increase in the ratio of 4f₇2/¹ to 4f₅2/¹ contributions in the ground state with increasing hybridization between the 4f levels and the conduction states.