Table of contents

An account is given of transmission electron microscope studies of the crystal structure of Y₂Cu₂O₅ discovered within samples of YBa₂Cu₃O_7-x and Y_1.85Sr_0.15CuO_{4- delta} ceramic compounds. Conventional and high-resolution electron microscope investigations confirmed that Y₂Cu₂O₅ is isostructural with Ho₂Cu₂O₅ and has a space group P2₁nb with lattice parameters a=1.247 nm, b=1.081 nm and c=0.3495 nm. The nature of planar defects discovered within the crystallites of Y₂Cu₂O₅ is discussed. It is proposed that small amounts of barium that do not react to form a solid solution with Y₂Cu₂O₅ segregate to the lines of defects. The displacement vector for the planar defects was found to be ¹/₂(014).

The phonon free energy is calculated for several silicon carbide polytypes using a valence overlap shell model. The three most commonly observed polytypes, i.e. (2), (23) and (3), are shown to be thermodynamically stable, with (2) and (3) as the low- and high-temperature phases, respectively, and (23) in between. The long-ranged phonon effect splits the multiphase degeneracy between phases (2) and (3) to which SiC is extremely close at T=0 K, and its stabilises (23) with a significantly large stability region of 500 K. The calculations of the interatomic displacement-displacement correlations demonstrate how the long-ranged interatomic interactions arise from the phonons. The formation theory of longer-period polytypes is also discussed.

The relaxed structures of the (1), (2), (23) and (3) polytypes of silicon carbide are calculated using the pseudopotential total-energy technique with norm-conserving pseudopotentials and the local density approximation to the exchange-correlation energy. A 'tension model' is proposed to account for the atomic forces and stresses of the ideal structures and the results of the detailed relaxed structures. The authors also deduce the force field due to an isolated antiphase boundary from the calculated atomic forces of the ideal structures. The energies associated with these relaxations are about 1 meV per SiC pair of atoms per antiphase boundary. In order to calculate it, they have developed a new formulation, which should be of wider use in calculating relaxation energies. They discuss the different effects of longitudinal and transverse relaxations on the stability of the polytypes, particularly (23) as a possible intermediate phase between (2) and (3).

A self-consistent formulation for the force field and the associated dipole force tensor is presented. In the tight-binding approximation of the Hartree-Fock scheme, the variation of the total energy of the solids-up to first order in the atomic displacement-is obtained in terms of neutral quantities with respect to the frozen crystal. Improvements from previous estimations concerning dilute substitutional and interstitial alloys are reported. Also, the authors present-for the first time-a derivation of the force field arising from the presence of the surface of a transition metal.

The electronic band structures of the lattice mismatched InAs/GaAs system has been calculated for three different lattice constants using ab initio pseudopotentials. The effect of strain on the band discontinuities has been investigated with respect to the interface interlayer separation as well as the strain. The superlattice electronic structure is examined within the context of interface states and charge localisation.

The Car-Parrinello molecular dynamics-density functional scheme derives reliable forces from the energy functional of Kohn and Sham. The authors describe an alternative approach based on a related energy functional that can be assigned a value for any density, specifically a superposition of site densities moving with the nuclear sites. 'In-out' fluctuations of the site densities are described by parameters that are treated as dynamical variables in a Lagrangian formulation. An important feature of the scheme is its adaptability with respect to the method used to solve the electronic structure part of the problem. Explicit calculations for the H₂ molecule illustrate how the scheme can be used with the electrons described in terms of a localised orbital basis.

The total decay rate of the ⁶I_7/2 term level of Gd³⁺ in YOCl and LiYF₄ is studied as a function of temperature. This rate is described as a termal average of the difference contributions of the crystal field components. The difference between the rates in YOCl and LiYF₄ is discussed.

The energy levels of a double-quantum-well system can be expected to occur in closely spaced pairs, due to 'tunnelling', so raising the possibility that oscillations can occur at the tunnelling frequencies. This paper gives a detailed theory of the excitation of such a pair by an incident electron wavepacket, using a time-dependent formalism and a one-dimensional system of barriers. It is confirmed that transient oscillations can be expected to occur in currents and charge densities near quasi-resonances. It is further reasoned that to increase the temporal coherence of an incident pulse, it is desirable to have inelastic scattering into a quasi-bound state.

The longitudinal magnetoresistance has been measured in the spin-glass and reentrant spin-glass states of Au-14 at.%Fe and amorphous (Fe_1-xNi_x)₇₇Si₁₀B₁₃ alloys by a new technique which involves applied fields of less than two gauss. The measurements have concentrated on the paramagnetic-ferromagnetic transition where in both crystalline and amorphous systems a curious negative magnetoresistance has been found over a small temperature range, which rapidly changes sign as the ferromagnetic state becomes established with decrease of temperature. From measurements of the field dependence of the magnetoresistance near the Curie temperature, the negative contribution, as well as the positive contribution, is found to continue to develop into the ferromagnetic state. The model used by Beck (1985), modified with the addition of a pin-glass type background matrix, is proposed from the study for the ferromagnetic state of these alloys.

Infrared reflectivity spectra of the reaction products K_xMoO₃ prepared by a solid state reaction ((x/2)K₂MoO₄+(1-x)MoO₃+(x/2)MoO₂ to K_xMoO₃; x=0.1-0.5) have been measured over the wavenumbers 400-4000 cm^-1 at room temperature by means of Fourier transform infrared (FT-IR) microscopy. Although X-ray diffractometry reveals that all reaction products are not of single phase, it turned out that three phonon peaks appear around 950 cm^-1 in the spectra, and some spectral changes are observed depending on the potassium composition of the predominant phase in the reaction products which is determined by an EDAXSW 9100 spectrometer. The FT-IR microscope also confirms the 'phonon stiffening' in the transition from the 'blue bronze' (K_0.3MoO₃) to the 'red bronze' (K_0.33MoO₃), indicating a discontinuity in the plot of the wavenumbers for the three phonon peaks as a function of potassium composition. It is concluded that such a discontinuity is caused by a possibility that the Mo-O bond distances are shorter in K_0.33MoO₃ than in K_0.3MoO₃.

Absorption spectra are reported for Se adsorbed in faujasite (X or Y) as a function of x, where x is the Al-to-(Al+Si) ratio. The amount of adsorbed Se increases with increasing x for small x and is saturated for x>or approximately=0.1. The results are interpreted using the model in which the finite number of Se atoms are physically adsorbed by each permanent dipole which consists of (AlO₄)^- in a zeolite framework and cation. The energy of the lower absorption edge changes with x and the spectral shapes are classified into three types. Spectral differences are also observed for Se incorporated into Linde type A and Linde type L lattices. From a comparison with the spectrum of the isolated Se chain in mordenite, the structures of Se in A and L are expected to be isolated clusters and double parallel chains, respectively.

A low temperature spectrum with periodically repeated phonon structures, induced by the localised vibration model in nitrogen doped GaP has been observed. Experiments and analysis show that all of the phonon sidebands from nitrogen-bound exciton luminescence come from multi-phonon optical transition processes described by Huang-Rhys' theory.

The authors report the observation of photoluminescence in antiferromagnetic and paramagnetic MnO. Two strong emission bands centred at 1.66 and 1.25 eV are observed below the Neel temperature, 118 K, in MnO crystals. Above the Neel temperature, the emission band centred at 1.66 eV vanishes, while the band centred at 1.25 eV persists and remains in the paramagnetic phase at least to room temperature. The photoluminescence is interpreted in terms of a model in which the emissions originate from the impurity-perturbed Mn²⁺ excitons. In the model, the observed emissions arise from the transitions assisted by magnons and the short-wavelength magnetic excitations. The excitation spectra, the time-resolved emission spectra, and the heat-treatment effect on the emission spectra are also reported and discussed.

Using a previously developed, relativistic theory of photoemission the authors calculate the off-normal photocurrent emerging from the (1,1,1) face of a platinum single crystal irradiated by normally incident, circularly polarised photons. They study the dependence of the photoemission intensities on the helicity of the photons and the spin and direction of the emitted electrons. The results support the recent suggestion of Oepen and co-workers (1986) that photoemission experiments with circularly polarised light can provide useful information about the spin structure of the electronic spectra of solids, even if the spin polarisation of the photocurrent is not measured.

Angle-resolved photoelectron spectra have been measured at room temperature (i.e. well below the Curie temperature) from the (001) surface of an Fe₃Pt single crystal for a variety of emission angles using unpolarised 21.2 eV radiation. Corresponding spectra were calculated by means of a one-step theory of photoemission together with the bulk band structure and the one-dimensional DOS. The fairly good overall agreement reached between theory and experiment firstly provides an explanation of the observed features in terms of direct bulk inter-band transitions from majority and minority spin states and secondly favours, amongst local density exchange-correlation approximations, an X_alpha version with alpha =0.7 around 2 eV below E_F and a somewhat smaller alpha closer to E_F.

The electronic band structure of the CuAu II superlattice was calculated and compared with the electronic structure of CuAu I as well as with results from low-temperature specific heat experiments. One of the differences found in the present study between the density of states of CuAu II and that of CuAu I concerns the position of the dip in the density of states curve at the Fermi level. This explains the large difference between the electronic specific heat of CuAu II and that of CuAu I and yields excellent agreement with experimental low-temperature specific heat data. The electronic properties of different atomic layers were also studied. Calculated local densities of states show marked differences between successive atomic layers. In the atomic layer nearest to the antiphase boundary the Cu and Au d bands attain their lowest and highest locations respectively and the charge transfer from Cu to Au atom attains its maximum value.

The authors consider a generic model for a disordered Fermi liquid. By means of an infinite resummation of the loop expansion they show that there is a phase transition involving the spin degrees of freedom which can be identified with a new fixed point in the renormalisation group flow equations. The critical exponents obtained for 2<d<4 are apparently exact. The large-disorder phase is described as an incompletely frozen spin phase with no long-range order and subdiffusive spin transport. The results are in good agreement with experiments on Si:P.

The electronic structure at the interface between bulk GaAs(001) and short-period superlattices of (AlAs)_n(GaAs)_m has been calculated using ab initio pseudopotential techniques. The results show that the valence band offsets at such interfaces are very similar to those obtained experimentally for random alloy systems, but superior transport properties are anticipated for the ordered systems.

The two-dimensional point contacts formed in the inversion layer of GaAs-AlGaAs heterojunctions demonstrate steps in their conductance versus gate potential dependence. It is shown that the step structure depends on the topology of the contact which can be changed from one corresponding to zero boundary conditions, to that appropriate for cyclic ones. The step structure would follow the relation G=(2e²/h)n in the first and G=(2e²/h)(2n+1) in the second case (n=0,1,2,. . .).

The authors study the Ising ferromagnet in a random field following the renormalisation of the distribution of random fields. The disorder in the local field induces fluctuations in the exchange couplings which are taken into account. The method allows a straightforward determination of the exponents associated with the zero-temperature fixed point which governs the critical behaviour along the critical line and in particular the critical slowing down. They find the correlation length exponent v=1.0 which is in very good agreement with numerical calculations. They also obtain the crossover exponent for small random fields.