Table of contents

A lattice relaxation model is employed for calculating the local distortions and local hydrostatic strains around Eu²⁺, Gd³⁺ and Tm²⁺, ions at the cubic sites in fluorites. The results turn out to be almost entirely consistent with the existing empirical results based on EPR and ENDOR data. Comparisons are also made with previous calculations in the literature, and certain discrepancies are noted.

The valence band densities of states and band gaps of SiN_x have been calculated by the Kittler-Falicov method in the one-atom-cluster Bethe lattice approximation. The DOS agrees reasonably well with the photo-emission spectra but the band widths are smaller. The variation of the band gap with x shows the experimentally observed increasing rate of increase with x but the widening of the gap is related to a rapid recession of E_c while the corresponding slow recession of E_v is absent. It is concluded that short-range order plays the dominant role in the electronic structure of SiN_x.

The Peierls instability in chain-like substances is reexamined and a two-step Peierls transition theory is developed. The weak inter-chain coupling makes the CDW transition and the associated metal-insulator transition occur at different temperatures and allows the CDW to coexist with the metallic feature. The resistivity anomaly in TaS₃ between 215 and 270 K and the pressure-induced metal-insulator transition in (DMe-DCNQI)₂Cu are well explained.

Current measurements in CDF₂ crystals doped with 0.001-0.1 mol.% Y and with electron concentrations of order 10¹⁵ cm^-3 are studied in the temperature range 10-60 K by applying electric fields up to 5*10⁴ V cm^-1. The current-voltage-temperature characteristics are examined according to the Apsley-Hughes model for hopping conduction. The value of the density of states at the Fermi level and of the localisation radius are obtained. Deviations from theoretical trend are observed at low applied fields, showing the existence of a conductivity drop that is attributed to the formation of collective ordered electronic structures, characteristics of compounds having low electronic density.

Monte Carlo simulations of site percolation on 2 D and 3 D regular random resistor networks with free boundary conditions have been performed. An iteration-relaxation procedure has been applied to estimate the network voltage distributions from which conductance and relative noise have been estimated. In the noise calculation the authors have used the formula derived by Rammal and co-worker (1985). Critical exponents for the conductance and relative noise have been calculated using a finite-size scaling method. The 2D results obtained are in good agreement with the recent estimates of other authors. In the 3D case they have calculated the relative noise exponents k=1.47+or-0.04 and b=1.35+or-0.04 which they have found to lie within the error bar limits of the theoretical upper bound derived by Tremblay and coworkers (1986) from a 'nodes-links-blobs' model.

The tunnelling of a wavepacket through a resonant tunnelling system is studied using the sequential tunnelling model. Fermi's Golden Rule cannot be applied to this process because the wavepacket is a coherent superposition of states which have different energies and in the Fermi Golden Rule it is assumed that contributions to the transition rate from states at different energies add incoherently. A time-dependent analysis shows that the occupancy of the resonant level increases as t², where t is the time after the wavepacket is incident on the barrier, for small values of t. If the tunnelling from the resonant level to the electrodes is ignored the occupancy of the resonant level is constant for large values of t which is expected because tunnelling from the wavepacket to the resonant level can only occur during the time that the wavepacket is in contact with the resonant tunnelling system. When a sequence of wavepackets is incident on the resonant tunnelling system the occupancy of the resonant level at large times reaches the value predicted by the Fermi Golden Rule.

The influence of microwave radiation with frequency ranging from 0.3 to 12 GHz on the quantum corrections to the conductivity in polycrystalline silicon films is studied. The heating effect and the dynamical suppression of these corrections are separated. The dynamical destruction of the weak localisation is described theoretically with parameters deduced from the negative magnetoresistance measurements.

The authors have calculated the normal-state resistivity rho _ab (parallel to the Cu-O plane) due to the electron-phonon scattering mechanism, and the phonon-assisted hopping conductivity sigma _c (perpendicular to the Cu-O plane) in the high-T_c oxide Y-Ba-Cu-O. The calculated results show that while both rho _ab and sigma _c are nearly linearly temperature dependent, the slopes d rho _ab/dT and d sigma _c/dT are slowly varying functions of the temperature. Using a set of parameters extracted from the experimental data for Y-Ba-Cu-O and McMillan's electron-phonon coupling parameter lambda =0.45, the authors' results for both rho _ab and sigma _c compare quantitatively with the experimental data.

The excess electrical conductivity, Delta sigma , above T_c in single-phase (within 4%) Ba₂LnCu₃O_{7- delta} compounds, with Ln=Y, Ho and Sm, has been measured in the reduced temperature range 5*10^-4<or approximately= epsilon <or approximately=1; this extends previous results almost one order of magnitude closer to T_c. This allows the authors to observe, for the first time, four different Delta sigma ( epsilon ) regions, with different critical exponents. These regions are analysed in terms of the Aslamazov-Larkin (AL) theory for thermal fluctuations in BCS superconductors and also using some dynamical scaling ideas. The results suggest that the Ginzburg-Landau mean-field region is located in the high-temperature range. In that case, the corresponding critical exponent is of the order of -¹/₂, in agreement with 3D AL superconductivity, although the strong intrinsic anisotropy, the polycrystallinity and the local defects of the samples may cause the disagreement with the AL model observed for the amplitude of the fluctuation effects.

Microwave absorption studies of the new high-T_c superconducting systems Tl-Ba-Ca-Cu-O (T_c( rho =0)=118 K) and Bi-Ca-Sr-Cu-O (T_c( rho =0)=84 K) were conducted using an X-band EPR spectrometer. The main features observed were (i) a sharp change in the leakage current at T_c; (ii) intense low-field (<50 G) absorption below T_c; and (iii) a largely field-independent absorption, the intensity of which was found to increase with increasing external magnetic field strength up to 10 kG. The dependence of the intensity on magnetic field is explained in terms of magnetic-field-induced decoupling of grains and field, and temperature-dependent lambda _MW/ lambda , where lambda _MW is the microwave penetration depth and lambda is the London penetration depth.

The authors show how the exact interface free energy F for the Ising antiferromagnet on the triangular lattice with infinite first-neighbour and finite second-neighbour interactions can be determined analytically. From considerations of the angular dependence of the interface free energy they conclude that the transition is not second order. An upper bound for the transition temperature is given.

The (110) surface of CdTe has been studied by LEED using a newly assembled experimental database of I(V) curves collected by a new high-speed computer-controlled diffractometer. Many of the (110) surfaces of similar zincblende semiconductors have similar structures, but in the past there has been some debate between two computing top-layer structures. The paper provides strong evidence for one of these alternatives and presents a refinement of 11 structural and nonstructural parameters that define the surface. The surface-layer rumple was determined to be -82 AA. Experimental data collected from diffraction geometries well away from normal incidence proved fundamental to the improvements arrived at in the present work. The solution was derived by the application of a searching algorithm tailored for use in LEED based on the Hooke and Jeeves method (1961) of unconstrained optimisation.

Using the principles of the theory of distortion-induced activity of forbidden modes of molecular units in crystals, a quantitative theory of distortion-induced activity of forbidden Raman modes of molecular units in different crystal orientations is discussed. An example of the observation of nu ₁^s and nu ₁ⁿ modes (A_g symmetry) of (NH₄)₂SO₄ in B_2g and B_3g polarisations has been discussed to demonstrate the usefulness of the results of the theory.

The Gd³⁺-nearest-neighbour V(Cl^-) paramagnetic centre in ThCl₄ doped with Gd³⁺ is a very sensitive probe for the normal-to-incommensurate phase transition at T_I=70.6 K. The details of the electron paramagnetic resonance (EPR) lineshapes and their dependence on the temperature contain much information on the static and dynamic phenomena which occur in the incommensurate phase and in the vicinity of T_I. At present, the authors analyse them by means of classical models. Using symmetry considerations and the superposition model, they construct a local spin Hamiltonian which involves the local phase and the amplitude up to second order in the modulation wave. Clearly, the modulated terms of the spin Hamiltonian are not in phase. However, the temperature transition is neatly marked by a change in regime from a normal phase line which coexists down to T_I-4 K to an incommensurate phase line. This is analysed in terms of motional averaging by large and inhomogeneous phase fluctuations.

A simultaneous analysis of the ³⁵Cl NQR transition frequency and of the molar heat capacity temperature dependences has been carried out for solid methyl chloride. The values for the various deduced quantities (Debye frequency, effective rotary mode frequency, etc.) are found to be very reasonable and produce an excellent description of the data, therefore giving the authors confidence in the models and method employed. The NQR transition frequency shows a thermal activated process which is assigned to the hindered rotations of the methyl group.

The problem of soliton degrees of freedom in physisorbed adlayers, as distinct from thermal degrees of freedom is considered. The authors propose making the separation between the two by applying a separate fine- and coarse-grained scaling for the phonon and domain wall motion respectively. This distinguishes the two different length scales in the temperature regime where both modes are present. The contribution to the free energy of domain wall fluctuations about the saddle point is calculated to lowest order in powers of the displacement from the static configuration. The wall fluctuation spectrum is found to have a small gap, justifying earlier theories which treated the walls as fixed. They also indicate how the saddle-point Hamiltonian is used to determine the properties of the incommensurate-commensurate transition.

In the paper, for the polaron bound to a fixed Coulomb origin located at the centre of a polar slab, the energies of the ground state and the excited state as well as the effective mass are investigated by means of the Lee-Low-Pines variation technique. From consideration of the interactions between an electron and both the bulk longitudinal optical (LO) phonon and the surface optical (SO) phonon, the self-energy and the effective mass are derived as functions of slab thickness. Taking GaAs as an example, the authors discuss a number of interesting features.

The authors develop a semi-classical theory of the intra-sub-band longitudinal electronic excitations in a quasi-(1+1)-dimensional semiconductor superlattice. By taking account of the frequency dependence of epsilon _s( omega ), they find the coupled intra-sub-band plasmon-optical phonon bands. A quasi-(1+1)-dimensional semiconductor superlattice with a complex unit cell is also investigated; two possible plasmons are found. Finally, excitations in an isolated pair of one-dimensional quantum well wires are compared with those in the pair of two-dimensional electron gas layers.

A Monte Carlo simulation of transient currents in thin layers with a macroscopically nonhomogeneous spatial trap distribution (STD) has been performed. Multiple-trapping band transport of electrical carriers for exponentially distributed depths of traps has been investigated for several STDs: the exponential distribution given by N_t(x)=N₀ exp(-x/D) exp(-E/kT_c), N_t(x)=N₀ exp(-(Lx)/D) exp(-E/kT_c), N_t(x)=N₀(exp(-x/D₁)+exp(-(L-x)/D₂))exp(-E/kT_c) and a Gaussian distribution with maximum trap density in the middle of the layer thickness L given by N_t(x)=N₀ exp(-(x-L/2)²/D²) exp(-E/kT_c). Remarkable deviations from homogeneous-trap-density characteristics have been found. Some peculiarities of transient currents resulting from trap nonhomogeneity have been pointed out. Energy and spatial distributions of the trapped carriers for chosen times have also been calculated.

The comparison of the physical properties of two salts of DIMET (dimethyl-ethylenedithiotetrathiafulvalene) shows strong differences in both the electronic and the magnetic behaviour. These differences are shown to be in contradiction with the electronic dimerisation deduced from transfer integral calculations for these two salts. Starting from an accurate examination of the anion distribution along the stacking axis in these two salts, and using a simple model of Coulomb interactions, it will be emphasised that the contradiction no longer exists if the transfer integrals are calculated taking the potential of the anions into account.

Electronic energy spectra are obtained near an interface between materials of different effective mass, in a magnetic field parallel to the interface. The effective-mass difference is shown to couple the electron's motion along the field to its transverse motion. The coupling is described in terms of an effective mass for transverse motion. An explicit formula is obtained for this effective mass. Its behaviour illustrates clearly the existence of magnetic bulk-like states and interface states near the interface.

The Helmholtz free-energy functional of the correlation functions for quantum spin-1/2 systems is studied, using field theoretical methods. It is shown that it does not coincide with the first Legendre transform of the effective action obtained by a Hubbard-Stratonovich transformation. It is instead related to the second Legendre transform of the original action, thereby confirming that approximations for this functional are conservative in the Baym and Kadanoff sense. The critical behaviour of the Ising model in a transverse field is discussed as an example.

Magnetic susceptibility measurements of Eu₂Co₁₂P₇ show a ferromagnetic phase transition at the Curie temperature of T_C=151+or-0.5 K, which is attributed to the magnetic order of the cobalt sublattice. ¹⁵¹Eu Mossbauer spectra reveal an unusual magnetic hyperfine structure splitting of the Van Vleck ion Eu³⁺. The temperature dependence of the magnetic flux density at the ¹⁵¹Eu nuclei was determined and compared with theoretical values. The exchange field acting on the europium ions may be assumed to follow the Brillouin function, where the best agreement is achieved for J=1. An exchange constant of mu _BB_exch(0)/k=7.5+or-0.1 K was deduced from the saturation value of the magnetic flux density B_sat=19.0+or-0.1 T. The possible contributions of Fermi contact terms of the magnetic flux density are discussed.

A systematic study of the infrared photoluminescence from deep centres in polycrystalline CdSe layers is presented. The authors observe two emission bands at about 0.92 mu m and about 1.22 mu m. The evolution of the band shape and emission intensity quenching with temperature is analysed using a semiclassical model, and the characteristic parameters of the centres are obtained. The results are shown to be consistent with transitions involving selenium vacancies V_Se and oxygen impurities O_Se.

In the high-T_c superconductors YBa₂Cu₃O_7-x and HoBa₂Cu₃O_7-x the Doppler-broadened positron annihilation lineshape parameter is studied as a function of temperature. Anomalies are detected around the transition temperature found by resistance measurements, giving indirect support for an electronic mechanism for superconductivity. The positron lifetimes in these compounds are measured at room temperature and are found to be similar. The origins of the several lifetimes found and their intensities are discussed.