Table of contents

Infinite-range quantum spin glass models with arbitrary spin are studied within the Matsubara time formalism. As an example the dynamic spin self-interaction R(t, t') and the resulting phase diagrams are presented for the quantum Ising model in a transverse field. Some results, such as the phase transition temperatures, are also reported for the XY and the Heisenberg models.

X-ray diffraction analyses of Fe/Au(001) superlattice films have revealed a body-centred tetragonal (BCT) structure with c/a=1.1 for Fe layers thinner than 1.0 nm when Au layer thicknesses are 3.0 nm. The perfect in-plane lattice matching was confirmed by off-axial measurements. The observed in-plane lattice parameter, a=0.2875 nm for BCT Fe, is close to the lattice dimension of bulk BCT Fe. However, profile analysis of axial scans have indicated that the Fe(002) lattice spacing along the growth direction has expanded. Such a tetragonal distortion cannot be explained by the normal elastic theory. The observed BCT structure of Fe would be a transient state on the BCC-FCC deformation path stabilized by a size effect or an interface effect. The tetragonal distortion is almost relaxed when Fe layers are thicker than 2.0 nm.

The authors have investigated the consequences of multi-hole localization at defect sites on the GaP(110) surface: the relaxation of the lattice and the emission of atoms due to bond breaking. It is shown that the combination of localization of two-hole states on a defect with cascade excitation results in emission of an atom from the defect. The results support the mechanism suggested by Hattori et al. (1990) of defect-initiated emission of Ga atoms under laser irradiation, of which the yield is a superlinear function of laser fluence.

The authors have performed tunnelling calculations for the field-ion microscope using a the JWKB method of Haydock and Kingham (1981) and the model potentials introduced by Homeier and Kingham (1981). They have found that Homeier and Kingham overestimated the cross-surface variations in the tunnelling rate constant by a factor of up to five. The reasons for these inaccuracies are analysed and they give the results of more accurate calculations.

The effect of interlayer coupling on the magnetic properties of layered antiferromagnets is studied by the use of nonlinear spin-wave theory. The sublattice magnetization, internal energy, specific heat and magnetic susceptibilities are calculated for different interlayer coupling strengths over the whole range of temperatures, and the asymptotic expressions for these quantities are given in two low-temperature regimes distinguished by a characteristic reduced temperature Theta ₀ which reflects the character of the layered structure. It is shown that the temperature dependences of the physical quantities have a crossover from three-dimensional to two-dimensional behaviour with increase in the reduced temperature from one regime ( Theta << Theta ₀< Theta ₁) to another ( Theta ₀<< Theta << Theta ₁) for small interlayer coupling strengths. The correction due to spin-wave interactions is also discussed.

The author studies the phase transitions of finite and infinite ferroelectric systems using the Ising model in a transverse field. The mean-field layer approximation is used and the results for the Curie temperature are expressed in terms of general determinants. The effects of surface modification are introduced through a surface exchange constant J₀ and a surface transverse field Omega ₀. It is shown that above a critical curve in J₀- Omega ₀ space, surface polarization is possible, even when there is no phase transition in the bulk. The dependence of the Curie temperature on the film thickness is obtained for two types of surface modification.

The photoluminescence in Si nanostructures is studied theoretically through the effects of size, shape and condition of the cluster surface on the energy gap of the Si cluster. Si clusters with a spherical shape or columnar shape are considered. The effect of the condition of the cluster surface is investigated through the chemisorption of hydrogen atoms at the surface. It is found that the energy gap of a cluster decreases with increase in cluster size. For the cluster to simulate an Si column, the calculated energy gap can fall into the energy region of visible-light luminescence with decrease in the cluster size. In the size range (less than 20 AA) considered, the energy gap of the cluster is strongly dependent on the shape, and the surface condition might be important for the determination of the electronic structures of Si nanostructures. These results might be useful in the understanding of photoluminescence in porous Si. The electronic structures of clusters are also investigated in detail.

The local structure of GaAs oxides, grown on GaAs (100) wafers by simple exposure to air after chemical etching as well as by heating in air at 250 degrees C and at room temperature in ozone (O₃) atmosphere, has been studied by glancing-angle X-ray absorption spectroscopy. The reflection geometry makes it possible to collect the EXAFS (extended X-ray absorption fine-structure) signal related to a thin surface layer (about 30 AA) in such a way as to reduce the unwanted substrate contribution and to collect the signal arising from the oxide grown at the surface. The oxide composition and thickness change as the surface varies from about 5 to 30 AA. An As-enriched oxide is produced by the 30' O₃ exposure while the Ga oxide component is more abundant for the sample heated at 250 degrees C in air.

The dependence of magnetic circular dichroism (MCD) in core-level photoemission on the experimental geometry and direction of electron emission has been studied experimentally using photoemission from the 2p and 3p levels of iron. The MCD has been measured both along emission directions of high symmetry, and along general emission directions within a single mirror plane. The observed angular variations cannot be accounted for by model calculations based on absorption, or based on angle-resolved emission into photoelectron states of a single, oriented atom. They are consistent with a model where the photoelectron states of the crystalline symmetry appropriate to couple to the detector are used.

The magnetic permeability of sputtered permalloy thin layers has been investigated by a specially designed experimental set-up. The investigation shows that the magnetic permeability depends strongly on the argon pressure during deposition Higher permeability of the sputtered layers is obtained for argon at pressure less than 5*10^-2 mbar.

An EXAFS study of potassium niobate was performed at the niobium K edge between 300 and 4.5 K. It is shown that there is no significant modification of the local environment of the niobium atoms within their oxygen octahedra with temperature. In the orthorhombic phase as well in the rhombohedral phase, each niobium is shifted by about 0.216 AA from the central position in the octahedron, along a (111) axis. This confirms the so-called 'eight-site model' used for ferroelectric perovskites. The Nb-Nb and Nb-K distances are also considered. The complementarity of X-ray or neutron diffraction techniques sensitive to the long-range order and EXAFS spectroscopy sensitive to the local order is discussed.

The fast linear muffin-tin orbital coherent potential approximation method allied with density functional theory (LMTO-CPA-DFT) is used to calculate electronic structure and cohesive properties of Ni-Al random alloys on an underlying FCC lattice in all concentration intervals. Binding curves obtained in the calculations are used in the Debye-Gruneisen analysis to determine thermal properties and temperature-dependent Connolly-Williams cluster interactions. The calculated lattice constants, bulk moduli, enthalpies and free energies of formation are in good agreement with experiment. The globally and locally relaxed cluster interactions are compared with the results obtained from the total energy-band calculations for elements and ordered phases.

Thin silicon-on-insulator (SOI) film formation during 25 keV O⁺ ion implantation has been investigated by cross-sectional transmission electron microscopy, Auger electron spectroscopy and scanning electron microscopy. The top Si layer thickness in as-implanted samples increased with increasing substrate temperature from 280 to about 500 degrees C and the current density from 10 to 450 mu A cm^-2 at a constant dose of 2*10¹⁷ ion cm^-2. Substrate heated to about 485 degrees C showed remarkable in situ annealing effects during ion implantation; resulting in the oxygen depth profile and the increase of the Si-O bonding. The SOI structure was formed in a wide range of the current density at about 485 degrees C. Shallow holes appearing in the surface at substrate temperatures higher than 485 degrees C scarcely influenced subsequent SOI structure formation. Deep holes appearing above 530 degrees C made it difficult to produce the SOI structure.

Stable lattice dynamical models of orthorhombic C₂H₂ are developed from those devised by Hirshfeld and Mirsky (1979) for the cubic phase. These satisfactorily reproduce measured phonon frequencies but cannot account for the crystal cohesive energy. Models are also identified based on the work of Nyberg and Faerman (1989) the principal difference being bond- rather than atom-centred short-range interactions. These yield acceptable crystal energies. All models have physically realistic molecular quadrupole moments.

The DC conductivity of BF₄^--doped polypyrrole films has been measured in the temperature range 77-305 K and could be well represented by Mott's variable-range hopping mechanism. The conductivity data were used to extract the coupling constant gamma , the effective polaron mass m_p, the effective dielectric constant epsilon _p the Debye temperature Theta _D, the polaron radius r_p, the hopping distance R, the density of states at the Fermi level N(E_F) and the hopping energy W, whose magnitudes confirm the formation of small polarons in polypyrrole.

The authors present a low-temperature study of the charge and spin dynamics of holes, at density approximately 10¹⁶ cm^-3, in the narrow-band semiconductor InSb. The conductivity and Hall coefficient are used to characterize the charge dynamics, and nuclear spin-lattice relaxation is used to monitor the electron spin dynamics. They have two samples; the first is cadmium-doped, p-type, uncompensated, of density 1.5*10¹⁶ cm^-3 and the other is an initially identical sample, but additionally compensated by neutron transmutation doping to reduce the hole density to 7.2*10¹⁵ cm^-3. This dramatically increases the mobility in the hopping regime, by reducing correlation effects. An unusual nuclear relaxation rate enhancement at low field and at low temperatures is monitored for all three nuclei, ¹¹⁵In, ^121,123Sb, over a wide field range; high field quenches the effect. The authors discuss possible nuclear relaxation mechanisms and speculate that the effect is due to electron spin fluctuations associated with the amorphous antiferromagnetic nature of the sample in which the effect is observed, which has a dopant density close to the critical density for the metal-non-metal transition.

The shifted-1/N-expansion method has been used to study the energy levels for a 2D donor impurity in the presence of an arbitrary magnetic field. Exact analytical expressions for the energy levels at zero- and high-field limits are obtained. The calculations are carried out to the third-order correction of the shifted-1/N-expansion energy series. The results appear to be in excellent agreement with those of Martin et al. (1992).

Presents and discusses the high-resolution optical spectrum associated with ⁵I₈(A₁) implies ⁵F₅(E), ⁵I₈(A₂) implies ⁵F₅(E) transitions of a tetragonal Ho³⁺ centre in CaF₂. The lowest crystal-field levels in the ground multiplet are two singlets separated by approximately 1.7 cm^-1. With the closeness of the states, the hyperfine interaction gives rise to strongly mixed wavefunctions and anomalously large pseudoquadrupole splittings. The mixing also results in an enhanced magnetic moment associated with the hyperfine components of the singlets and the magnitude is found to depend on the hyperfine level. Consequently, there are separate superhyperfine resonances associated with each of the four non-degenerate hyperfine levels and these resonances were observed by ODNMR techniques when holeburning in the various optical hyperfine lines. More than 30 superhyperfine resonances were detected and they are assigned to interstitial and groups of near-neighbour F^- ions.

A theoretical investigation of the photostimulated thermomagnetic effect by electrons in a semiconductor superlattice (SL) in the lowest miniband is presented. The possibility of controlling the thermopower alpha , the electron thermal conductivity chi and the electroconductivity sigma of the SL with the help of laser radiation is indicated. The parameters alpha , chi and sigma were found to oscillate. The Hall constant, the Righi-Leduc and the Maggie-Righi-Leduc effects were calculated. The prospect of using the SL as a good-quality and highly efficient thermoelement is also proposed.

The phonon drag and electronic diffusion contributions to the tensor M which determines the heat flux U=M.E is calculated for a two-dimensional electron gas in a perpendicular magnetic field B. The drag component is obtained using Boltzmann transport and diffusion is included using the lowest-order cumulant approximation to describe scattering between Landau states with different orbit centres. The 2D Landau levels have Gaussian model lineshapes with RMS width gamma =CB^1/2 which is the only adjustable parameter. At temperature T=5.02 K, drag dominates the predicted M_yx(B) values which are in good agreement with new data for Si MOSFETs taking C=0.6 meV T^-12/. The calculated M_xx values are in worse accord with the data because the predicted drag contribution M_xx^g is zero. Both M_xx and M_yx reveal magneto-oscillations originating from fluctuations in the density of states at the Fermi level. The authors show that at T=5.02 K, setting M_xx^g=0 has little effect on the accuracy of the magnetothermopower S_xx(B) but yields poor approximations to the off-diagonal term S_yx(B). At T=1.47 K, these thermopower components are diffusion-dominated. The predicted values are comparable to experiment although the magneto-oscillations are overemphasized in S_yx and underestimated in S_xx.

Reports on detailed measurements of the thermoelectric power coefficient, L(T) identical to S(T) sigma (T), relating the thermoelectric power S(T) to the electrical conductivity sigma (T) in three polycrystalline Y₁Ba₂Cu₃O_{7- delta} samples. All the samples have almost the same composition ( delta <or approximately=0.10) and they are single-phase to within 4%, but they have very different structural inhomogeneities at long length scales, i.e. at scales much larger than the superconducting correlation length in all directions. Measurements in different single-phase samples indicate that all the critical behaviour of S(T) above the superconducting transition is driven by sigma (T). Therefore, these results strongly suggest, within experimental resolutions of Delta L/L<or approximately=2%, that L(T) in Y₁Ba₂Cu₃O_{7- delta} compounds is not affected, over the entire mean-field-like region (MFR) above the superconducting transition, by thermodynamic fluctuations of the superconducting order parameter amplitude (OPF). The reduced temperature behaviour of the thermopower excess Delta S( epsilon ) is the same for all the single-phase samples studied here, i.e. fractal effects are not observed in the MFR, in agreement with the authors' previous paraconductivity results. Also, the OPF effects arising on S(T) through sigma (T) are confirmed to be essentially three-dimensional (3D) over the entire MFR.

The authors study the spectral properties of a localized particle (a deep core level, or a heavy particle hopping in a solid), coupled to conduction electrons that are described by a marginal Fermi liquid hypothesis. The main result is that, in this model, the core level line shape in an X-ray photoemission experiment shifts with temperature. The spectrum is consistent with the decrease of the hopping rate with decreasing temperature obtained by Zhang et al. (1992). In the presence of a distribution of energy levels for the hopping particle, the temperature dependence of the hopping rate is found to be less pronounced. The observability of both the shift in the X-ray photoemission spectrum and the decrease of the hopping rate depends on the strength of the interaction. A rough estimate of the value of the interaction parameter indicates that the effect might be observable in X-ray photoemission experiments.

The authors examine the dynamical properties of the 2D+or-J Edwards-Anderson Ising spin glass in the anomalous phase which corresponds to the Griffiths phase in the diluted ferromagnetic system. By using Monte Carlo simulation they find that the autocorrelation function can be described as a function of a power of scaled time. This new scaling includes both the stretched exponential law for the intermediate stage and the logarithmic law for the early stage of relaxation. The relaxation time obtained from the scaling is found to diverge exponentially with exponent proportional to T^-2.

The authors study the dynamic spin correlation function of a spin-1 antiferromagnetic chain with easy-plane single-ion anisotropy. They use exact diagonalization by the Lanczos method for chains of length up to N=16 spins. They show that a single-mode approximation is an excellent description of the dynamical properties. A variational calculation allows them to clarify the nature of the excitations. The existence of a two-particle continuum near-zero wavevector is clearly seen, both in finite-size effects and in the dynamical structure factor. The recent neutron scattering experiments on the quasi-one-dimensional antiferromagnet NENP are fully explained by the results.

The electronic structure and local magnetic moment of BCT-FeN compounds were calculated by the first-principles self-consistent cluster method. For the alpha '-martensite, the author found that the magnetic moment of the first-neighbour Fe atoms of N was reduced while that of the second-neighbour Fe atoms was increased compared with those of pure BCC-iron; this was attributed to the concomitant effects of p-d interactions and tetragonal elongation of BCT-FeN, and was in good agreement with experiment. For the Fe₁₆N₂ structure, the calculated average magnetic moment is smaller than that of experiment.

Four kinds of S-type domain and twin boundaries in the ferroelectric R3m and antiferroelectric Pbam phases of plate-like PbZrO₃ crystals with complicated twinned structures have been described. Conditions for thermal stability of S-boundary orientations in perovskites have been formulated.

The authors report the experimental results of a Raman scattering study of magnons in the layered triangular-lattice Heisenberg antiferromagnet VCl₂. The magnetic phase transition of this compound is said to reflect the mechanism of the dissociation of the Z₂ vortex proposed by Kawamura and Miyashita (1984). As well as two phonon lines, which agree with a group theoretical analysis, three lines are detected at low temperatures; however, they disappear around T_N=36 K. Two lines among these three are attributed to the two-magnon process on the assumption that the Neel state forms a three-sublattice structure, i.e. a 120 degrees spin structure in the ac plane. The rapid decrease of their intensity above T_N is unusual; this phenomenon, the authors believe, should be understood on the basis of the generation of Z₂ vortices. However, the origin of the other line is hard to explain using conventional spin-wave theory.

Precise photoluminescence measurements on an RbCaF₃:Mn²⁺ sample containing only 400 p.p.m. of Mn²⁺ have been carried out in the 10-300 K temperature range. The results are compared with those obtained in other fluoroperovskites doped with Mn²⁺. The analysis of the ⁶A_1g(S) to ⁴T_1g(G) excitation peak at room temperature leads to a Mn²⁺-F^- distance R=213.3 pm which is close to that derived from the experimental isotropic superhyperfine constant, A_s. The plot of the first moment of the emission band, M₁, against temperature reveals a slight but sensible change of slope at T=193 K which is associated with the O_h¹ to D_4h¹⁸ structural phase transition of the host lattice. Furthermore, at T=40 K, M₁ undergoes an abrupt increase of approximately 100 cm^-1. This fact supports the existence of another phase transition involving an increase Delta R/R approximately=0.2% upon cooling, and thus a situation which is similar to that detected in the structural phase transition of KMnF₃ at T_c3=81.5 K. To the authors' knowledge this is the first time that clear evidence of both phase transitions in RbCaF₃ has been achieved through an optical probe. Finally the variation of the ⁴A_1g(G), ⁴E_g(G) peak, E₃, along the fluoroperovskite series is analysed.

The X-ray emission K and L_2,3 bands of sulphur and phosphorus as well as their X-ray K absorption spectra are obtained for the semiconductors TlAsS₂, Tl₃AsS₃, Tl₃AsS₄, Tl₃PS₄ and Tl₃TaS₄. The upper part of the valence band in the compounds investigated is formed of the S p states mainly, but the S s states are localized near the bottom of the valence band and hardly take part in the bonding. Comparison of the X-ray spectra of S and P in the isoelectronic and isostructural compounds Tl₃AsS₄ and Tl₃PS₄ has made it possible to determine the energy position of the p states of arsenic in Tl₃AsS₄. The energy distributions of the valence electron states of sulphur and its neighbours in all the compounds investigated are analysed and the covalent-ionic mechanism of chemical bonding is proposed.