Table of contents

We show by analytic calculation and numerical simulation that the effect of trap displacements are amplified in mixtures of Bose-Einstein condensates, consistent with experimental observations. We also investigate the relative stability of inner and outer vortex states as the minima of the trapping potentials are displaced along the axis of rotation.

The role of coupling between two quantum wells in TE optical modal gain is analysed within the self-consistent solution of the Poisson, Schrödinger and 4×4 Luttinger-Kohn equations. The many-body effects of bandgap renormalization, coulombic scattering interactions and a non-Markovian distribution are also included. The analysis is performed for a 1.55 µm InGaAsP/InP lattice-matched system grown in the [001] direction. It shows that electrostatics significantly changes the modal gain in both amplitude and spectrum. The gain amplitude is larger when electrostatic effects are included due to better charge localization in the wells. The gain spectrum also changes due to the modification of the heterostructure potential and hence different coupling between the wells.

Employing ab initio calculations we have determined the structural parameters of the paraelectric and ferroelectric phases of KH₂PO₄, which are in good agreement with experiment. The calculations reveal that the O-O bond length and the coordinated motion of the P and K atoms have a large effect on the double-well potential for H, controlling the distance, δ, between the two equilibrium positions of the H along the O-O bond in the paraelectric phase. The calculations provide new evidence that the ferroelectric phase transition in H-bonded crystals has also a displacive feature.

An undocumented photoluminescence centre at 1.789 eV has been characterized in diamonds grown by a high-pressure high-temperature technique. It could only be observed in nitrogen-free crystals grown in a Fe-containing chamber. The centre is unique for diamond due to its room-temperature linewidth, which can be as small as 1 meV. Polarization, temperature and time-resolved measurements reveal that the corresponding defect has trigonal symmetry, a 3.7 meV splitting in the ground state, vibronic modes of 30 and 48 meV, very weak electron-phonon coupling and a lifetime of 7 ms at room temperature. The centre is attributed to an interstitial Fe related defect.

Upon exposing Lu₂SiO₅:Ce³⁺ scintillators to daylight, the material shows undesired intense afterglow lasting for several hours at room temperature. This effect is studied by means of illuminating Lu₂SiO₅:Ce³⁺ with visible and ultraviolet light. Conditions for trap filling and the relation with photoconductivity phenomena and Ce³⁺ luminescence quenching will be discussed. Autoionization of so-called Ce2 centres following the excitation to the lowest 5d level seems to play an important role in the mechanism.

We have investigated dynamical properties of coherent folded longitudinal acoustic (FLA) phonons generated in (GaAs)₁₀(AlAs)₁₀ superlattices (SLs) with 20, 100 and 195 periods. In the time-domain signals of the SL with 20 periods, the coherent FLA phonon mode with the Raman-inactive B₂ symmetry at the Brillouin zone centre, which is not observed in the SLs with more than 100 periods, is detected in addition to the Raman-active modes with the A₁ symmetry at and near the zone centre. The appearance of the B₂ mode is caused by the break of the mode symmetry due to the finiteness of the total number of periods. Moreover, it is found that the bandwidths of the coherent FLA phonon modes observed in the Fourier transform spectra depend on the finiteness of periodicity and the wavevector, which is discussed from the aspect of the relaxation of the wavevector conservation and the propagation of the phonon wavepacket through the SL region.

We present a review of recent progress in determining the surface structure of quasicrystals, with emphasis on their connections to mathematical tiling models. The review focusses in particular on the five-fold surface of icosahedral Al-Pd-Mn and the ten-fold surface of decagonal Al-Ni-Co. We also assess their potential as templates for the formation of two-dimensional quasicrystalline overlayers with reference to recent investigations of atomic and molecular adsorption.

The Ni(110)c(2×2)-Sn surface phase has been investigated by the combination of quantitative low energy electron diffraction (LEED) with the aid of tensor LEED multiple scattering simulations, and medium energy ion scattering using 100 keV H⁺ incident ions. The structure is found to involve substitution of half of the outermost Ni atoms of the clean surface by Sn atoms, but the resulting single layer NiSn alloy is corrugated, with the Sn atoms being 0.40±0.03 Å higher above the underlying Ni substrate than the outermost Ni atoms. The results are discussed in the context of previous structural studies of similar surface alloy phases; a weak trend for the amplitude of the corrugation in Ni/Sn surface alloys to become smaller as the surface layer packing density reduces may be consistent with previous ideas of the role of the depletion of valence electron density in the surface layer and the associated surface tensile stress.

Reflection anisotropy spectroscopy (RAS) is combined with scanning tunnelling microscopy in a study of the Na/Cu(110)(1×2) surface reconstruction induced on the thermally roughened Cu(110) surface. The roughening transition of Cu(110) generates a surface morphology consisting of a high density of exclusively monatomic height steps. This surface is further characterized by the irreversible loss of the 2.1 eV RAS feature and an enhancement in the response around 4 eV. Na deposition onto the roughened surface results in the formation of the (1×2) reconstruction. We show that the occupancy of the surface state near the Fermi level at on Cu(110) is not a prerequisite for surface reconstruction.

The in situ, at-temperature, real-time monitoring of open-volume defect formation, migration, coalescence and annealing has long been possible in bulk solids by measuring the Doppler broadening of annihilation radiation arising from the implantation of energetic positrons from a radioactive source. However, equivalent measurements on vacancy-type defects in thin films or within ~10² nm of a solid surface have not been made, principally because of the distorting influence on the data of surface annihilations. This paper describes the first measurements known to the authors of in situ, at-temperature annealing studies of near-surface open-volume defects, using as an example a silicon sample implanted with 50 keV Si⁺ ions. The technique involves the measurement of the fraction of controllable-energy positrons which diffuse back to the surface and there form positronium. The applicability and limitations of this method are discussed.

The attenuation rate of high-frequency sound waves (phonons) in a bulk metal was calculated by Pippard assuming the free-electron model and the deformation-potential coupling of the electron-phonon interaction. We study the correction to the Pippard theory in a metallic superlattice. A remarkable feature that we predict is the existence of the resonant absorption of the phonons by electrons. This happens due to the alteration of the band structures of electrons arising from their Brillouin-zone folding.

A general approach is presented for calculating the dynamic conductance of coupled quantum dot systems. To consider the long-range Coulomb interaction, we introduce the capacitances between the leads and dots, gate electrodes and dots, and dots and dots. The displacement currents are also taken into account. Our results fulfil charge and current conservation requirements, as well as current and charge response invariance under an overall potential shift. We apply our approach to a one-dimensional quantum dot array by use of the non-equilibrium Green function techniques. The numerical results are presented for a three-quantum-dot system. Three-peak resonant structure of the alternating-current conductance is observed at low temperature. The effect of the capacitance on the frequency-dependent conductance is obvious, but less so at low frequency. The low-frequency conductance shows either capacitive or inductive behaviour depending on the chemical potential of the electron reservoirs, but sufficiently large capacitances may change this situation.

Ni nanowire arrays were produced by electrodeposition of Ni into self-assembled porous anodic alumina templates. With different anodization voltages, the wires show different surface morphologies. The temperature dependence of the coercivity and activation volume of wires with regular shape can be explained by thermal activation over an energy barrier with a 3/2 power dependence on the field. The wires with irregular surface morphologies show abnormal temperature dependences of the coercivity and activation volume. Possible mechanisms for these behaviours are discussed.

A tight-binding simulation of the atom-by-atom deposition of amorphous carbon (a-C) at 100 eV incident energy is presented. More than 500 atoms were deposited. Chains are observed to form on the surface, some of which are sputtered. The good agreement with the experimental sputter frequency data and observation that all such clusters are linear provides strong support for the existence of these chains and the direct emission model of sputtering. The bulk of the grown film is a-C with a tetrahedral bonding fraction of 20%. Experiments have shown that at this incident energy of 100 eV, tetrahedral a-C is the preferred structural form rather than the a-C produced by this simulation. This discrepancy is attributed to the short range of the interatomic potential.

An abnormal sequential disordering process was observed in Sc-Ni multilayers upon room temperature 200 keV xenon-ion irradiation: the Ni lattice became unstable and entered an amorphous state faster than the Sc lattice did. Since there was negligible difference in mutual solid solubility and no significant difference in melting point between the two metals, this abnormal process was attributed to the fact that the damage density in the Ni layer was calculated to be about 2-3 times that in the Sc. A damage gradient model was thus proposed in which the Sc atoms diffused much faster along an uphill damage gradient towards the Ni layer than vice versa, resulting in the Ni lattice exceeding its critical solid solubility and collapsing into a disordered state faster than the Sc lattice.

Raman spectra of ZnTe were measured under hydrostatic pressures up to 15 GPa at T = 300 K. Results for the frequencies of first- and second-order Raman features of the zincblende phase (0-9.5 GPa) are used to set up a rigid-ion model of the phonon dispersion relations under pressure. Calculated phonon densities of states, mode Grüneisen parameters and the thermal expansion coefficient as a function of pressure are discussed. The effect of pressure on the widths and intensities of Raman spectral features is considered. Raman spectra of high-pressure phases of ZnTe are reported. These spectra indicate the possible existence of a new phase near 13 GPa, intermediate between the cinnabar and orthorhombic (Cmcm) phases of ZnTe.

We report on structural changes in monoclinic tetracyanoethylene, C₂(CN)₄ as studied by in situ high-pressure (0-5 GPa) neutron powder diffraction experiments. Structural parameters were obtained by Rietveld profile refinement of the diffraction pattern up to P = 2.5 GPa. Above this pressure the width parameter is found to diverge, indicating pressure-induced disordering at 2.5 GPa. The recovered sample, on release of pressure from 5 GPa, is found to have transformed to a graphite-like amorphous structure. The transformation to a graphite-like material has been verified by an independent high-pressure x-ray diffraction experiment.

Hydrogen storage in the Pd₁₂ cubo-octahedron cluster is studied theoretically using a self-consistent density-functional scheme with a norm-conserving pseudopotential. A cubo-octahedron Pd₁₂ cluster is assumed to be a model for the new PdH structure having superabundant vacancies that was found recently. The cluster calculation predicts that octahedral and tetrahedral sites neighbouring a vacancy in the face-centred cubic lattice are duplicated. The configuration entropy for hydrogen sites is estimated within the cluster model and it is thought to be the cause of the formation of the superabundant vacancies. Since this structure does not lose its configuration entropy when the hydrogen concentration becomes high, it is suitable for use as a hydrogen-storage medium.

A second-generation potential energy function for solid carbon and hydrocarbon molecules that is based on an empirical bond order formalism is presented. This potential allows for covalent bond breaking and forming with associated changes in atomic hybridization within a classical potential, producing a powerful method for modelling complex chemistry in large many-atom systems. This revised potential contains improved analytic functions and an extended database relative to an earlier version (Brenner D W 1990 Phys. Rev. B 42 9458). These lead to a significantly better description of bond energies, lengths, and force constants for hydrocarbon molecules, as well as elastic properties, interstitial defect energies, and surface energies for diamond.

Electrical conduction is studied along parabolically confined quasi-one-dimensional channels, in the framework of linear-response theory, for elastic scattering. For zero magnetic field an explicit multichannel expression for the conductance is obtained that agrees with those in the literature. A similar but new multichannel expression is obtained in the presence of a magnetic field ∥ perpendicular to the channel along the x-axis. An explicit connection is made between the characteristic time for the tunnel-scattering process and the transmission and reflection coefficients that appear in either expression. For uncoupled channels a Landauer-type expression is obtained that tends to a conductance of N parallel channels. In addition, this expression accounts explicitly for the Hall field and the confining potential, and is valid, with slight modifications, for tilted magnetic fields in the (x,z) plane.

Non-equilibrium effects in layered superconductors forming a stack of intrinsic Josephson junctions are investigated. We discuss two basic non-equilibrium effects caused by charge fluctuations on the superconducting layers: (a) the shift of the chemical potential of the condensate and (b) charge imbalance of quasi-particles, and study their influence on I-V curves and the position of Shapiro steps.

In this paper we study the ground state ferromagnetism in a doubly orbitally degenerate Hubbard model generalized by taking into account correlated hopping and inter-atomic exchange interaction. We find that an important role in the stabilization of ferromagnetism is played by the intra- and inter-atomic exchange interactions as well as correlated hopping, which allows us to describe the metallic paramagnetic-ferromagnetic transition with a realistic relationship between the above-mentioned exchange interactions. The expression for magnetization and the criterion of ferromagnetic ground state stability are derived. The obtained results are compared with some experimental data for magnetic materials.

A detailed thermodynamical investigation of the quasi-one-dimensional sulphur-based organic salts (TMTTF)₂PF₆ and (TMTTF)₂Br is presented in the temperature range from 30 mK to 7 K. In this part (part I), we consider the general aspects of the low-temperature specific heat of these materials in relationship with their ground states and we compare them with those previously measured for selenium members of the same family. All these compounds exhibit very similar thermodynamical behaviour, despite a variety of electronic ground states: spin-Peierls for (TMTTF)₂PF₆, commensurate antiferromagnetic spin modulation for (TMTTF)₂Br, incommensurate spin-density wave for (TMTSF)₂PF₆ (TMTSF≡tetramethyltetraselenafulvalene). We show that the low-energy excitations which are predominant below 1 K have a similar character to that observed in glassy systems, at least on short timescales of measurements. The dynamical aspects of the non-equilibrium thermodynamics will be presented separately in the following part (part II).

Using the nonlinear perturbation method, we study a crossover between quantum and classical regimes for the escape rate. We present a general formula for determining whether the escape rate changes smoothly around the crossover temperature or not. Applying it to tetragonal and hexagonal symmetries, it is found that the crossover is mostly of first order.

Small-angle x-ray scattering measurements on ball-milled Fe_yCu_1-y giant-magnetoresistive alloys (0.141⩽y⩽0.45) have been performed using synchrotron radiation. For samples with different nominal compositions, prepared under exactly the same conditions, the scattered intensity recorded as a function of the wavevector varied systematically. For this reason, the strong scattering signal was attributed mainly to composition fluctuations in the crystalline grains. The system was treated as a two-phase model consisting of Fe-rich regions in a homogeneous Cu matrix, with composition-dependent relative volume fractions. Real-space analysis of the scattering was performed in terms of a volumetric distribution of spherical particles. Results indicate that the main contribution corresponds to 2 nm scattering objects of well-defined density contrast. The invariant Q was calculated to assess the variations in electron-density contrast as a function of the Fe content in the samples. Calculations based on these results allowed the determination of 0.35 at.% iron concentration in the iron-rich phase for all the nominal compositions studied.

The effect of spin mixing due to thermal spin waves and the temperature dependence of the hot-electron spin polarization to the collector current in a spin-valve transistor (SVT) has been theoretically explored. We calculate the spin dependence of the collector current as well as the temperature dependence of the magnetocurrent at finite temperatures to investigate the relative importance of the spin mixing and the hot-electron spin polarization. In this study the inelastic scattering events in ferromagnetic layers have been taken into account to explore our interests. The theoretical calculations suggest that the temperature dependence of the hot-electron spin polarization makes a substantial contribution to the spin-dependent magnetotransport at finite temperatures in the SVT.

It was shown that on applying two radio-frequency fields, both of the same amplitude, one rotating at the frequency ω^I for nuclei inside the diffusion barrier and one rotating at the frequency ω^S for nuclei outside the diffusion barrier, the Hartmann-Hahn condition will be reached, which results in conservation of Zeeman energy in the spin-diffusion process and destruction of the spin-diffusion barrier. This technique can be used to detect the nuclear magnetic resonance signal from the nuclei located near paramagnetic impurities.

Fe_73.5-xCo_xSi_13.5B₉Cu₁Nb₃ (x = 13.5 and 60 at.%) nanocrystalline alloys were studied by means of x-ray diffraction, ⁵⁷Fe Mössbauer spectrometry and thermomagnetic measurements. For the first time, the compositions of both the Fe, Co, Si nanocrystalline and the residual amorphous phases were determined on the basis of combined structural and magnetic hyperfine data. The Co content of the nanocrystalline phase was found to be the same as the Co content of the as-cast alloy. The hyperfine magnetic field distribution of the intergranular phase shows the emergence of a bimodal behaviour, consistent with a two-cluster-like model. Such behaviour is attributed to the presence of iron-rich and iron-poor zones located in the bulk of the intergranular region and at the periphery of the crystalline grains, respectively.

The antiferroelectric (AFE) phase transition of Rb₃D_xH_1-x(SO₄)₂ was studied using x-ray diffraction, optical birefringence, and nuclear magnetic resonance. The orientation dependence of the resonance lines deduced from the quadrupole-perturbed ⁸⁷Rb nuclear magnetic resonance of Rb₃D(SO₄)₂ single crystals indicates slight deviations from the monoclinic symmetry in the paraelectric and the AFE phases. The dynamical critical exponents as deduced from measurements of the spin-lattice relaxation times depend on the deuteron concentration. Additionally, we have carried out x-ray single-crystal diffraction as well as optical birefringence measurements and find clear evidence for a structural phase transition in Rb₃D(SO₄)₂. The low-temperature space group of this compound is A2.

The reflectance spectra of the Tm₃Al₅O₁₂ and Yb₃Al₅O₁₂ single crystals have been studied at room temperature. Fifteen infrared-active modes, out of the 17 theoretically predicted, have been experimentally observed. The infrared data have been analysed by means of the Kramers-Kronig transformation to yield the complex dielectric function, the complex refractive index, the absorption coefficient, as well as the frequencies of the longitudinal (ω_LO) and transverse (ω_TO) long-wavelength T_1u modes. Furthermore, the experimental data are compared and discussed in the light of theoretical lattice dynamical calculations based on the rigid-ion model.

Thin-film phosphors of Sr₂B₅O₉Cl doped separately with Tm³⁺, Tb³⁺ and Mn²⁺ were deposited by spray pyrolysis of aqueous solutions. Because these phosphor films were activated at 600 °C, they are suitable for use on glass substrates. The films containing Tm³⁺, Tb³⁺ and Mn²⁺ exhibited blue, green and yellow cathodoluminescence (CL), respectively. The chromaticity coordinates, dominant wavelength and purity were determined for each phosphor. Films deposited on aluminosilicate ceramic plates exhibited enhanced CL intensity compared to those on glass plates. The CL luminance and efficiency of each phosphor depended on the electron beam voltage and current density. Saturation effects were observed as the current density increased.

Earlier neutron powder diffraction measurements on the CsCl compound TbMg showed that a canted magnetic structure is stabilized at T = 4.2 K. This structure mixes ferromagnetic and antiferromagnetic, ⟨½ 0 0⟩, components. Accounting for the anisotropy, several consistent models of structures could be proposed. In order to identify the best one, we performed x-ray diffraction measurements on a single crystal. A set of charge satellites originating from multipolar 4f scattering and related to the ⟨½ 0 0⟩ wavevectors were observed. Analysing the experimental data, within the framework recently proposed for multipolar scattering in cubic systems, a model is selected for the magnetic structure of TbMg.

ZnO-based varistors containing 0-10 000 ppm Ga were prepared by the mixed oxide route. Disc-shaped samples were sintered in air at 1030 °C for 2 h. All products were of high density (>94% theoretical). Gallium addition led to a reduction in grain size from 12 µm to approximately 8 µm. From I-V characteristics the non-linear coefficients (α) were determined to be ~38. The doped varistors exhibited donor-like behaviour for Ga contents up to 1500 ppm Ga and acceptor-like behaviour at higher levels. Above the 1500 ppm transition doping level, the leakage current density decreased and breakdown fields were consistently high.