Table of contents

The conditions which allow the self-trapping of a quantum particle to nucleate a local heterophase region in the host fluid are explored theoretically. A previously neglected aspect is emphasized: the curvature of the surface separating the coexisting liquid and vapour leads to a surface pressure difference across that surface which has contributions from the surface tension and the quantum particle. Results of calculations for an electron in liquid neon, for conditions in which a stable vapour bubble is nucleated, are used to give an example of the phenomena explored.

The crystallization behaviour of the metallic glass Cu_73.9Ni_9.2Sn_3.5P_13.4 has been investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Three exothermic peaks occur at 216, 295 and 365 degrees C. Based on the exothermic peaks and observations from XRD at various stages of crystallization, the crystallization sequence and the structure of the crystallization phase of the glass is described.

Measured density profiles of colloidal suspensions in the gravitational field may be inverted, within the local density approximation, to yield the osmotic equation of state. The feasibility of the procedure is tested by Monte Carlo simulations of the density profiles for two simple models of interacting colloidal particles.

The hop rate of a localized particle in newly proposed marginal Fermi liquid is examined. It is found that due to the disappearance of the quasi-particle spectral weight at the Fermi surface, the hop rate behaves quite differently from that in all other materials. In particular, the hop rate increases with temperature at finite T, contrary to that in all other known materials. Furthermore, localization can occur at extremely low T. The experimental consequence of this prediction, as well as its connection with muon spin relaxation, is also discussed.

The magnetic structure of a Pt₃Fe alloy changes from antiferromagnetic to ferromagnetic as a result of plastic deformation. The recovery processes of the magnetic transition have been studied in a Pt₃Fe alloy deformed plastically by 36% strain. Three recovery processes have been discovered; the spontaneous magnetization decreases in 473 K and 673 K annealing and disappears in 1073 K annealing. Each recovery process is discussed from the viewpoint of the atomic migration and the dislocation displacement through the activation process.

The authors discuss the interaction between two-level systems and the phonon field in noncrystalline dielectric solids, and they calculate the quantum transition matrix elements of such an interaction. After studying the Boltzmann equation for phonon transport, they obtain the phonon distribution function. From a direct calculation of the heat current, and as a result of obtaining the thermal conductivity of noncrystalline dielectric solids, the T² relation is obtained.

The authors argue that for real, freely suspended liquid films the undulation mode (transverse sound mode) has a linear dispersion relation of the usual sound-like structure, i.e. a damping contribution proportional to the wavevector squared (k²). Thus, there is no need to introduce non-linear fluctuation effects, which would be necessary if the regular damping mechanism ( approximately k²) were absent, and which would lead to singular contributions in the damping.

The electronic band structure for the ferrimagnetic state of Mn₂Sb has been calculated using the augmented plane-wave method. The energy splitting between the spin-up and spin-down bands cannot be described by a rigid splitting of the nonmagnetic band. It has been found that the width of 3d band of Mn(II) atoms is narrower than that of Mn(I) atoms and the hybridization between the 3d states of Mn(I) and Mn(II) is relatively weak compared with that in the nonmagnetic state. The calculated magnetic moments are in good agreement with the observed values. The density of states for the ferrimagnetic state of Mn₂Sb as well as that for the ferromagnetic state of MnAlGe are compared with recent photoemission measurements.

A new scenario is proposed for a class of disordered interacting electronic systems undergoing a metal-insulator transition. The scenario applies in the absence of spin-flip mechanisms to systems that are not nearly ferromagnetic. At the phase transition both the spin susceptibility and the coefficient of the linear term in the specific heat diverge with identical exponents, and the electrical conductivity vanishes continuously with a different exponent. Approximate values are given for the critical exponents of transport and thermodynamic properties in three-dimensional systems.

The possibility of coherent splitting of a ballistic electron beam in a high-mobility two-dimensional electron gas (2DEG) system is discussed. The authors propose a new version of a planar quantum-point-contact device which is able to produce two coherent and highly collimated divergent beams. The performance of this device is studied by solving the Schrodinger equation of the 2DEG system.

The structure factor S(q) and the direct correlation function c(q) of rare gases are expected to have a nonanalytic mod q mod ³ term at small q. This term is due, in addition to the well known long-range r^-6 tail of the two-body interaction, also to the three-body Axilrod-Teller interaction. The coefficient c₃ of the mod q mod ³ term of c(q) has a particularly simple state dependence, T^-1 in temperature and linear in density, when the AT interaction is present. If no three-body interaction is present C₃ is density independent. Therefore the experimental determination of the density dependence of c₃ will give unequivocable evidence of three-body forces. On the basis of the integral equation method the authors determine that it will be easier to detect the mod q mod ³ term if the scattering data are analysed in term of c(q) and for measurements at density and temperature not far from the critical point. Under these favourable conditions scattering measurements in the range 1-4 nm^-1 will give a precise determination of the mod q mod ³ term. The role of retardation, which changes the r^-6 tail into r^-7, on the small-q behaviour of S(q) and c(q) is also evaluated and this effect is predicted to be measurable for q<0.5 nm^-1.

The authors have studied the influence of a long-ranged force between a wall and a fluid on the wetting properties of the wall-fluid system. The system with short-ranged forces, i.e. cut-off Lennard-Jones potentials only, has been studied extensively in previous simulations, revealing the presence of a wetting and a drying transition. The authors have increased the cut-off radius of the wall-fluid potential by a factor of 5 in the simulations and re-inspected these transitions. The wetting transition turns out not to be much affected by the long-ranged potential but the drying transition is suppressed.

The phase diagram of an extended Josephson junction in the magnetic field-temperature plane is constructed. It contains a floating incommensurate phase, which corresponds to the usual intermediate state, high-order commensurate phases and a liquid-crystal-like disordered phase. It is shown that the disappearance of the Josephson effect is a weak first-order phase transition connected to bulk superconductivity breaking. The influence of defects is also considered.

The authors consider the defect contribution to phase transition anomalies in various physical quantities in the situation when the system is quenched from the high-symmetry phase. The main emphasis is placed on the contribution of the metastable domain structure arising at the time of quenching. It is found that the formation of the domain structure may cause a sharp change in the frequency dependence of the sound absorption coefficient as well as the emergence of a narrow nonLorentzian central peak in the inelastic light scattering spectrum. The recent conclusion that the temperature dependence of the elastic light intensity during cooling from the high-symmetry phase through the phase transition temperature is monotonic is supported but also amended.

The authors have performed an experiment on the modulus exponent of porous plates of plexiglas, aluminium steel and brass which covers materials used in most previous measurements by others with similar sample preparation, but different sample thicknesses. Comparisons of the results from this experiment with those of previous work show that the elastic exponent is highly thickness sensitive. Therefore, when measurement of elastic exponent is carried out in order to compare with theoretical predictions, one must make sure that such a thickness effect is negligible.

Ultrasonic attenuation has been measured as a function of temperature approximately over the range 10-300 K at 5, 10, 15 and 45 MHz in a single crystal of a Pd₈₅Pt₁₅ alloy. The measurements have been carried out for five different propagation modes, both in the H-free and the H-loaded (H/(Pd+Pt)=0.29 at.%) state of the material. An anelastic relaxation has been observed at around 225 K (f=10 MHz), whose activation energy W and limit relaxation time tau ₀ are: W=0.24+or-0.02 eV; tau ₀=(1.4+or-0.5)*10^-14 s. The mode dependence of the relaxation strength reveals the existence of a hydrostatic relaxation due to reactions between dipoles associated with H-H and H-Pt complexes. The results can be accounted for in terms of stress-induced changes in the short-range order of both the H-H and the H-Pt bonds (or antibonds) as well as of the occupancy probabilities of octahedral interstitial sites by H.

The temperature dependences of the first moments of phonon Raman scattering lines are studied for Fe_1-xZn_xF₂. The diamagnetic dilution diminishes the spin-phonon coupling strength of phonons with both A_1g and E_g symmetry in proportion to the decrease in the ordering temperature T_N. Moreover, the E_g mode shows an asymmetrical lineshape for Zn concentrations x>0.3. It is explained by an unresolved superposition of two modes, one of which is ascribed to perturbed ZnF₂ with broken translational symmetry.

A two-dimensional system of classical dipoles on a triangular lattice with isotropic in-plane ordering is studied both in the spin-wave approximation and by Monte-Carlo simulation. It is shown that the mean-square displacement ( theta ²), unlike in the two-dimensional XY model, is finite in this system in the thermodynamic limit-not divergent logarithmically. This boundedness of ( theta ²) is caused, as demonstrated, by the long-range nature of the potential.

The order-disorder phase transition which occurs in YBa₂Cu₃O_7-x is studied by the Monte Carlo method. The results are in agreement with those obtained by the modified cluster variation method. At x<0.5 and low temperatures the phase described by the reduced wavevector (¹/₃, 0, 0) is obtained on the phase diagram. The problem of establishing thermodynamic equilibrium in this system is also discussed.

The effect of hydrostatic pressure on structural phase transitions in some fluorotitanates and fluorosilicates of the ABF₆.6H₂O family is studied. It is shown that it is possible to divide the crystals of this family into two groups differing from each other in their values of entropy change and shifts of the phase transition temperature T₀ under pressure. In some compounds the high-pressure phases and triple points were established. The phase diagram for FeSiF₆.6H₂O can be considered as a unified diagram describing the possible phase transitions in the crystals studied. The effect of deuteration is investigated in different phases.

Electronic ground-state properties of planar, periodic, infinite polythiophene chains are calculated using a first-principles density-functional method. The author concentrates on a set of geometries differing in the size of the carbon-carbon bond length alternation along the backbone, thus studying a transition between a quinoid and an aromatic structure. Whereas the lowest unoccupied orbitals are found to have pi symmetry the author finds valence energy bands of sigma symmetry unusually close to the Fermi level. It is demonstrated that the valence band structures offer a consistent interpretation of experimental photoelectron spectra. An important result is the lack of a band-gap closure when passing from the aromatic to the quinoid structure, although the band gap is diminished. The results are used in deriving a model Hamiltonian with which the author subsequently studies the pi -electron density of states for solitonic and polaronic excitations. The above-mentioned lack of band-gap closure leads to the absence of near-mid-gap states for solitons. Polarons induce states asymmetric in the gap and, comparing with experimental results for doped and photo-excited polythiophene, the author estimates the width of the polaron to be large (of the order of eight thiophene units which, however, might be slightly modified when including correlation effects), and suggests that bipolarons and not polarons are the charge carriers in doped polythiophene in the dilute limit.

The authors study the influence of electron-phonon coupling on the resonant dynamical Stark effect in Cu₂O. They start from a multi-band density matrix theory involving directly forbidden dipole transitions and directly allowed quadrupole transitions. The use of the multi-band formalism is necessary for the inclusion of the observed polarization effects. From a microscopic treatment of the electron-phonon interaction they derive a frequency-dependent transverse relaxation for the excitons and a renormalization of the optical transition matrix elements resulting in a Fano effect. They derive an expression for the differential transmission, taking into account a parametric sum frequency generation. Theoretical and experimental data are compared, showing satisfactory agreement.

With both longitudinal optical (LO) and surface optical (SO) modes included, the influence of electron-phonon interactions on the quantum confined Stark effect in quantum wells is investigated by means of the variational technique. For an infinite GaAs-GaAlAs quantum well with a shallow donor located at its centre and subjected to an external electric field, the energy shifts of a conduction electron are calculated approximately. It is found that the presence of an ionized impurity decreases the energy shift. The LO mode and SO mode effect will give significant corrections to the Stark energy shift individually and their influences on the shift are just opposed.

The localization of eigenfunctions in finite samples of the 1D Anderson and Lloyd models is quantitatively described by the information length. This quantity is numerically investigated for both models and it is found to scale with the size of the sample and the disorder according to a simple law.

Excitation measurements of the ²E(D)-²T₂(D)Cu²⁺ luminescences in ZnS and CdS crystals are presented, revealing new excitation processes of the centres. In ZnS sharp resonances observed on the low energy onset of the charge transfer band at 1.17 eV enable one to identify a transient shallow acceptor state of the Cu²⁺ centre in ZnS. An energy transfer process by photoexcited holes between Cu²⁺ and Fe²⁺ centres yields an accurate value for the deep Cu²⁺-acceptor position in ZnS of (1.293+or-0.005) eV. In CdS the deep Cu²⁺-acceptor position is (1.20+or-0.02) eV above the valence band. Additional excitation bands just below the excitonic bandgap are observed and interpreted as transitions between the conduction band and the transient shallow acceptor state. The binding energies of these states are determined to be 119 and 94 meV for ZnS and CdS, respectively. The recombination energy of the transient shallow acceptor state is transferred efficiently to the excited ²E(D) state, leading to the internal Cu²⁺ luminescence.

The authors have performed the realistic tight-binding band calculations for Fe/Cr/Fe sandwiches by taking into account the interface randomness within the coherent potential approximation. It is shown that the interface randomness yields significant modifications in electronic and magnetic structures near the interface. Magnitudes of local magnetic moments on Cr and Fe atoms near the interface become larger by increasing the degree of the interface randomness. They discuss the implications of their calculation for the magnetoresistance recently observed in Fe/Cr multilayers.

The magnetoresistance of three-dimensional amorphous metals with strong spin-orbit scattering is often difficult to fit to present theories of weak localization and electron-electron interaction. Especially, the fitted values are sensitive to various assumptions in the fits and only quite wide ranges for the parameters controlling the phenomena can be given. Two methods to extract the parameters in a less sensitive way are discussed. A good agreement with theory is found. The principal behaviour for the studied strong spin-orbit scattering system is identical to what is found in amorphous metals with weak spin-orbit scattering.

The non-linear properties of mechanical normal-superconducting contacts are investigated. It is shown that the near-boundary superconductor conductivity jump due to direct current through the heterosystem would induce not only the known N-shaped non-linearity in I-V characteristics (Ta-(SnPb) heterosystems) but also the S-shaped non-linearity in I-V characteristics (In-Bi heterosystem). This conclusion follows from the conditions required to observe self-oscillations. The nature of the non-linearity in the In-Bi heterosystem which is non-traditional for conductors is analysed using self-oscillation parameters.

Detailed studies on the mid-infrared-active phonon spectra of YBa₂(Cu_1-xM_x)₃O_{7- delta} ; M=Co, Fe and Zn for 0<or=x<or=10%, have been carried out from 30 K to room temperature. The phonon mode near 580 cm^-1 softens at the superconducting transition temperature (T_c), as reported earlier. The new observation is a marked difference in the temperature dependence of the phonon in Co- and Fe-doped compounds as opposed to Zn-doped and undoped material. The authors observe that the phonon begins to soften at temperatures well above T_c-in some cases approximately 2T_c( approximately=180 K)-in the Co- and Fe-doped YBa₂Cu₃O_{7- delta} , whilst no such behaviour is detected in Zn-doped and undoped samples. They argue that this precursor phonon softening above T_c in Co- and Fe-substituted samples is due to superconducting fluctuations. Their proposal is that the transition-metal-ion substitution affects the dimensionality of the system and the ramifications of this are crucial for explaining the observed differences between Zn/undoped YBa₂Cu₃O_{7- delta} and Co/Fe-doped compounds. Their observations cast doubt on the idea that 2D superconductors like YBa₂Cu₃O_{7- delta} show a Kosterlitz-Thouless behaviour. Instead, it appears that T_c to 0 K and fluctuations are reduced for the fully decoupled layers.

The derivative of the microwave absorption versus DC magnetic field strength is observed near zero field in the superconducting state of single crystals of Bi₂Sr₂CaCu₂O_8+x. The absorption intensity decreases as the DC magnetic field is rotated away from the crystallographic c axis, which indicates preferential orientation of weak link current loops parallel to the largest crystal facet. The hysteresis of the low field absorption is also anisotropic because of the anisotropy of flux pinning barriers. Direct microwave absorption measurements at higher fields show a continued increase in absorption above 0.1 T which is associated with flux motion. The temperature, field and orientation dependence of this absorption are reported and interpreted in terms of flux motion. Barriers to flux motion in the surface regions of the crystal are estimated.

A perturbed gamma gamma angular correlation (PAC) study of the electric quadrupole interaction (QI) of ¹¹¹Cd probes on substitutional V sites in the C15 Laves phase HfV₂ has revealed a pronounced thermal hysteresis of the QI and the ¹¹¹Cd fraction on regular V sites: when samples are cycled through 700 K in vacuum, the quadrupole frequency nu _Q on the way down is found to be systematically larger than that on the way up at the same temperature. Simultaneously a sharp irreversible drop of the regular site fraction occurs upon cooling from 700 K. This behaviour indicates that HfV₂ reacts with water vapour at T<or=700 K, dissociating residual H₂O into its atomic components. HfV₂ acts as a strong getter for the hydrogen, which is absorbed upon cooling below 700 K and again desorbed at T>800 K. The oxygen, however, is absorbed only in trace amounts. These conclusions are supported by a mass spectroscopic desorption measurement and a comparative PAC study of QI in the hydride HfV₂H_0.1, for which the activation energy for the hydrogen diffusion has been determined to be E_a=0.070(25) eV.

The two-state Ising model with nearest-neighbour bilinear exchange constant J⁽²⁾, three-spin coupling constant J⁽³⁾ and four-spin coupling constant J⁽⁴⁾ is studied using the effective-field approximation. The temperature variation in spontaneous magnetization is studied for different coordination numbers z and for various values of the parameters alpha =J⁽³⁾/J⁽²⁾ and alpha '=J⁽⁴⁾/J⁽²⁾. It is seen that, in the absence of three-spin coupling, there exists a critical value of alpha ' (say, alpha '_c) above which the magnetization becomes double valued at and beyond the Curie temperature T_c, forming a bulge near T_c which indicates the occurrence of a first-order transition. The expressions for alpha '_c for z=3, 4, 6 are derived and it is found that alpha '_c increases as z increases. In the absence of four-spin coupling it is found that first-order transitions appear for alpha >or=0. The magnetization curves are also studied for the case when all the interactions are present. The results are discussed with reference to those obtained from the molecular-field approximation.

The influence of phonon fluctuations on the dynamics of kink-like domain-wall excitations in the one-dimensional Ising model with a transverse field (s=¹/₂) was investigated. It was found that such non-linear excitations behave almost like classical Brownian particles. This is the consequence of the emission and absorption of acoustic phonons excited by domain walls propagating through the vibrating lattice. The effective friction and diffusion constants are also evaluated. It was established that the overall process has the character of Cherenkov-type radiation.

The electron paramagnetic resonance (EPR) spectrum of descloizite (Pb(Zn,Cu)(VO₄)(OH)) has been studied at room temperature using conventional methods. The powder and single-crystal spectra revealed hyperfine structure of Cu²⁺ in axial site symmetry. The measured values g/sub ///=2.023, g_{perpendicular to} =2.222, A/sub ///=110*10^-4 cm^-1 and A_{perpendicular to} =-56.2*10^-4 cm^-1 indicate that the Cu²⁺ ion is sited in a compressed octahedron (D_4h) so that ²A_1g is the ground state. The experimental data were used to determine the constants P and K.

An EPR study of a new S₂^- defect in RbCl is presented. A comparison with the S₂^- centre in RbBr, previously found by Vannotti et al. is made. For both salts a clearly resolved superhyperfine structure was observed, which has not yet been reported. The interpretation of the spectra was complicated by the simultaneous presence of the ⁸⁵Rb (I=5/2) and ⁸⁷Rb (I=3/2) nuclei. An excellent quantitative agreement between simulation and the experimental data was obtained, using only two parameters, i.e. the linewidth for a single superhyperfine component and the ⁸⁵Rb (I=5/2) splitting. The analysis is based on the interaction of the unpaired electron on the S₂^- ion with four equivalent Rb nuclei. The model is consistent with an S₂^- ion replacing a single Cl^-, respectively Br^-, ion.

The authors have studied the propagation of pseudosurface acoustic modes on the (11(mean)1) plane of GaAs. From a calculation of the surface density of phonon states, two resonances are found in the continuum of modes, corresponding to the pseudosurface acoustic modes, called PSM and HFPSM. Experimental evidence for these modes is given by Brillouin spectra taken for different directions on the (11(mean)1) plane of a GaAs crystal. They found a good agreement between the experimental spectra and the theoretical cross sections, calculated taking into account both the ripple and the elasto-optic contributions to the scattering process.

The influence of the changes in the structure of near-surface region of a lithium niobate Z-cut single crystal grown from a congruent melt on the characteristics of the low-energy cathodoluminescence has been investigated. It is shown that radiation spectra strongly depend on the structure of the near-surface region. The luminescent properties of the crystal annealed in air at 950 degrees C in contrast with those of the polished crystal are found to be stable against the bombardment of the crystal by electrons with energy at least up to 1.3 keV. The effect of electron bombardment of the crystal face on the radiation spectra taken from the opposite face was observed.

An empirical pseudopotential calculation has been carried out to study the anisotropy of Compton profiles between (100) and (111) directions of GaP. Compared to a purely local potential an l-dependent non-local potential is seen to explain the experimental anisotropy rather well. The contributions of the first and the second bands to the total measured anisotropy are found to cancel each other. This is found to be true even for Si. This means that the measured Compton profile anisotropies in compound and elemental semiconductors cannot be viewed in terms of the differences in the nature of the lowest band. Core-orthogonalization correction was found to have negligible effect on the Compton profiles.

The surface core-level shift of the Pd(100) single-crystal surface has been measured from high-resolution Pd 3d core-level spectra. The surface atoms are found to have 0.44+or-0.03 eV lower binding energy than the bulk atoms. The result is compared with theoretical estimates.

The isothermal relaxation process in a beta -Cu-Zn-Al single crystal which follows quenches from different T_q temperatures has been studied by means of an isothermal calorimetric technique at three T_a temperatures. The relaxation has been attributed mainly to a reordering process which takes place through a vacancy mechanism. The dissipated energy during the process has been measured for the different T_a and T_q temperatures. The authors have found that the relaxation process is not of a single exponential type and that, in all cases, the recorded curves giving the dissipated thermal power, W(t), against time show long tails. The kinetics of the process have then been characterized by means of a relaxation time tau , defined as the time at which the product tW(t) versus time shows a maximum. Assuming an Arrhenius law dependence of tau on T_a and T_q, they have obtained effective activation energies of 0.76+or-0.03 eV and 0.43+or-0.03 eV for vacancy migration and formation respectively.

EELS experiments of H₂ physisorbed on noble metals at low temperature have indicated that the ortho-para (O-P) conversion can be very fast on non-magnetic catalysts. Two processes are described in which the molecular nuclear angular momenta are dissipated by the emission of metal electron-hole pairs through a metal-molecule spin-orbit coupling. Both processes rely on electron momentum transfer to the metal but in the second one the virtual electron jump-back and forth from the metal to the molecule as induced by the electron Coulomb interaction-considerably strengthens the O-P rate and brings it close to the experimental values observed on Cu catalysts.