Table of contents

We report the first study of interfacial wetting at the liquid-vapour surface of metallic gallium-bismuth alloys approaching the miscibility gap. Ellipsometry has been used to probe the interface continuously with increasing temperature along the coexistence curve. Below the monotectic (eutectic) temperature T_mono a liquid Ga-rich phase is in equilibrium with solid bismuth and the real part epsilon ₁ of the complex dielectric function measured at the liquid surface clearly reflects Ga-like behaviour. Above T_mono the second liquid Bi-rich phase becomes stable and epsilon ₁ changes from Ga-like to Bi-like behaviour. We conclude that the Bi-rich phase, which has the higher mass density, now covers the low-density phase. This can be consistently interpreted as complete wetting of the interface between the vapour and the Ga-rich phase by a coexisting Bi-rich layer.

Electron energy loss spectra have been investigated for Sc, Ti, V and their oxidized surfaces. The position and shape of the measured peaks associated with the 3p electron excitation are found to be consistent with 3p⁶3d^a4s^m-3p⁵3dⁿ⁺¹4s^m transitions calculated for isolated Sc, Ti and V ions.

We report ESR measurements of K_xKCl_1-x melts at 800 degrees C which for the first time have been studied over a wide composition range approaching the nonmetal-metal transition (10^-4<or=x<or=10^-1). The concentration has been varied precisely by in situ Coulometric titration. The observed motional linewidth narrowing gives evidence that fast electronic exchange processes prevail in the nonmetallic melts. The spin susceptibility exhibits strong deviations from a simple Curie limit and its magnitude strongly indicates that only a small fraction of excess electrons exists in paramagnetic states. Both the spin dynamics and the spin susceptibility show clear changes near x=0.05 indicative of the onset of a Mott-Anderson transition. Over the whole nonmetallic regime the ratio of spin susceptibility and electronic conductivity is found to be constant.

The lattice location of Fe atoms in Fe-doped LiNbO₃ is determined experimentally employing X-ray standing waves. At a bulk concentration of 7.48*10¹⁸ cm^-3 Fe is found to occupy sites close to the ferroelectric Li position; shifted by 0.18 AA in the direction of the -c axis. Electronic structure calculations confirm this result insofar as computed spectroscopic data are more consistent with the respective experimental values for Fe located in the vicinity of the Li site than for Fe occupying the Nb lattice position.

Lithium oxide has been suggested as a suitable breeder blanket material for fusion reactors. Tritium ions and lithium vacancies are created by neutron irradiation, forming bulk defect complexes whose exact character is experimentally unclear. We have used ab initio total energy pseudopotential methods to study the structure and relative energies of tritium as a substitutional defect, and of the separate tritium interstitial and lithium vacancy. For all stable defect geometries, the formation of an OT^- complex with an O-T bond length of about 1 AA is found to be energetically favoured. In the case of the substitutional defect this bond is found to point towards the vacant Li site, but the direction is fairly free for the interstitial case. The binding energy of tritium to a lithium vacancy is found to be 1.3 eV. Structural relaxation effects are included throughout, and are found to significantly affect the relative energies of different defect geometries. The effects of zero-point fluctuations are estimated and found not to be very significant. The most probable migration path of interstitial tritium is identified as a jump between nearest-neighbour oxygen ions, with an activation energy of 0.45 eV, in agreement with experimental evidence. The results suggest a picture of thermally assisted diffusion of tritium interstitials and lithium vacancies along the anion and cation sublattices respectively, with the preferential trapping of the two defects into substitutional complexes.

The model of plastic deformation in amorphous systems based on the concept of pointlike structural defects is investigated in the three-dimensional case. The yielding limit is calculated and the propagation of shear bands is investigated. The relation between the structural defects and the low-frequency localized vibration modes is also discussed.

Incoherent inelastic neutron scattering spectroscopy was used to characterize the optic-vibrational density of states (DOS) of the octahedrally coordinated deuterium (D_o) and hydrogen (H_o) atoms in LaD_2.25, LaH_2.25 and LaH_2.03. The DOS exhibits a temperature- and concentration-dependent behaviour consistent with that observed previously for the analogous beta -TbH_2+x system. At low temperature, the H_o DOS for LaH_2.03 is fairly sharp with minor spectral sidebands, indicating that the H_o atoms are predominantly isolated, with some atoms residing in short-range-ordered domains. Increasing the H_o (or D_o) concentration to LaH_2.25 (or LaD_2.25) yields a dispersion-broadened bimodal DOS characteristic of the H_o (or D_o) I4/mmm long-range order that develops in the octahedral sublattice at low temperatures and these higher H_o (or D_o) concentrations. For LaH_2.25 at higher temperature (340 K), a broad, somewhat asymmetric DOS is suggestive of an H_o sublattice that is now largely disordered yet still possesses some degree of short-range order.

A semiconductor-metal transition occurring just below the Curie temperature T_C was observed in single crystals of perovskite La_1-xSr_xMnO₃ for x>or=0.175. For x<or=0.15 the crystals exhibit only semiconducting behaviour. These transport properties are closely connected with the electron transfer mediated by the double exchange interaction between Mn³⁺ and Mn⁴⁺ ions. The electronic state of manganese in these mixed systems is well illustrated by ⁵⁵Mn nuclear magnetic resonance study. A maximum in the magnetoresistance (MR) was observed near T_C for the crystals with x>or=0.13 whereas huge MR persists well below T_C for the x=0.10 sample, which is probably due to a non-collinear spin configuration.

A theoretical discussion of the retarded electromagnetic modes of an effective magnetoplasma film is described. The effective film is characterized by an effective dielectric tensor, and we deal with the Voigt configuration (propagation transverse to the applied magnetic field) for both a symmetric and an asymmetric film. The numerical results show that for a symmetric effective film, dispersion curves for both surface modes and guided waves are reciprocal, omega (+k)= omega (-k), but the field profiles are different for +k and -k. The dispersion curves for the asymmetric effective film are non-reciprocal, and the field profiles are different. Numerical illustrations are given for a GaAs/AlAs superlattice.

A new method of investigation of the Auger ionization probability, based on the analysis of the static characteristics of a tunnel MIS emitter Auger transistor, is proposed. The main advantages of this method are monoenergetic electron injection and very simple energy control. The ionization probability (quantum yield) for silicon was first determined as a function of electron energy E_e in the near-threshold range. The Auger effect in Si is noticeable even for E_e approximately=1.2-1.5 eV. The data obtained in the present paper are in good agreement with some experimental and theoretical results published for E_e>2 eV.

Intersubband optical transition matrix elements between quantized hole states in semiconductor quantum wells, calculated from the 4*4 Hamiltonian, are derived, taking into account the position dependence of Luttinger gamma parameters. Furthermore, the transition matrix elements between states obtained within the axial approximation, from the 2*2 Hamiltonian, are also derived. Numerical calculations indicate the importance of taking the position dependence of Luttinger parameters into account for bound-free transitions.

We have investigated the field dependence of the magnetoresistance, Hall effect, thermal conductivity and thermoelectric power on Co-Ag granular films made by electron gun evaporation under ultrahigh-vacuum conditions. A large field dependence correlated with the giant magnetoresistance is observed in all the transport properties. The dependence is anisotropic for 42at.% Co-Ag between parallel and perpendicular fields. At zero field, a resistivity minimum is found that varies with annealing temperature. The behaviours of the magnetothermal conductivity and the magnetothermoelectric power are generally consistent with previous studies. The extraordinary Hall resistivity exhibits an anomalous field dependence and changes sign with temperature.

Data on the Raman scattering of F and F⁺ centres in CaO are recorded. Symmetry assignments of the Raman effect of F⁺ centres are in agreement with the F⁺ emission spectrum measurements of Hughes, Pells and Sender. Furthermore, polarization results show that the frequency separation between T_2g and E_g modes is about 17 cm^-1. The vibrational modes of F centres are also discussed.

First- and second-order Raman spectra of the Al_xGa_1-xSb alloy for x=0.14 have been measured. High-resolution Raman spectra performed at 80 K allowed us to resolve the TO-LO splitting of both GaSb-like and AlSb-like modes. The values of TO-LO splitting we measured suggest the occurrence of charge transfer from the Ga-Sb bond to the Al-Sb bond, which in this alloy is mainly governed by the different cation electronegativities. The AlSb-like LO peak is substantially broader than its GaSb-like counterpart, reflecting a much higher degree of disorder in the AlSb sublattice as a consequence of the low Al concentration of the sample studied in this work. In the second-order Raman spectrum, three frequency regions can be clearly distinguished, containing, respectively, overtones and combinations of GaSb-like modes, combinations of GaSb-like and AlSb-like modes, and overtones of AlSb-like modes.

The oxygen isotope shifts of the vibrational modes of interstitial oxygen in silicon, obtained from isotopically enriched silicon samples, are compared with the predictions of a calculation based on a bond-centred location of the O atom, as is obtained from first principles. This comparison confirms the existence of a Raman-active mode in which the O atom is weakly involved and of a resonance producing a broadening and a small shift of the ¹⁷O asymmetric mode.

XAFS measurements of the Ga K edge and the Nb K edge have been carried out for amorphous Fe₇₃Ga₄Nb₃Si₁₁B₉ alloy after annealing at various temperatures in order to reveal the roles of Ga and Nb in the nanocrystalline formation. XAFS results show that: there is no definite Ga segregation, unlike Cu in Fe₇₃Ga₄Nb₃Si₁₁B₉ (FINEMET) XAFS spectra of Ga for the samples annealed above the alpha -iron precipitation temperature, T_x1, indicate that Ga atoms sit substitutionally at bcc iron sites. Curve fitting analyses of the XAFS of Ga show that coordination number after annealing drops at T_x1 but it rises again above T_x1, which suggests that at the beginning of alpha -iron precipitation, Ga atoms are inclined to be excluded toward the boundary where B and Si are much more abundant than other atoms. Such a transient exclusion effect of Ga can be attributed to the nanocrystalline formation of these alloys. Nb in Fe₇₃Ga₄Nb₃Si₁₁B₉ seems to play the same role as that in FINEMET, i.e. retarding crystal growth of alpha -iron and suppressing Fe-B compound formation.

Transformation matrices of phonon and phason strains under symmetry groups of two-dimensional (2D) quasicrystals (QCs) which are three-dimensional solids periodically stacked by aperiodic planes have been derived by using group representation theory. Quadratic invariants have been calculated for all 2D QCs of rank 5 and rank 7.