Table of contents

Using neutron diffraction, the first direct experimental evidence is given that ⁴He does not solidify continuously on the surface of MgO. This result is confirmed by isopycnals measured in the vicinity of the bulk melting curve. Thus the importance of substrate symmetry for continuous nucleation is illustrated.

The atomic dynamics of the metallic glass Mg₇₀Zr₃₀ and the same sample after crystallisation was investigated at room temperature using neutron inelastic scattering techniques. For the first time the dispersion of short-wavelength collective density excitations could be measured in the glassy metal for a few momentum transfers near the first maximum of the static structure factor. From the intensity distribution in the spectra the authors conclude that the modes observed are transverse collective density excitations in the glassy system (transverse phonons in the polycrystal). They compare their findings with molecular dynamics and analytical calculations on the same system.

A double-minimum potential, constructed by placing two Morse functions back to back, is used to calculate the single-particle proton energy levels in KDP. The potential well parameters are estimated from infrared and crystallographic data. The temperature and pressure dependence of the proton tunnelling frequency is computed. The soft-mode Raman scattering data of Percy (1973) are re-analysed, allowing for the temperature dependence of the tunnelling frequency and for its modulation by the oxygen-oxygen thermal motion.

Superfluid ³He-A in a torus is modelled by using a geometry which permits azimuthal superflow. Stable stationary textures are found in the presence of a magnetic field by numerically solving the equations of motion of l.

Conduction electrons in the low-temperature phase of crystalline WO_3-x (x>or approximately=0.0001) are found to be paired diamagnetically, forming bipolarons due to joint lattice distortion. They can be dissociated with light in a broad band of wavelengths peaked at 1.1 eV, as monitored by the appearance of W⁵⁺ ESR and a corresponding change in DC conductivity. The transport activation energy is 0.18 eV in the paired state and decreases upon dissociation. The bipolaron binding energy is larger than 0.36 eV. Separated electrons recombine in a bimolecular process controlled by diffusion in the conduction band.

The experimental results of Henry and Lang (1977) for the B centre in GaAs are compared with theoretical calculations of the multiphonon capture rate, the radiative capture rate and the Auger recombination rate, made within the static coupling scheme in an effective phonon energy dipole approximation. It is found that a theoretical fit, fully consistent with the experimental data, is not possible on the basis of the multiphonon capture mechanism alone, in contrast to the work of previous authors, although this remains the dominant mechanism around room temperature and above, where the capture cross section is strongly temperature-dependent. At low temperatures the dominant contribution is that of an Auger process. In particular, a good fit of both the magnitude and the temperature dependence of the cross section is found for an electron concentration of 10¹⁶ cm^-3, an effective overlap mod FF mod ² approximately 3*10^-3, an effective phonon energy of 0.018 eV and a lattice adjustment energy of 0.238 eV: values which are shown to be in good accord with what little evidence is available.

The author reports dark-current measurements in single-crystal PDA-TS in the traps-filled limit (TFL) for conduction perpendicular to the polymer chain direction c. If the value thus obtained for the electron mobility perpendicular to the chain is taken together with previously measured values of the ratio of the mobilities for motion perpendicular to and along the chain direction, broad agreement is found with the values for the mobility along the chain direction as determined by Reimer and Bassler (1978) and by Chance et al. (1978) but not with the much higher values obtained by Donovan and Wilson (1979).

Crystals of yttrium aluminium garnet (YAG) emit a broad luminescent band peaking at 270 nm under X-irradiation at 4K. Exploitation of the magneto-optical properties of this emission makes possible measurement of EPR spectra of an excited state of the emitting centres. It is suggested that at least three different centres are responsible for the 270 nm emission and the relevance of the results to rare earth-doped YAG phosphors is discussed.

The binding energies of all core levels in the N and O shells for the elements Hf to Bi have been measured by means of electron spectroscopy. The binding energies obtained are compared to previously reported values.

The EPR characterisation of p-type CuInSe₂ single crystals shows them to be predominantly contaminated by Fe; the Fe defect is present as a cation-substituted Fe³⁺ ion. The spin Hamiltonian parameters of this defect were determined at T=4.2K to be mod g/sub /// mod =2.040, mod D mod =0.3044 cm^-1 and mod a+²/₃F mod =0.0172 cm^-1.

Disorder effects in amorphous GaAs and GaP are investigated by EXAFS measurements at the K edge of Ga and As for GaAs and of Ga only for GaP, performed with the DCI synchrotron radiation at LURE. The samples are unannealed films with controlled stoichiometry prepared by flash-evaporation under ultrahigh-vacuum conditions. In the case of GaAs, the EXAFS data are analysed by a classical Fourier filtering technique, using crystalline powders as a reference material. The results are compared with those obtained in the same experiments on evaporated amorphous Ge films. No contribution from second neighbours can be detected in either set of amorphous data. The mean first-neighbour distances around Ga and As absorbing atoms are found to be identical, in contrast to previous observations, and larger than the crystalline distance by 0.01 AA only. The spread of these distances around their mean value, deduced from the EXAFS amplitudes, supports the conclusion that the amorphous GaAs samples are chemically ordered. In the case of GaP, the EXAFS data are compared with computed model curves constructed within different assumptions about chemical disorder characteristics. From the study of the amplitude, there is some indication of the presence of Ga-Ga wrong bonds, with a length slightly larger than the Ga-P normal bonds. The difficulties in EXAFS analysis related to disorder are discussed.

Measurements of the effects of hydrostatic and uniaxial pressures on ultrasonic wave velocities are used to obtain the six third-order elastic constants of yttrium aluminium garnet (YAG). Results are used to investigate whether the shear isotropy and the conformity with the Cauchy relation (indicative of central forces), which hold for the second-order elastic constants of YAG, extend into the region of finite strain which determines higher-order elasticity. The compression has been extrapolated up to 150*10⁸ Pa using the Murnaghan equation of state. The vibrational anharmonicity of YAG and yttrium iron garnet (YIG) are compared through the acoustic-mode Gruneisen gammas computed from the pressure dependences of the elastic constants. The attenuation of transverse microwave ultrasonic waves propagated along the (001) direction (the mode used in acoustic devices) due to anharmonic interactions with thermal phonons, calculated at the limits omega tau >>1 and omega tau <1, is shown to be in good agreement with previous experimental attenuation measurements for this mode.

Methyl tunnel splittings and torsion frequencies have been measured by inelastic neutron scattering for seven acetates. The parameters of the hindering potential V₃ cos 3 phi +V₆ cos 6 phi where phi is the methyl rotation angle, were determined from the spectra. The ratio V₆/V₃ is similar (about 0.15) for six of the samples though sodium acetate appears anomalous. The dependence of this ratio on the form of interatomic potentials is discussed.

The authors measured the six lowest phonon dispersion branches in alpha quartz at room temperature for the ( xi 00) and ( xi xi 0) directions. The qualitative agreement with lattice dynamical calculations by Barron et al. (1976) is good. Among the quantitative differences the most striking appears in the acoustic modes in the ( xi xi 0) direction. The symmetric ('longitudinal') branch has a lower initial slope (4.70 km s^-1) and then one antisymmetric ('transverse') branch (5.4 km s^-1). This unusual behaviour, which appears at relatively small phonon wavelengths observable by inelastic neutron scattering is in contrast to the long-wavelength sound velocities calculated from elastic constants which give 5.724 km s^-1 for the symmetric and 5.114 km s^-1 for the upper antisymmetric branch. They explain this observation by the different sound velocities in the first and zero sound regimes.

A cubic field coupling to s mod s mod ², in n-component spin models induces a bicritical crossover from n-isotropic to Ising-like (m=1) critical behaviour for 1<n< infinity , but to classical behaviour in the limit n to infinity . The mechanism for this is explained and related to the van der Waals or infinite-range crossover. By following the analysis of Nelson and Domany (1976), the bicritical scaling function for the parallel susceptibility in d dimensions is obtained correct to order epsilon =4-d and for general (m,n) in a form correctly reproducing all limits; the effective exponent, gamma _eff, is examined numerically in various regimes.

The dielectric response of a quasi-one-dimensional system of interacting electrons has been investigated. In the authors' model the particles move freely in one direction and are tightly bound in the transverse direction. Calculation of the Coulomb matrix elements leads to an effective one-dimensional electron-electron interaction nu (q) which is less singular for small q than the 1/q² potential assumed by previous authors. As a result, the excitonic instability which they find even for a rigid positive background does not arise in their calculation. The calculations are based on the approximation of Singwi, Tosi, Land and Sjolander (1968). As expected in this approach, the pair correlation function at small distances remains positive over a much larger range of densities than that for which the compressibility sum rule is satisfied. The static dielectric function is negative around q=2k_F, which suggests the possibility of charge ordering when the background is allowed to be polarisable. If nu (q) is replaced by a delta -function potential, a numerical calculation of the energy for the exact solution of Yang may be made. On comparison with the result obtained with the method of Singwi et al. it is found that the approximate scheme gives good correlation energies for high densities.

An explanation of the three photon-assisted bands observed in Ag⁰:KCl and Ag⁰:KBr is reported. The two lowest energy peaks are assigned to 4d¹⁰ 5s to 4d⁹ (e_g) 5s² and 4d¹⁰ 5s² to 4d⁹ (t_2g) 5s² transitions respectively while the remaining peak is assigned to the 5s to 6s transition of free Ag⁰. This assignment allows the measurement of the cubic field splitting Delta for the internal 4d orbital in the unrelaxed 4d⁹ 5s² configuration. Such a value ( Delta approximately=0.6 eV) is related to the expected screening effects of the two 5s electrons upon the 4d level. It is shown that the intensity of the three bands is obtained through a T_1u vibration mode and that while in the case of the 5s to 6s transition the intensity is likely to be stolen from the 5s to 5p transition. The intensity source for the two other peaks however can only be the t_1u⁶ ( pi ) 4d¹⁰ 5s to t_1u⁵ ( pi ) 4d¹⁰ 5s² charge transfer transition. All the observed levels of Ag⁰:KCl and Ag⁰:KBr have been placed within the band scheme of the host lattice, showing that the 6s level is located within the conduction band but above the vacuum level. Finally, following the estimations of Harrison for the band gaps of alkali halides, a simple explanation for the position of the 5s level of Ag⁰:KCl is given.

The zero-field splitting parameter D of the ⁶S_5/2 ground state of Mn²⁺ ions with Cl^- ligands is discussed. Ab initio calculations of D, based on the relativistic spin-spin and Blume-Orbach splitting mechanisms, and an electrostatic model of the crystal field, are in poor agreement with experimental measurements for Mn²⁺ ions in a representative group of six chloride host compounds: CdCl₂, MgCl₂, CsMgCl₃, CsZnCl₅, TMCC and (CH₂NH₃)₂CdCl₄. In contrast, the empirical superposition model gives a consistent account of the experimental data for six-fold coordinated Mn²⁺ ions in this group of compounds. The dependence of the intrinsic parameter b₂ on ligand distance R is found to be b₂=-0.235 (R-R₀) cm^-1, with R₀=2.692 AA, suggesting that the zero-field splitting involves a cancellation effect between two competing mechanisms. The superposition model is used to predict zero-field splittings of D=+0.019 and +0.036 cm^-1 in the low-dimensional antiferromagnets TMMC and (CH₃NH₃)₂ MnCl₄ respectively.

The coherent potential approximation is applied to a study of the AC electrical conductivity of substitutionally disordered off-diagonal binary alloys with the BCC host lattice. The validity of the concept of the optical relaxation time for intraband transitions is discussed. It is found that anomalous behaviour of the optical conductivity is observed in cases where the Fermi level of the system lies in the minimum of the DC conductivity.

The de Haas-van Alphen (dHvA) effect has been studied in the refractory compound TiB₂ using a field modulation technique. Observed dHvA oscillations in the (1010), (1120) and (0001) planes have frequencies of the order of 10⁶ G. The angular dependence of the dHvA frequencies closely resembles that of an electron Fermi surface (FS) of semi-metallic ZrB₂, though the frequencies of the former are smaller than those of the latter. It is proposed from an analogy with ZrB₂ that TiB₂ is a semi-metal and has a ring-like electron FS around the K point on which nearly ellipsoidal FS are joined together. The oscillations corresponding to the hole FS could not be observed due to the sample limitation. The carrier concentration of electrons calculated from the observed FS was estimated to be about 0.023 per unit cell.

Measurements of the temperature dependence of the resistance of high-purity specimens of n-type InSb as a function of magnetic field are reported in the ranges T=4.2-0.3K and B=0-7.5 T. Unless the specimen is suitably etched, the huge increase of resistance with increasing magnetic field and falling temperature is limited by surface conductance, sometimes to a maximum resistance as low as approximately 100 k Omega . In this respect etching in H₂O₂/HF solution is more effective than in CP4-A. Bulk conduction is found to be governed by two activated components. The first is attributed to freeze-out of carriers from conduction band to donor states and the second to thermally activated hopping via donor sites. Energies controlling freeze-out seem to correspond to the transition 1s to 1p₀ rather than 1s to CB. Hopping energies of a few tenths of a meV increase with magnetic field but slowly decrease with doping. As expected, the pre-exponential hopping parameter is a strong function of magnetic field and doping. Below approximately 0.8K, the temperature dependence of the hopping component becomes weaker in accordance with the increasing importance of long-range hopping. Surface conduction seems to be almost entirely controlled by an activation energy that depends strongly on the particular surface condition.

The extended Hubbard model which includes nearest-neighbour interactions has been studied including the finite-lifetime effects. It has been shown that the problem in general is equivalent to a three-component alloy problem with wavevector-dependent self-energy. In the special case of small intersite-correlations, the solutions have been shown to be equivalent to the coherent potential approximation for the ternary alloy problem. The alloy problem has then been solved to obtain the effect of intersite correlations on the metal-insulator transitions.

The width and shape of the conduction electron spin resonance line in InSb is investigated theoretically. The free-electron dynamic susceptibility is obtained by using a stochastic approach, with both spin and orbital relaxation processes taken into account within the relaxation-time approximation. The line is broadened not only by the spin relaxation mechanisms (electron-impurity and electron-electron spin-flip scattering, as well as the Dyakonov-Perel process due to the inversion asymmetry of the crystal), but mainly by the dependence of the g-factor on electron energy. It is shown that the non-elastic electron transitions due to electron-electron collisions narrow the line by one order of magnitude and lead to the symmetric line shape. Thus, the interaction among electrons plays an essential role in the spin resonance phenomenon in small-gap semiconductors. The agreement of the collision narrowing theory with the experimental data is good.

The early formation and subsequent growth of evaporated silver films on atomically clean cleaved InP (110) surfaces have been probed using low-energy electron diffraction, Auger spectroscopy and angle-resolved photoelectron spectroscopy. Experimental data obtained would suggest that the silver film at room temperature grows initially as islands or rafts with possible nucleating sites at surface steps. Facet formation of the silver film is found to occur where mainly silver (111) facets are obtained. The formation of the Schottky barrier has been probed using photoemission and it is found that in this system the barrier is largely formed for a silver coverage which is equivalent to only about 10-20% of the surface. The abruptness and adhesion at the interface has also been considered. Strong adhesion is obtained for silver deposited at room temperature on the atomically clean cleaved InP (110) surface. This strong adhesion has been attributed to interdiffusion at the interface and for the etched InP surface the oxide layer presents leads to a considerable reduction of the interfacial mixing at room temperature. The mechanisms influencing the Schottky barrier formation have been considered and the defect model possibly involving indium vacancies initiating at step edges is thought the most likely reason for the pinning of the Fermi level at around 0.5 eV below the conduction band edge for silver contacts on the atomically clean cleaved InP (110) surface.

Detailed measurements are reported of the variation of spin-dependent resonance (SDR) signals at Si surfaces as a function of modulation frequency, light intensity, temperature and microwave power. These cannot be accounted for by the mechanisms assumed in current theories. A new model is proposed in which the resonant variation of the photoconductivity is explained in terms of two mechanisms, spin-dependent recombination and spin-dependent trapping, taking place at multivalent centres. An expression is obtained for the SDR signal, which for the first time is capable of explaining its observed magnitude. The model allows the calculation of the number of paramagnetic centres in different experimental conditions. Its validity is tested by comparison with experimental results and by the consistent results obtained in the calculation of capture coefficients in various experimental conditions.