Table of contents

Reports an interatomic potential for lead fluoride which is based on the shell model of lattice dynamics. The potential provides a good description of the experimentally determined dispersion curves in the beta (cubic) phase, the measured Raman frequencies in the alpha -phase and the formation energy of anion Frenkel defects in this solid. The authors suggest the use of this potential for studying other anion transport properties in PbF₂.

The critical exponent beta at the normal-incommensurate phase transition in the mixed system (Rb_0.995K_0.005)₂ZnCl₄ has been determined from the behaviour of the primary satellite intensity. The measured value of 2 beta is 0.70+or-0.01, in good agreement with the two component isotropic vector model. The index is not affected by the replacement of a small proportion of the Rb ions in Rb₂ZnCl₄ by K ions.

(111) stress of sufficient magnitude to transfer all conduction electrons into a single valley has been applied to two Ge:As samples of densities 4.2 and 5.0*10¹⁷ cm^-3 at 4.2K. The Knight shift has been used to monitor the change in the density of states at the Fermi surface. The results support the idea that an impurity band is present at these densities.

On the basis of the elastic-body approximation including surface relaxation effects, the effect of size on the vibrational eigenfrequencies of a small particle is discussed. It is shown that softening of the eigenfrequencies does not occur with the decrease of particle size. The mean square vibrational displacement of a small particle is calculated to specify its size and temperature dependences. The calculated results agree well with the observed ones, which confirms the validity of the model of the lattice vibrations.

The authors study numerically the transition by breaking of analyticity which occurs in the incommensurate ground state of the Frenkel-Kontorova model (1938) when the amplitude lambda of its periodic-potential V(u) is increased beyond a critical value lambda _c. A brief review of the properties of this transition and its connection with the standard map is given. They consider four quantities which are critical when lambda goes to lambda _c from upper values: the gap in the phonon spectrum, the coherence length of the ground state, the Peierls-Nabarro barrier and the depinning force. The numerical method is discussed and the authors show in particular that the mapping method is unpracticable for lambda > lambda _c. They observe the transition by breaking of analyticity and show that in the stochastic region ( lambda > lambda _c) the ground state is never chaotic. Nevertheless in this region metastable chaotic states can be obtained. The numerical calculations, performed with a ratio of the atomic mean distance to the period of the potential V(u), l/2a=(3- square root 5)/2 equivalent to the golden mean, show that critical exponents can be defined for the four critical quantities studied. Their values reveal two scaling laws which are empirically explained.

Phase transitions in MnTiF₆.6H₂O and ZnTiF₆.6H₂O have been observed by studying their IR spectra at different temperatures. The librational modes of the water molecule change significantly around the transition temperature.

The evidence of non-linear terms in Landau's two-fluid theory of helium II, provided by some non-linear wave experiments, is assessed. The non-linear relationship between heat flux and wave amplitude is found. The standard discontinuity relations derived from the energy equation and superfluid equation of motion are compared with recent experimental results on shock waves.

The band structure of bis-(1,2-benzoquinonedioximato)nickel(II), Ni(bqd)₂, is investigated by means of crystal orbital calculations based on a semiempirical INDO model. The nature of the metal-metal interaction in the stack as well as the intercell coupling between the 3d centre and the ligand atoms is analysed in detail. There is no evidence for an attractive Ni-Ni interaction; stabilising contributions in the intercell potential are predicted for the coupling between the positivated Ni centres and ligand atoms with a surplus of charge. The net interaction energy is decomposed into increments of physical significance. A large number of ligand bands is predicted on top of the Ni 3d bands; strong metal-ligand mixing in the latter band set leads to significantly reduced 3d amplitudes in most of the transition metal bands (3d_z2, 3d_xz, 3d_yz). The partially oxidised Ni(bqd)₂ polymers belong to the class of organic metals, the charge transport is due to ligand functions of lone-pair and pi character.

Reports a self-consistent non-local pseudopotential calculation for the electronic structure of alpha -Sn. The method includes spin-orbit interactions. Based upon this method the author has applied the local-density-functional-momentum-space formalism of Ihm et al. (1979) to calculate the total energy, bulk modulus and equilibrium lattice constant of this material. In contrast to earlier results for the ground-state properties based on the self-consistent local pseudopotential method, the present calculations are found to be in better agreement with experiment.

The authors describe the first direct application of the configuration interaction method to a calculation of the optical spectrum of an extended solid. The two-particle spectrum of the valence and conduction bands is employed as the basis, and procedures are carefully chosen to maintain size consistency. A simplified description of LiF by these methods, using only on-site particle-hole interactions, already gives very good predictions for the optical spectrum including the main exciton resonance.

Deep level defects produced by quenching from 1175 degrees C silicon doped with Au, Ag, Fe, Cu or Ni have been observed using transient capacitance spectroscopy. In both Au- and Ag-doped samples a hole trap at E_v+0.48 eV was observed, with an electron trap at E_c-0.28 eV present in the Au-doped material. In quenched, Fe-doped samples three hole traps were observed (E_v+0.32 eV, E_v+0.39 eV, E_v+0.40 eV), in agreement with previous measurements. In both Cu- and Ni-doped samples, one hole trap was observed (E_v+0.53 eV for Cu, E_v+0.33 eV for Ni), which was also seen in the as-diffused material. Undoped samples displayed a quenched-in, hole trapping defect at E_v+0.43 eV. All defect states, with the exception of the E_v+0.40 eV Fe-interstitial centre, were neutralised to a depth of about 7 mu m by a 2 h exposure at 200 degrees C to a low-pressure hydrogen plasma. Results are also given for the motion of the E_v+0.40 eV Fe-related centre and the E_c-0.28 eV Au-related centre under the action of the electric field in reverse-biased junction diodes. Using a simplified treatment mobilities at 25 degrees C of 9*10^-15 cm²V^-1s^-1 for the E_v+0.40 eV state, and 2*10^-15 cm²V^-1s^-1 for the E_c-0.28 eV state were estimated.

The electrical conductivity and dielectric constant of V₂O₅-TeO₂ glasses of compositions 10-80 mol% V₂O₅ have been measured in the frequency range 0.1-100 kHz and in the temperature range 77-400K. A frequency dependence according to the relation sigma ( omega )=A omega ^s has been observed, and values of the exponent s are less than unity (between 0.95 and 0.80 depending on the composition). The data of AC conductivity have been analysed in the light of different theoretical models based on quantum-mechanical tunnelling (QMT) and hopping over the barrier (HOB). The analysis shows that neither QMT nor HOB models can explain the data quantitatively. A linear dependence of the conductivity and a temperature-independent s at low temperatures, and a non-linear increase of conductivity at higher temperatures for higher frequencies agree respectively with QMT and HOB models. It is difficult to estimate the absolute value of tau ₀ (characteristic relaxation time of the system) or to arrive at a definite conclusion about the conduction mechanism by the Butcher and Morys model (1973) owing to experimental uncertainties in the measured values epsilon '( omega ) and the lack of knowledge of epsilon _infinity . A detailed investigation of epsilon ' as a function of composition for these glasses has been made. A non-linear variation of dielectric constant with concentration has been observed. The abnormally high dielectric constant ( approximately 30) of the low composition V₂O₅-TeO₂ glasses may be attributed to the characteristic role played by the glass former, TeO₂.

The light-scattering width of a perfect dislocation in an insulating crystal is calculated. The contribution of the strain field around the dislocation to the change in dielectric constant is considered along with the core contribution. The strain-field contribution to the scattering width is inversely proportional to the wavelength of light. In most of the dielectric crystals considered here, the contribution of the former to light scattering is much more than that due to the latter. The estimated turbidity associated with the light scattering by dislocations in alkali halide crystals is of the same order as observed experimentally for pure crystals.

In 1979, Jamison and Nurmikko reported that high infrared light intensities led to self-induced opacity in narrow-gap n-type semiconductors. They attributed the effect to an avalanche generation of excess carriers by hot, photoexcited conduction electrons and subsequent intervalence band absorption. Here the authors present a rate-equation model for the distribution of hot carriers in the case of InSb at 77K. In the non-parabolic conduction band they introduce single- and two-photon absorption as hot-carrier excitation processes, LO-phonon emission and impact ionisation as energy-loss mechanisms. The assumption of two-photon absorption is necessary for the establishment of a sufficient population above the threshold energy for impact ionisation. They estimate the two-photon absorption rate from a numerical simulation of Jamison and Nurmikko's experiment.

By measurements of resonant Raman scattering combined with absorption and luminescence at low temperature, excitons and phonons are investigated in AgBr_1-xCl_x for a wide range of composition. One-phonon scattering due to intra- and intervalley exciton relaxation involving various optical and acoustic phonons occurs and is interpreted in terms of a modified model used to describe resonant two-phonon scattering in AgBr and AgCl. The analysis of the energy-dependent cross section of dispersive and non-dispersive phonon scattering suggests the existence of various quasi-free exciton states for x>or=0.2 that are characterised by finite effective-mass values but lie lower in energy compared with the free-exciton absorption. For the first time the phonon mode behaviour is studied for phonons at different points outside the centre of the Brillouin zone and discussed within a recently developed model.

In the high-T_c A15 compound V₃Si the authors found a Raman-active mode at a frequency of about 2 Delta . Peak frequency and intensity of this gap mode are strongly temperature-dependent. The temperature-dependence peak frequency compares well with the gap energy of the BCS theory. Thus one can investigate the superconducting gap with the help of Raman scattering. The intensity of the gap mode increases by about a factor of 20 if the temperature decreases from 14K to 5K. From the spectra of the gap mode and phonon mode in V₃Si and Nb₃Sn at various temperatures the authors draw the conclusion that the gap mode is coupled to the phonon mode.

Experiments are reported for the temperature dependence of the two-magnon Raman scattering in FeF₂ (T_N=78K). New results were obtained for the polarisation characteristics of the scattering, which was observed for temperatures up to approximately=4 T_N. A theoretical analysis was developed by extending earlier work to include the three dominant exchange interactions in a temperature-dependent formalism valid up to approximately=³/₄ T_N. The theory and experiment were found to be in good agreement for spectra with (ZX)=(YZ) polarisation. However, the spectra in (YX) and (ZZ) polarisations showed an additional feature at approximately=143 cm^-1 that was not purely two-magnon in origin. After subtraction of this band from the (YX) spectrum the authors again obtained agreement with theory. Possible origins of the additional scattering near 143 cm^-1 are proposed.

The theory of the Brillouin scattering from the surface of a semi-infinite medium, developed previously, is extended to coated surfaces of cubic materials. The Brillouin cross section is obtained for any scattering geometry and for p and s polarisation of the incident and scattered light. The formulae for the cross section contain contributions from the ripple and the elasto-optic scattering mechanisms for the film as well as for the substrate. The theory is applied to a silica film deposited on crystalline silicon and the behaviour of the cross section against the film thickness is investigated. The authors find that there are strong interference effects among the various contributions to the cross-section, so that the intensity of the spectral lines dramatically oscillates by changing the thickness. Although in general the ripple scattering mechanism is the dominant one it is found that the elasto-optic coupling in the silica film is not negligible and sometimes is even prevailing. The discrete phonon spectrum (Rayleigh, Sezawa, Lamb modes) and the continuous spectrum (resonances, mixed modes) are both studied.