Table of contents

Some features of the field-induced spin-density-wave phases (FISDW) in quasi-1D conductors, such as the transition line and the variation of the SDW wave-vector, are derived from the study of the susceptibility of the 2D electron gas with open Fermi surface under a magnetic field.

Analysis of conductivity data taken over the temperature range 4.2-0.3K for a compensated sample of InP on the insulating side of the metal-insulator transition in terms of the relation sigma = sigma ₀exp(-(T₀/T)^s) indicates that s=¹/₂, in agreement with the Efros and Shklovskii (1975) Coulomb gap prediction.

The authors have performed a numerical study to derive qualitatively the observed temperature dependence of the spin-flip Raman spectra in semimagnetic semiconductors. Their calculation also predicts a low energy peak of the spectrum, and its relation to experimental evidence is discussed.

The ionic disorder in single crystals of the fluorite-type solid solutions Ba_1-xLa_xF_2+x (with x=0.209 and x=0.492) has been studied in the temperature range from room temperature to 800 degrees C by diffuse neutron scattering, ionic conductivity, and specific heat measurements. From the diffuse neutron scattering it was found that the disorder was dominated by 222 clusters, which at low temperatures (T<400 degrees C) were ordered along the (100) direction in aggregates of up to four 222 clusters. The correlation between the different 222 clusters in the aggregates is slowly lost when entering into the fast ion phase. The lifetime of the 222 clusters could not, even at the highest temperatures, be determined by neutron scattering ( tau >>10^-10s), in agreement with NMB results which suggest a jump frequency below 75 MHz. The temperatures at which the steepest slopes are found in the loss of correlations and in the conductivity coincide at approximately 650 degrees C. At this temperature no clear anomaly is observed in the specific heat. Based on these findings the authors propose a conduction mechanisms where F^- ions are moving through the lattice by means of rearrangements of the 222 clusters.

X-ray scattering experiments have been carried out in order to investigate the mechanism of the phase transitions in CsSnBr₃. The perovskite crystal CsSnBr₃ undergoes three successive phase transitions at T=19, 1 and -26 degrees C. The crystal symmetry sequentially transforms from cubic to tetragonal to tetragonal to monoclinic with decreasing temperature. These phase transitions are associated with the freezing of rotational modes of SnBr₆ octahedra around the three principal axes. Just above T₁, rod-like critical diffuse scattering was observed along the M to R zone boundary caused by the rotational mode. The phase transition at 19 degrees C is caused by the condensation of the M₃ phonon mode. The second phase transition at 1 degrees C is associated with the condensation of the Z₁ mode along the (001) direction of the tetragonal lattice, which is derived from the R₂₅ mode in perovskite crystal. The third phase transition at -26 degrees C takes place with the condensation of one of the doubly degenerate modes Z₅^x and Z₅^y modes.

The electronic properties of a large class of point defects in alpha -quartz are calculated using a fist-principles tight-binding method. The point defects examined include all atoms from the first three periods of the periodic system (hydrogen to argon) and they are treated as interstitials and as silicon and oxygen substitutes as well as ideal silicon and oxygen vacancies. For most of the point defects deep levels are found in the band gap. In some cases agreement with experiment is seen to be good.

AC conductivity is studied in crystalline germanium irradiated by 1.5 MeV electrons at liquid helium temperatures. A sufficient degree of damage occurs to allow the observation of nearest-neighbour hopping conductance. In n-type germanium the formation of complex defects by annealing of the samples at 60 and 110K leads to a hopping conductance obeying Mott's law ( sigma ^infinity exp-b/T¹4/). Two different values of b are observed in the temperature region 4-70K, the lower one being attributed to electron-electron interaction (variable-number hopping) and the higher one to variable-range hopping.

Capacitance bridge measurements on irradiated germanium at low temperatures are reported. It is argued that beam-activated migration of some primary defects occurs in germanium.

The authors present a study of the effects of the non-local dependence of the exchange and correlation energy in the Wigner crystallisation of an electron system. Calculations are made in the density functional formalism, using the functional models recently developed by themselves. The non-local treatment gives higher values of r_s for the transition than does the LDA, improving the comparison with computer simulations. Moreover, the unphysical shapes of the density distribution given by the LDA are completely corrected by their non-local model.

The temperature dependences of the heat capacity for Cs₂NaNdCl₆ and of the unit cell parameters for Cs₂NaNdCl₆ and Cs₂NaPrCl₆ have been investigated over a wide temperature range including the temperature at which the phase transition O_h⁵-C_4h⁵ occurs. Analysis of the results has been performed in terms of thermodynamic theory. The coefficients of the thermodynamic potential expansion and the values of the tilt angles of B³⁺Cl₆ octahedra have been determined.

Calculations are presented for the normalised velocity autocorrelation function, its power spectrum and self-diffusion coefficient as well as the self-dynamic scattering function for a model of liquid rubidium via the Mori formalism. The frequency spectrum and the self-diffusion coefficient respectively are in good agreement with the molecular dynamic calculation and experimental results. The self-dynamical structure factor exhibits the characteristics which are expected of simple liquids as shown, for instance, by the closeness of some of the parameters calculated from it to their known values in the hydrodynamic limit.

The authors studied the adsorption-desorption behaviour of Ar and Xe on the Si(111) 7*7 reconstructed surface. Using thermal desorption spectroscopy (TDS) one can clearly distinguish between adsorption in the monolayer range and higher coverage. Desorption from the second and higher layers show up in a non-saturating desorption peak of zero-order kinetic at lower temperatures while in the monolayer range they observe a saturating behaviour of the desorption rate with a complex structure at higher temperatures. Here the adsorbate-substrate interaction is dominant and different adsorption sites can be resolved demonstrating the rough character of the 7*7 reconstruction. This interpretation was confirmed by a controlled in situ change of the 7*7 reconstruction using a saturation precoverage of atomic hydrogen. The concentration and binding energy of the different binding sites are obtained by fitting a theoretical desorption model to the data. The relevance of the model and alternative models is discussed.

The stable state of an excess electron in solid Ne, Ar and Kr is studied by evaluating the energies of the free state (conduction band minimum) and of a localised state inside a microcavity (a bubble). The electron wavefunction is directly orthogonalised to all occupied states in the solid, that the lattice is represented by pair potentials. It was found that in solid Ne there is a large bubble containing an electron with a slightly higher energy than for the free state. The energy difference decreases as the lattice expands at high temperature such that at liquid density the localised state becomes marginally more stable. In solid Ar and Kr, the free state is clearly the stable state.

Transmission and reflectivity measurements on thin epitaxial samples of Pb_1-xMn_xTe (x<or=0.02) are analysed in the vicinity of the absorption edge using a novel numerical procedure which takes full account of internal interference and dispersion. The spectral dependence of the absorption coefficient is well described by a two-band k.p model. The resulting energy gap increases by up to 40% for x=2% at low temperatures as compared to PbTe. From far infrared investigations, no significant influence of x is found on the lattice dynamical properties.

The electronic structure of SmS is determined by means of a self-consistent APW calculation which includes both the occupied and the unoccupied 4f bands. The Coulomb splitting gives rise to the semiconducting phase in this Sm compound as well as the hybridisation of the empty 4f orbitals with with other extended states in the conduction band. Another calculation is performed that involves reducing the crystal volume by up to 0.87 V, where-upon a metallic phase appears because the occupied 4f bands are mixed with the conduction band.

The density of states is calculated for nearest-neighbour tight-binding bands in rectangular and tetragonal models of a crystal as a function of the band anisotropy. The results for the rectangular lattice are obtained in terms of a complete elliptic integral, and those for the tetragonal lattice by numerical integration of such an integral. The densities of states are continuous functions of the anisotropy which change into those characteristic of lower dimensionality in appropriate limits. However, the quasi-one-dimensional regions for the two lattices differ markedly because of the different residual dimensionalities, with three-dimensional features being particularly persistent. Care is therefore needed to obtain realistic quantities such as transport coefficients from models with reduced dimensionality.

The transverse and longitudinal magnetoresistance and the Hall voltage of both semiconducting (x=0.2) and semimetallic (x=0.135) Hg_1-xCd_xTe have been measured in pulsed (<35T) and continuous (<15T) magnetic fields in the temperature range between 4.2 and 1.4K. The conductivity tensor has been analysed as a function of temperature and magnetic field. This is found to be thermally activated in a way that is indicative of magnetic freeze-out. For the semiconducting material, the activation energy is determined as a function of the magnetic field. The magnetoresistance of the semimetallic material follows the T¹4/-law of Mott indicating conduction by hopping in the impurity band.

The authors interpret the large currents in the c direction (perpendicular to the chains) reported in Hg_{3- delta} AsF₆ below 4K as being due to superconducting fluctuations near its mean-field T_c. They show that the unique structure of this material and the strong anisotropy of the electron-phonon interaction lead to fluctuating currents in the c direction, with a critical behaviour characteristic of a one-dimensional system. They show that the unique form of the BCS Hamiltonian gives rise to superconductivity in this quasi-on-dimensional metal, without an interfering Peierls transition.

Estimates of the tricritical point (T_t, (J₄/J₂)_x) within an effective-field treatment are obtained for the Ising model with pair and quartet interactions. The lattices considered are the FCC, BCC and SC lattices with conventional quartets, and the SC lattice with square quartets. A correlated effective-field method is used to obtain a criticality relation between an effective field lambda and the coupling ratio J₄/J₂ for which the criticality equation has no solution for lambda . T_t is found from the phase boundary equation. If is found that lambda =0 at the tricritical point, for the FCC lattice.

The Monte Carlo simulations based on the 'heat bath' algorithms are implemented for the following classical spin systems: (i) the continuous-spin Ising model; (ii) the XY model and (iii) the Heisenberg model.

Nuclear spin rates of relaxation due to magnetic dipole coupling between spins undergoing relative translational diffusion in single crystals will depend on the orientation of the applied static magnetic field with respect to the crystal axes. The general form of the orientation dependence is calculated for the nuclear spin correlation functions J^(q)( omega ) and hence for the relaxation rates and the second moment of a rigid-lattice absorption line broadened by dipolar interactions. The only assumption concerning the diffusional motion is the principle of detailed balance and the orientation dependence is obtained using the transformation properties of spherical harmonics in terms of rotation matrices. Explicit results are obtained for the correlation functions and relaxation rates for each point group and the form of the relaxation rates in the low-frequency (high-temperature) limit is deduced.

The angular dependence of the anomalous transmission of Mossbauer radiation at resonance through a perfect single-crystal platelet of ⁵⁷FeBO₃ was studied for a pure nuclear reflection. Using a primary-beam divergence of 4", rocking curves of the Laue reflected and the Laue transmitted beams were simultaneously recorded. For the transmitted beam, a pronounced angular peak of about 8.5" half-width was observed, which was similar to the peak for the reflected beam. The peak in the forward direction is the first direct observation of anomalous transmission obtained by an angular scan in the case of a pure nuclear diffraction. In order to substantiate this result an energy analysis of the forward beam was performed. The beam was set at resonance and was analysed either without the crystal, or with the crystal set in an off-Bragg position. It was demonstrated that the energy distribution of the gamma -quanta anomalously transmitted at the Bragg position was qualitatively the same as that of the primary beam. Precisely those gamma -quanta at resonance, which in the off-Bragg transmission experience the strongest absorption, gave rise to the observed forward peak in the Laue transmission. The observation of the transmission peak combined with the analysis of its energy distribution constitute direct evidence for the suppression effect.

For pt.I see ibid., vol.19, no.14, p.2557 (1986). The angular dependence of the anomalous transmission of Mossbauer radiation through a perfect single-crystal platelet of ⁷FeBO₃ was studied for a pure nuclear reflection. The cases of intermediate and weak resonance absorption were realised at energy distances Delta E=+or-2 Gamma and +or-6 Gamma off resonance, where Gamma is the natural linewidth. With increasing distance Delta E an increase of the Laue-reflected peak was observed, caused by an additional contribution of the second wavefield associated with anomalously strong absorption. For the Laue transmitted beam, light-dark and dark-light intensity profiles were observed at energy distances Delta E=+2 Gamma and -2 Gamma , respectively, which are characteristic for anomalous transmission in the case of intermediate absorption. At Delta E=+or-6 Gamma , minima were observed in the Laue transmission rocking curve, which can be ascribed to an intensity transfer into the Laue reflected beam. The approximate compensation of the transmission minima by the reflection peaks is an indication of the enhancement effect, which reveals itself in pure form only at negligible resonance absorption. The combined results of parts I and II demonstrate, that in Mossbauer dynamical diffraction the different cases of strong, intermediate and weak absorption can be easily realised in one and the same crystal sample.

The complex dielectric function epsilon ( omega ) along the a axis of PbHPO₄ crystals is determined between 0.3 and 10 GHz by time-domain spectroscopy in the paraelectric and ferroelectric phase near the transition temperature T_c=309K. The main component of the dielectric dispersion mechanism related to proton ordering is found to be located in this frequency region. Analysis of the data in the paraelectric phase leads to the conclusion that the dielectric mechanism is a heavily damped soft mode with a frequency-dependent damping function.

A theory for the time-resolved secondary radiation from the F centre excited by a photon pulse is formulated on the basis of a vibronic model. It includes the ls ground state and 2s and 2p excited states with a pseudo-Jahn-Teller-type electron-phonon interaction. The transient behaviour of the intensity and polarisation of the radiated photon is calculated as a function of the time interval between the excitation and the detection. It is found that this transient behaviour enables one to distinguish the hot luminescence from the ordinary luminescence more clearly than previously. In particular, the depolarisation due to hot luminescence becomes more prominent in the transient polarisation spectrum than in the case of stationary excitation, where the hot luminescence is usually masked by the tail part of the predominant ordinary luminescence.

The authors report a photoluminescence study of the 811 meV no-phonon line structure in platinum-doped silicon. Zeeman and piezospectroscopy measurements show that the centre responsible for the luminescence has C_2v symmetry. The transition is tentatively identified as occurring between internal levels of substitutional Pt⁰ with an orthorhombic distortion.