Table of contents

Presents results for the case of a 4*4 impurity superlattice model of deep levels in semiconductors. In particular the authors consider the case of gallium arsenide and make comparison with deep level optical spectroscopy measurements for the EL2 level and show the degree of agreement to be good. They also consider the effects of phonon coupling and show them to be important, in particular in the case of photoionisation to the valence band.

A synthesis of Hartree-Fock and spin density functional theories yields a fully correlated many-electron energy minimisation method which avoids orbital self-interaction without requiring the use of localised orbitals. The new method is designed primarily for use with extended systems and may facilitate calculation of the band structure of semiconductors and insulators.

The stability conditions for replica-generated solutions of Ising spin glass models with general spin magnitude (S), anisotropy energy and gaussian distributed exchange are derived and discussed. The phase diagrams for integer S and half-integer S are argued to be qualitatively different when the anisotropy constant is negative. The spin glass phase (without symmetry breaking) of the S=1 model is shown to be everywhere unstable and the authors argue that the symmetric solution yields wrong first-order transition lines.

The critical behaviour of a compressible dilute Ising system has been studied here using Monte Carlo simulation techniques for a simple cubic lattice of size 20³. The discontinuity in the transition behaviour of the compressible Ising system is seen to be removed by the presence of lattice (spin) dilution. The observation agrees with the renormalisation group epsilon -expansion prediction.

Two resolved ferromagnetic resonance absorption lines are observed for a bulk single crystal specimen of GdAl₂. Theoretical interpretation yields an exchange constant A=0.23*10^-6 erg cm^-1 and surface anisotropy constant K_s=0.57 erg cm^-2.

Results of the absorption of laser radiation between 2.5 mu m and 2.8 mu m in solids containing complexed water molecules are presented. They are interpreted in terms of multiphonon absorption involving anharmonic coupling of vibrational states with various hindered rotations of molecules in the solid state.

In bootstrap percolation the sites on a lattice are first occupied with probability p, then the occupied sites with less than a specified number m of occupied neighbours are successively culled until a stable configuration is reached. For m=4 on a cubic lattice, a previous computer study has yielded a first-order transition, in agreement with an exact solution on a Cayley tree; however, this study could not unambiguously detect the critical fluctuations present in the Cayley-tree solution. The authors present a phenomenological theory of the time dependence of the percolation probability as neighbours are culled. This mean-field theory is constructed to be consistent with the Cayley-tree results in the static limit and predicts a 'critical slowing down' of the order parameter as p to p_c⁺ with r varies as (p-p_c)^-x and x=¹/₂. The authors present computer data for m= infinity on a simple cubic lattice which shows such critical slowing down, but with x=2.3+or-0.7. This suggests that on real lattices critical fluctuations in the bootstrap transition may be superimposed on the order-parameter discontinuity, as predicted by the Cayley-tree solution.

Neutron diffraction has been used to study the thermal parameters at the Au and Cu sites in ordered AuCu₃ at temperatures from ambient to 653K, i.e. within 15K of the order-disorder transition. The method consists of measuring the weak superlattice reflections out to high scattering angle (Q_max the maximum momentum transfer, is 21 AA^-1) with short-wavelength (0.53 and 0.72 AA) neutrons at the D9 diffractometer at the Institut Laue-Langevin. The data have been analysed with a method involving the Gram-Charlier expansion of the restoring potentials at the atomic sites. The analysis shows clearly the need to include fourth-order anharmonic terms. To understand these analyses the authors have evaluated the probability density functions and show the modifications of the harmonic potentials at each site.

A calculation is made of the angular spreading of a beam of phonons injected into ⁴He at low temperatures. This is achieved by solving the Boltzmann equation for the phonon density assuming that this quantity can be described by a temperature that depends on the direction of the beam and its distance from the heater. The calculated intensity is compared with the measurements by Page (1977).

Phonon scattering in phosphorus-doped silicon has been studied using a heat-pulse technique in which backscattering is measured. With crystal sizes of the order of 10 mm and with phosphorus concentrations up to 1.8*10¹⁶ cm^-3, it has been shown that single-phonon scattering is predominant. A theoretical curve has been fitted to measurements of backscattered pulse amplitude as a function of input pulse power, and from this fit the authors have been able to deduce the constant A for isotopic Rayleigh scattering in pure silicon (A=0.43 s^-1 K^-4), the deformation potential E_u associated with the bottom of the conduction band (E_u=8.6 eV), and the Bohr radius r₀ of the phosphorus impurity (r₀=16 AA). These results are in agreement with those derived by other workers from thermal conductivity measurements.

The good agreement previously obtained between theory and experiment for the g-factor of cadmium arsenide when proper account was taken of the 'free-electron' term has led the authors to investigate the same question in a variety of other semiconductors: HgTe, HgSe and the graded-gaps alloys Cd₃As_2-xP_x, Hg_1-xCd_xTe and Hg_1-xCd_xSe. In all cases, the contribution of the free-electron term permits good agreement with available experimental data. The authors find that the free-electron contribution depends strongly on the band structure of the material, because it is quantised in terms of Landau states for the material. A general result appears to be that the free-electron term is considerably greater than two for inverted-gap materials, but usually less than two for direct-gap ones, in which the g-factor is in any case smaller. It may, however, even in this latter case, make a substantial fractional contribution to the total g-factor.

The metal-semiconductor transition due to the Coulomb correlation (CC) is described. The method is illustrated with two band structures of Er₂O₃, one including and one without CC effects. This is done by inserting a potential U(r) (the occupation potential) into the band structure and the metallic phase appears when U(r)=0. U(r) together with the potentials V_ex^+or-(r) produces two different splittings, 4f up arrow /4f down arrow and 4f_o/4f_e (occupied and empty bands). The potential U(r) does not keep the symmetries of the 4f_e states with respect to those of the 4f_o ones. Results show that the hybridisation with other extended states modifies the widths of these bands drastically.

Expressions are obtained for several significant contributions to the spin-Hamiltonian parameters which describe the ground state splitting of Gd³⁺. Numerical estimates provide a qualitative explanation of several features of the experimental results.

The main features of the experimental results on excess electron transport are analysed. These features suggest a simple model: the excess electron is localised on one atom (or molecule) and its transport is due to a thermally assisted hopping. The results of a molecular dynamics simulation of the model are presented. Analogy between the drift velocity v_d and the sound velocity v_s leads to a general expression for v_d including density, temperature, electric field and mass effects. The electron-atom interaction potentials Phi deduced from the field and density effects are in good agreement too. Application of the general expression at densities lower than the critical one shows that the thermally assisted hopping plays a decreasing role at decreasing densities. This means that the lifetime duration of the 'negative ion' becomes shorter than the mean time interval between two efficient collisions. The authors do not notice any aspect of the drift velocity data in dense heavy non-polar fluids disagreeing with the consequences of the model.

The destruction of ferromagnetic order by alloying in intermetallic compounds (T_1-x⁽¹⁾T_x⁽²⁾)_n(Fe_yM_1-y)_m is studied using correlated percolation theory. In these intermetallics M is Si or Al and T⁽¹⁾ (T⁽²⁾) are 4d or 5d transition metals, with m, n integers. Starting from a simple model for the electronic structure of these compounds, a mapping into the percolation problem is made. A critical frontier is derived for the existence of ferromagnetic order in the x, y phase diagram. The role of dimensionality and stoichiometry is discussed for some cases.

Within a real space renormalisation group framework (12 different procedures, all of them using star-triangle and duality-type transformations) the authors calculate accurate approximations for the critical frontiers associated with the quenched bond-diluted first-neighbour, spin-¹/₂ Ising ferromagnet on triangular and honeycomb lattices. All of them provide, in both pure bond percolation and pure Ising limits, the exact critical points and exact or almost exact derivatives in the p-t space (p is the bond-independent occupancy probability and t identical to tanh(J/k_BT)). The authors best numerical proposals lead to the exact derivative in the pure percolation limit (p=p_c) and, in what concerns the pure Ising limit (p=1) derivative, to a 0.15% error for the triangular lattice and to a 0.96% error for the honeycomb one; in the intermediate region (p_c<p<1), where the exact critical frontiers are still unknown, the worst error in the variable t (for fixed p) is estimated to be less than 0.27% for the triangular lattice and 0.14% for the honeycomb one.

Results from an investigation by Mossbauer spectroscopy of approximately 1 at.% ⁵⁷Fe in NiCl₂, NiBr₂, NiI₂ and CoI₂ are presented. In NiCl₂ and NiBr₂ the results are satisfactorily analysed by a crystal-field model, and new information concerning the magnetic spin arrangements has been obtained. At all temperatures below T_N for NiCl₂+⁵⁷Fe, and at temperatures between T_N and 23K for NiBr₂+⁵⁷Fe, the spins are along a hexagonal (210) direction. Below 23K the results from Mossbauer spectra of NiBr₂+⁵⁷Fe are consistent with a different magnetic structure. Optical spectra taken at several temperatures suggest that iron impurities modify the NiBr₂ helical arrangement. Mossbauer spectra of NiI₂+⁵⁷Fe show extreme line broadening; possible explanations are discussed. Lastly, in CoI₂+⁵⁷Fe the results are consistent with a magnetic structure in which the spins lie in the basal plane.

For ferroelectric CsH₂PO₄ and CsD₂PO₄ the specific heat has been measured using an AC calorimeter. The singularity of the specific heat at the phase transition point is logarithmic. The most characteristic feature of the behaviour is the existence of an anomalous specific heat over the wide temperature region from T_c-60K to T_c+60K. This anomalous specific heat is considered to be due to the one-dimensional fluctuation of the polarisation in these materials. The authors also compare the experimental results with the pseudo-one-dimensional Ising model.

Infrared and Zeeman infrared measurements of the tetragonal symmetry cerium fluoride ion and neodymium fluoride ion charge compensation sites in calcium and strontium fluoride are reported. The neodymium results are supplemented by optical absorption and fluorescence measurements. Crystal field analyses successfully account for the observed ²F₇2/(4f¹) and ⁴I₉2/, ⁴I₁₁2/(4f³) multiplet crystal field splittings and for the measured Zeeman splitting values.

Presents optical reflectivity spectra for several 3d transition-metal intercalates of the group-Va transition-metal dichalcogenides. They discuss these results with reference to other optical and transport data for such materials. They show that there exists a correlation between certain features of the reflectivity spectra and the particular 3d transition-metal intercalate in a given host material. Similar correlations have previously been found for other physical properties, which have been interpreted as indicating an important interaction in these materials between the conduction electrons and the localised electrons on the 3d transition-metal ions.

The authors analyse, within the framework of the soft-phonon theory and using the Green function method to calculate the phonon spectrum, how a film growing on a substrate crystal may produce an interface instability. Different effects related to the film thickness, the film-substrate coupling parameter and the parameter determining the relative position of the phonon bands are predicted.

Measurements of the photo-Hall effect, AC photoconductivity, and pulsed-laser photoconductivity have been carried out on epitaxial p-type GaAs:Cr grown by MOCVD. The temperature range investigated was principally 80 to 300K, although photo-Hall measurements extended down to 10K. The samples remained p-type under illumination with light in the wavelength range 1 to 3 mu m. Optical thresholds near 0.8 and 0.4 eV were observed and these were interpreted as Cr³⁺/Cr²⁺ and Cr⁴⁺/Cr³⁺ transitions respectively. The temperature dependence of the higher threshold was of the form (0.831+or-0.008-2.8*10^-4 T eV). The lower threshold was at (0.42+or-0.01)eV in the range 100 to 160K. The behaviour of the photoconductivity of steadily illuminated samples as a function of temperature exhibited four distinct regimes. A detailed explanation of this behaviour was made on the basis of a model involving the conversion of Cr³⁺ to Cr²⁺ and Cr⁴⁺ as the temperature was lowered. One consequence of this conversion was the profound weakening of the electrical screening of surface charges. The degeneracy ratio for the Cr⁴⁺/Cr³⁺ level could be only roughly determined: 0.5<or=(g₀/g₁)<or=2. Evidence of the capture of holes at low temperature by a cascade process was found.