Table of contents

The room-temperature crystal structure of squaric acid (H₂SQ), H₂C₄O₄, has been determined by X-ray diffraction at 2.75 GPa; and the lattice parameters have been measured at a number of pressures between 0.93 and 4.70 GPa. Under these conditions, H₂SQ is above the monoclinic-to-tetragonal transition temperature, T_c, in its tetragonal phase. The principal structural changes with pressure are shown to be compression of 2R, the length of the O-H-O bonds connecting the C₄O₄ groups, and a reduction of the separation between the planar H₂C₄O₄ sheets so formed. The C₄O₄ groups maintain their size and shape, but rotate a little in a sense that tends to reduce the compression of 2R. The form of 2R(P) is used to re-examine the relationship between the changes in 2R and T_c observed on deuteration of H₂SQ.

Electron energy-loss spectroscopy (EELS) is suggested as a possible probe for the longitudinal plasmon sub-bands in metallic superlattices. An outline of the theory leading to the dispersion relation and the scattering cross section is given. The treatment applies the dispersive hydrodynamic jellium model for the field quantisation of the superlattice longitudinal plasmons and standard scattering theory for the derivation of the cross section. The feasibility of the suggested EELS experiment is examined by application of the theory to a typical metallic superlattice.

The authors present a model with quenched disorder which in general has no thermal equilibrium state. Nevertheless, the model exhibits a freezing transition into an Edwards-Anderson phase in the absence of thermal noise. It is only in the exceptional case of detailed balance that thermal noise does not destroy the transition.

The inverse, e^-1(r, r'), in real space, of the dielectric response function for a medium is derived as a solution of the relevant Volterra integral equation. The result is applicable to any model of a solid for which a polarisation operator is defined.

The pressure-volume relationship for CeS has been determined at room temperature by X-ray powder diffraction in a diamond anvil cell up to 25 GPa. Special attention has been paid to the influence of possible uniaxial stress components and experiments in both liquid and solid pressure transmitting media have been carried out. No sharp volume transition has been found; actually, only a smooth variation occurs under pressure which can be described by Birch's equation of state with an initial bulk modulus B₀=82 GPa and its first pressure derivative B'₀=2.2. These results show that a continuous electronic transition takes place over the whole investigated pressure range.

The system of conducting electrons which interact through the long-range Coulomb force which in addition are coupled to the 2k_F deformed lattice is considered within the generalised Tomonaga model for spinless fermions. The T=0 gap Delta _L in the electron spectrum is evaluated using the self-consistent harmonic approximation for the excitation spectrum of the bosonised Hamiltonian. The resulting plasmons characterised by the maximum frequency omega ₀, acquire the gap Delta _L at T=0. The screening picture which is metallic for T>or=T_L^MF becomes semiconducting at T=0. However, to the leading order in Delta _L/ omega ₀ both T_L^MF and Delta _L are given by the expressions involving only the short-range part of Coulomb forces, i.e. the law of corresponding states Delta _L approximately=T_L^MF of the short-range theory is recovered.

Polarised far-infrared spectral emission from KBr films is observed around the restrahlung band at an emission angle of 45 degrees . The spectra observed at elevated temperatures show an abnormal line shape around the longitudinal optical (LO) mode frequency. In order to discuss the effect of anharmonic phonon interaction on the spectra, these are analysed using a dielectric response function in which the empirical phonon self-energy function is considered. The results suggest that the shape of the damping function changes with temperature as well as with an increase of magnitude, which implies that the higher-order contribution to the self-energies becomes important at high temperatures as well as the lowest orders. The re-normalised quasi-harmonic LO mode frequency remains fairly constant close to the harmonic mode frequency for all temperatures. More than 10% of deviation from the Lyddane-Sachs-Teller relation is obtained at high temperatures.

The temperature dependences of the mean square displacements and order parameters have been calculated using molecular dynamics for a series of ferrodistortive and antiferrodistortive phase transitions for two-dimensional lattices of varying size. The model system consists of a multiple-minimum effective one-particle potential centred on each lattice site and harmonic nearest-neighbour coupling. By allowing the walls of the one-particle potential to be quadratic the experimentally observed linear temperature dependence of the mean square displacements at high temperatures can be reproduced. Choosing as the origin the equilibrium atomic position for the high-temperature phase, the mean square displacements are observed to be smooth across the phase transition, T_c occurring in the vicinity of the point of inflection. This behaviour is in agreement with scaling theory, although mean-field theory predicts a spurious cusp-singularity at T_c. Comparisons are made with experimental data and with one-dimensional models.

The order parameters are found for the phase transitions I-II (283K) and I-I_g (170K) in cyanoadamantane along with possible domains of phases I and II and their stress-free boundary planes. A phenomenological model of the glassy phase I_g is then stated. A linear temperature dependence of a coupling coefficient in the free-energy expansion is proposed on the basis of the thermal variation of the phase II structure. A possible generalisation of the free-energy expansion in strongly first-order phase transitions is discussed.

The LMTO method is used to calculate the band structure of the alkaline-earth oxides CaO and BaO. The cohesive energy, the bulk modulus and the equilibrium lattice constant are also calculated. The results are compared with experiments and other theoretical calculations.

Recent experiments by Levi et al. (1985) and Long et al. (1986) in n-doped GaAs have produced path lengths in the range 300-500 AA at 77K for hot-electron decay. The scattering has been attributed to electron-electron and electron-phonon interactions. The authors have made a detailed theoretical study of electron-electron and electron-phonon effects in GaAs in the doping range n=10¹⁷-10¹⁸ cm^-3 where polar LO phonon and plasmon mode mixing effects are expected to be largest. In a previous study on electron-electron interaction where the authors neglected plasmon-LO-phonon mode mixing, they have shown that plasmon dispersion effects are important in the calculation of path lengths for hot-electron decay. Thus altered dispersion in the initial unmixed modes, mode dispersion in the final modes due to mode-mixing and interaction effects and dynamical screening by the electrons and the phonons will influence the damping of hot electrons in these structures. They use a temperature-dependent Green function approach within the RPA approximation to include lattice polarisation and electron-electron effects and evaluate a wave-vector and frequency-dependent screening function epsilon (q, omega ) which they can then use to evaluate the imaginary part of the electron self-energy and hence a path length due to combined electron-electron and electron-LO-phonon interaction.

The authors have given a method of calculation of the energy E_R and width Gamma _R of the lowest resonant energy state lying below the bottom of the Landau sub-bands in the presence of a Coulombic impurity (negligible screening). Coupled equations for the longitudinal part of the wavefunction are obtained and integrated numerically for the case of one open and one closed channel, using a new method of handling the boundary conditions. Numerical calculations have been carried out for the n=1 level in InSb using the parabolic model with fields of 2.5, 5, 7.7 and 10 T, the ratio Gamma _R/E_R varying from 0.014 to 0.0064. The method may readily be generalised to a more complex band structure (including non-parabolicity) and higher Landau levels.

Fluorescence line narrowing techniques are used to record the spectral energy transfer with the R₁ line of Cr³⁺ in emerald from 30-140K. In marked contrast to the situation in ruby the transfer is established to be non-radiative, two-phonon-assisted and dipole-dipole in nature.

A model is introduced for which the 'naive' mean-field equations (NMFE) for an Ising system, m_i=tanh ( beta Sigma _jJ_ijm_j+ beta h_i), became exact in any dimension. The model is solved for the infinite-range Ising spin glass (i) starting from the Hamiltonian and using the replica method without replica symmetry breaking, and (ii) starting directly from the NMFE using the method of Sompolinsky (1983). The solution is qualitatively similar to that of the Sherrington-Kirkpatrick model (1975). The Glauber model (1963) for the dynamics of the system is also discussed; the spin autocorrelation function exhibits a t^-1/2 decay everywhere in the ordered phase.

The Mossbauer spectra of powered hypersthene, a natural silicate mineral belonging to orthopyroxene group, have been taken over 1.3 to 300K. At temperatures above 77K they show asymmetric quadrupole peaks. This asymmetry arises from the overlapping of two quadrupole doublets from Fe^2-(⁵D) ions in two different sites. The quadrupole splitting, centre shift and their temperature dependence are appreciably different for Fe^2- ions in the two sites. The spectra around 4.2K shows the presence of electronic spin relaxation which is strongly temperature dependent. One observes a full magnetic hyperfine spectrum at 1.3K, a very weak presence of broad magnetic structure of 4.2K and only asymmetric quadrupole peaks at 5.8K. The estimated values of the internal magnetic fields at the two Fe^2- ions are approximately 125 and 20 kOe. It appears that the spin-lattice relaxation has been effectively suppressed, while the spin-spin relaxation is quite important. It is believed that the suppression of spin-lattice relaxation has resulted from a complete absence of intermediate energy ( approximately 100-500 cm^-1) electronic states which usually provide efficient paths for the exchange of energy with phonon spectrum. This is the first time that slow spin-spin relaxation for Fe^2- ions in the absence of an external magnetic field has been so clearly observed.

EPR measurements at X band and room temperature have been made of defects produced by implanting (100) and (111) silicon wafers with doses >10¹⁸ O^- cm^-2 using 300 keV O^- and 400 keV O₂⁺ ions and an implantation temperature of 500-600 degrees C. These doses are sufficient to form a buried SiO₂ layer. The main features of the EPR spectra are attributed to three types of defect, with (i) g=2.0003(3), (ii) g=2.0013(2), g_-=2.0082(2) with the axis parallel to any (111)-type direction and (iii) g=2.0054(3). The defects are suggested to be respectively (i) E'₁ centres in the a-SiO₂ layer, (ii) P_b0-like centres primarily at Si/SiO₂ interfaces of SiO₂ precipitates and (iii) amorphous silicon centres, possibly associated with oxygen. The angular dependence of the P_b0-like centre linewidth is measured and interpreted in terms of a spread in g-values. Annealing and etching studies have also been made.

For pt.I see ibid., vol.19, p.6417 (1986). EPR measurements have been made of defects produced by implanting a (100) silicon wafer with a dose of 0.75*10¹⁸ cm^-2 200 keV N^- ions at an implantation temperature of about 520 degrees C. A single nearly isotropic line with g=2.0039+or-0.0002 and peak-to-peak linewidth 0.80+or-0.04 mT is observed. It is attributed almost entirely to silicon dangling bonds which lie mainly within a buried amorphous layer, 0.21 mu m thick, which is rich in nitrogen. The defect concentration, which has an initial value of about 4.5*10¹⁴ cm^-2, is measured as a function of etching time and annealing temperature.

The structural phase transition in RbCdF₃ at 124K from cubic to tetragonal symmetry was studied by investigating the paramagnetic Mn²⁺ and Fe³⁺ probes with ENDOR. From the superhyperfine interactions the order parameter could be inferred. It is shown that the phase transition is weakly of first order. The lattice relaxation about the probe causes the order parameter to be a 'local' one, which differs from first to third shell of neighbours around the probe ion. The results demonstrate the limitations of the use of probes to study the intrinsic crystal properties.

The absorption spectrum of Nd₃Ga₅O₁₂ has been analysed at 4, 77 and 300K between 5000 and 35000 cm^-1 for which an energy level scheme of 104 Stark levels is derived. Both free-ion (12 parameters) as well as crystal-field parameters (9 for the D₂ symmetry of the Nd point site) of an effective operator Hamiltonian were fitted to the data and close correlation between experimental and simulated level energies was obtained with a mean quadratic deviation of 18 cm^-1 on the 364 kets basis in the 4f³ configuration.

A theory is presented to model the lineshape observed in electroreflectance spectra of quantum well structures reported recently by the authors and elsewhere. The existence of such spectra is shown to be a direct consequence of the sensitivity to a perpendicular electric field of the quantum well sub-band energies and the mechanism is thus qualitatively different from the Franz-Keldysh effect, which may explain the electroreflectance spectra of bulk materials. The importance of optical interference in determining the observed lineshape in multilayer samples is emphasised, and good agreement is obtained between the observed spectrum and the spectrum predicted by the model of a single 100 AA quantum well of Ga_0.8Al_0.2As/GaAs/Ga_0.8Al_0.2As. From a careful analysis of absorption, photoluminescence excitation and electroreflectance spectra for this sample the authors calculate binding energies for the heavy and light hole excitons of 11.6+or-1.3 meV and 10.1+or-1.1 meV respectively.

A certain kind of electron Raman scattering in semiconductors, which the authors have called 'interband-intraband electron Raman scattering' (IIERS), is discussed in detail. The IIERS process differential cross section is calculated without particular assumptions on the semiconductor band structure, allowing the study of a wide class of materials. Concrete applications are made to the cases of parabolic and Kane band structure models. The differential cross section of the IIERS is also studied for the critical points in the three-dimensional case. Special emphasis is made on the spectral frequency dependence for different polarisation of the incident and emitted light. Scattered light in the IIERS is displaced towards the low-frequency region providing spectral structure far from the incident light frequency.