Table of contents

The structure of TlGaSe₂ has been studied by triple-crystal X-ray scattering techniques. At room temperature in the paraelectric phase the structure is believed to be C2/c, but the authors found that some of the (h, h, l) Bragg reflections, those with h odd, are broadened along the c* direction. This is shown to arise because of faults in the stacking of the atomic sheets perpendicular to c*. A simple model is developed for the scattering which suggests that on average a stacking fault occurs in the single crystal about once every four planes. Between 117 and 110 K the crystal has an incommensurate structure with a wavevector ( delta , delta , ¹/₄) where delta approximately=0.02 reciprocal lattice units. At 110 K, delta jumps discontinuously to zero to produce a commensurate phase. These results confirm the earlier suggestions of an incommensurate phase in TlGaSe₂.

The hydrostatic pressure and temperature dependences of velocities of ultrasonic modes propagated in natural single crystals of MnS₂ have been measured. The elastic stiffness tensor components and the bulk modulus of this high-spin pyrite structure compound are much smaller than those of the low-spin isostructural crystal FeS₂. This is consistent with MnS₂ having substantially weaker attractive binding forces than those in FeS₂ as would be expected from its larger lattice parameter due in turn to the high-spin configuration of the manganese 3d electrons. The temperature dependences of the elastic stiffnesses measured between 4.2 K and 293 K show that the crystal softens elastically near the Neel temperature but other than this the elastic properties are very similar in the antiferromagnetic and paramagnetic phases. The hydrostatic pressure derivatives of the elastic stiffness tensor components are all positive at room temperature: there is no acoustic mode softening under pressure. The results are used to describe the anharmonicity of the long wavelength acoustic modes in terms of mode Gruneisen parameters in the elastic continuum approximation. The mean long wavelength acoustic mode Gruneisen parameter gamma ^el (=2.4) is much larger than the thermal Gruneisen parameter gamma ^th (=0.98) which implies that the mode Gruneisen gammas for optic modes must on average be much smaller than those for the long wavelength acoustic phonons.

The authors study a one-dimensional extended Hubbard-Peierls model in the infinite intracell Coulomb repulsion limit, which is equivalent to a system of spinless electrons. They consider first- and second-neighbor Coulomb repulsion, and two types of phonon are coupled to the electronic system: intramolecular and longitudinal, intercell vibrations. The special case of half an electron per cell is considered. The model is solved in the Hartree-Fock approximation by assuming a possible symmetry break of period two. The solution is obtained for all temperatures. They construct a phase diagram with respect to temperature and the system parameters. Three types of low temperature ordered phase appear: bond ordered waves (BOW), charge ordered waves (COW) and an intermediate phase, across which the transition BOW to COW is continuous. This phase corresponds to a ferroelectric state (F). As temperature is increased from zero, the system undergoes different phase transitions, following a particular sequence, according to its characteristic parameters. At high temperatures the system stabilises in a homogeneous (disordered) phase (H). For some parameters the sequence BOW to F to COW to H is possible. The last result is very striking, since it implies that, on increasing temperature, the symmetry of the system can decrease, as is the case for the BOW to F transition, where the inversion symmetry is lost. This special BOW to F phase transition also occurs in the non-interacting limit, where the results are exact.

A model solid with a double-well potential at each site and intersite coupling J is considered, where J is sufficiently small that kT_C is much less than the depth kT₀ of the well. The effect of the local breaking of symmetry that occurs around T₀ is investigated for a single site. Anomalies are particularly noticeable in the oscillation frequencies, mean squared displacement and heat capacity. There are examples of materials and computer simulations that display such behaviour.

The Fermi surfaces of the tungsten silicide alloys WSi₂ and W₃Si determined from semi-relativistic self-consistent band calculation are presented. The former consists of simple closed sheets, while the latter is constructed from two closed sheets, two multiply connected surfaces and, possibly, one electron pocket. The Fermi levels of these silicides are located in the dip of the density of states. Additionally, the relationship between the content of tungsten atoms in the alloys and its electronic properties is discussed.

Measurements of the temperature dependence of conductivity and magnetoconductivity are presented of barely metallic n-GaAs bulk crystals doped by the neutron-transmutation doping technique resulting in medium-compensated samples of fixed compensation degree in the immediate vicinity of the metal-insulator transition at low temperatures. At n to n_c the temperature dependence in tau _phi (the phase-breaking time) disappears and therefore at low temperatures the temperature dependence of the conductivity is determined by the electron-electron interaction and necessarily L_int is the smallest relevant length leading to a dependence sigma (T)=bT¹3/. The experimentally determined density of states at the Fermi level is quite different whether n=n_c is reached by an exactly controlled impurity concentration or by magnetic tuning with n=n_c(B)>n_c(0). This is interpreted by the influence of the magnetic field on the weak-localisation contribution and a change in the density of states at the Fermi level. When n=n_c is reached by a controlled impurity concentration, the experimentally obtained density of states at the Fermi level is in good agreement with the value estimated from the impurity concentration and the energy spread of donor states in the gap.

Photoemission spectra of the system La_1-xSr_xCoO₃ (0<or=x<or=0.5) have been measured. These deviate increasingly from those expected from a simple model as x rises, due to strong correlations between the holes in the system. The authors discuss the nature and origin of the many-body effects in the spectra, and show that they may to some extent be accounted for using a ladder approximation for the hole self-energy, once effects due to surface composition are included. They provide an estimate for the magnitude of the Hubbard U in this system.

The Schrodinger equation for an electron near a randomly rough metal-metal (or metal-vacuum) interface is approximately solved. The wavefunction and probability currents are averaged over all realisations of the interface characterised by its root-mean-square departure from flatness and the extent of its lateral correlation, and the results are used to set up simple boundary conditions for the Boltzmann transport equation. These are in the form of angle-dependent transmission and reflection coefficients, generalising Fuch's reflectivity parameter, but given as explicit functions of observable properties of the interface. These results are further applied to the calculation of the electrical conductivity of thin films and foils, and of double-layer metallic films.

A method of calculating the longitudinal magnetoresistance of ultrathin films or inversion layers in MOS structures with sufficient size quantisation of transverse electron motion is proposed within the framework of the theory of weak localisation. The cases when a single subband and several subbands of size quantisation are filled are discussed. It is shown that in all studied cases the magnetoresistance is negative and depends on the ratio H²/T, so that it can be distinguished experimentally from the H/T-dependences of electron-electron interaction corrections to conductivity. Extension of the obtained results to the magnetoresistance calculation of quasi-1D channel is also discussed.

A generalisation of Planck's law for the spontaneous emission from light emitting diodes with direct band gaps allows a calculation of the absolute spectral emission intensity to be performed when the applied voltage, temperature, and absorption coefficient of the light emitting region are known. For given values of these parameters the emission intensity is independent of the diode current and the quantum efficiency of the recombination process. GaAs LED spectra calculated on this basis are compared with measurements at 82 K and at 296 K, and are found to agree quite well. Differences are attributed to the uncertain knowledge of the absorption coefficient of the diode material.

The effects of tunnelling from the p states of the scanning microscope tip are considered. In the case of the graphite surface, it is shown that an appreciable tunnel current flows to the p_x and p_y components of the tip wavefunction and that this tunnel current significantly reduces the amplitude of the corrugations that should be measured on this surface. The results also show that the giant corrugations observed in scanning tunnelling microscope images of graphite cannot be explained by a tunnelling model.

The author compares three basic physical mechanisms which induce correlations between the copper spins of a single strong-coupling plane of copper oxide in the perovskite superconductors. Motions of holes by virtual Cu³⁺ excitations induces ferromagnetic correlations, hole motion by Cu⁺ excitations induces paramagnetism, and Heisenberg interactions induce antiferromagnetism. They study the smallest clusters which exhibit the phenomena and examine the competition between these effects by exact diagonalisation. The paramagnetism and antiferromagnetism are quite similar and readily coexist locally. The ferromagnetic correlations are rapidly destroyed by increasing the Heisenberg interactions which stabilise the paramagnetic state locally. The mechanism which leads to paramagnetism via hole motion by virtual Cu⁺ excitations is investigated by solving a one-dimensional chain topology using short-range valence bonds. The paramagnetic correlations seem the only phenomenon which is likely to be relevant in the experimental systems. The inclusion of the Heisenberg interactions may be considered a minor perturbation to the paramagnetic correlations induced by the hole motion, and so may be neglected to a first approximation for this limit.

The magnetisation induced by a localised spin-¹/₂ interacting with the itinerant electrons of a paramagnetic host metal is obtained for a known band Kondo model and for the 'other' sign of coupling. Using the formal Berthe ansatz solution to this model the ground state energy is found for both the interacting host electrons and for the same system with impurity. The host magnetic properties are enhanced by a Stoner-like exchange. The impurity susceptibility diverges inverse-logarithmically at vanishing applied magnetic field-a result not accessible via perturbation theory.

Pure hexagonal beta -Al₉SiMn₃ does not order magnetically down to at least T=4.2 K. Recently the authors have studied beta -Al₉Si(Mn_1-xFe_x)₃ using Mossbauer effect spectroscopy with x=0.28 (1987 Phil. Mag. Lett. 56 143-51) and discovered a new transition at about 80 K. Here they present neutron diffraction studies down to 5 K, which reveal no structural changes, so that this transition at 80 K is indeed of magnetic origin. The magnetic susceptibility on the alloy with this Fe concentration shows evidence of a small maximum near this temperature superimposed on a large Curie-like background. The height of this maximum above the background is consistent with an antiferromagnetic transition at this temperature involving mainly the Fe moments. In addition to this transition, the Curie-like background shows a strong maximum (cusp) near 4 K. The authors thus infer a magnetic double transition in this disordered alloy. The upper transition at T_C=80 K involves mainly Fe moments. The lower transition at T_g=4 K is spin-glass-like, and must involve both Mn and Fe moments. These magnetic transitions have been followed to lower Fe concentrations (x=0.10 and 0.20) by both Mossbauer spectroscopy (T_C) by the magnetic susceptibility (T_g). It is found that T_C decreases rapidly with decreasing Fe concentration, while T_g remains relatively constant. The saturation hyperfine field B_hf(0) decreases rapidly with decreasing x, indicating that the Fe atoms lose their magnetic moments in the limit of infinite dilution, and the important role played by Fe-Fe nearest neighbours in determining the magnetic properties.

The ligand and quadrupole interactions parameters have been measured at 4.2 K by ENDOR for three shells of ^35,37Cl, ¹³³Cs and ²³Na neighbours around Cr³⁺ and Yb³⁺ substitutional impurities. The ligand interaction parameters were analysed by means of expressions for spin densities obtained by the configurational interaction method. The quadrupole interaction parameters have been discussed in terms of a point-dipole model, and local deformation around the magnetic ions being taken into account.

Results are presented of neutron diffraction experiments on a series of solutions of NiCl₂ in D₂O. Contrary to results from a previous investigation, no change in the Ni²⁺ hydration geometry is found in the concentration range 0.1 molal to 2.0 molal. A simple model is introduced in an attempt to deduce information on the distribution of orientations of the hydrated water molecules.

Explains the distinction between using the fractal dimensionality and the grain size in describing the change of grain shape when the concept of fractal dimensionality is introduced to the study of the recovery and recrystallisation process in materials. The fractal dimensionality of pure Zn was measured in the recovery and recrystallisation process. From these results a relationship between the fractal dimensionality and energy and a physical interpretation of the dimensionality were deduced.

The low-frequency internal friction spectra Q^-1(T) of alpha -ScH(D)_x, 0<or=x<or=0.3, reveal a hydrogen-specific peak near T_p=200 K for H and near T_p=210 K for D, the peak temperature increasing slightly with increasing x. The dynamic modulus, G, obtained simultaneously shows a break Delta G/G, in the temperature dependence corresponding to the peak in Q^-1, with - Delta G/Gx approximately=0.3/atom H(D). The activation energy for the relaxation has been determined to be E_a=0.54(3)-0.61₅(3) eV, increasing with decreasing x. The phenomenon is attributed to a Zener-type process of stress-induced changes in the short-range- and long-range-ordered configurations of the hydrogen sublattice such as are established by neutron scattering.

The authors have collected and analysed the available data for the Hall effect and its dependence upon temperature in amorphous Zr-Co, Zr-Ni, Zr-Cu and Ti-Cu alloys. The differences between the measured Hall coefficients, R_h, and the suggested free electron values, R₀, have very similar dependences on the number of electrons in the early transition (TE) metal d band. At temperatures higher than those at which quantum corrections are important, R_H-R₀ is either constant (within the experimental error) or decreases slowly with the increasing temperature as the resistivity does. These observations are consistent with the proposition that in these alloys the positive Hall coefficient is due to the spin-orbit effect.

High resolution electrical resistivity measurements ( rho d rho /dT) are used to investigate in detail the two spin-reorientation transitions of the hexagonal ferromagnetic NdCo₅ compound which occur at T_SR1=242 K and T_SR2=283 K (T>T_SR2 gives spontaneous magnetisation, M_s, parallel to the c axis; T_SR1<T<T_SR2 gives a conical structure; T<T_SR1 produces M_s in the basal plane). The authors have obtained the temperature dependence of the angle theta (T_SR1<T<T_SR2) using rho (T) data. The results being in fair agreement with theta (T) derived from the anisotropy constants K₁ and K₂. However, the temperature derivative d rho /dT exhibits new (and very localised) delta -type singularities near T_SR1 and T_SR2. These new anomalies in d rho /dT are interpreted in terms of lattice softening effects at T_SR1 and T_SR2 and associated anomalies in the C₃₃ elastic constant.