Table of contents

Polycrystalline pyrolytic graphite was compressed in a diamond anvil cell to pressures up to 73 GPa. In situ Raman measurements reveal evidence of a phase transition at 15-20 GPa, consistent with earlier static high-pressure experiments on graphite. The sample was quenched by rapid downloading of the cell to ambient pressure. Micro-Raman investigation of the quenched sample reveals a broad range of previously unobserved intermediate carbon structures, sometimes coexisting with diamond. We also observe the quenching of the 'nanocrystalline' phase of graphite.

Sr₂RuO₄ is a superconductor with a similar structure to a high-T_c cuprate superconductor. Nevertheless, the superconducting state may have different symmetry than that of cuprate superconductors. Strong Hund's rule coupling favours triplet over singlet pairing, similar to ³He. A strong candidate is the odd-parity pairing state which is the two-dimensional analogue of the Balian-Werthamer state of ³He. Various experimental consequences and tests are analysed.

Spin-density-wave (SDW) binary Cr_1-xMn_x (x<<0.1-4.6% Mn) and ternary (Cr+1.3% Si)_1-xMn_x (x=0.17 and 0.6% Mn) alloys are found to exhibit spin-glass behaviour, which differs however from that of a conventional spin glass in that the magnetic susceptibility chi (T) is essentially independent of temperature T between the low-temperature maximum and the Neel temperature T_N, and in that the temperature of the maximum is essentially independent of the Mn concentration. Comparison of binary and ternary alloys having the same Mn content shows that the nature of the maximum in chi (T) depends on whether or not the SDW is commensurate. A model is proposed that explains the increase in chi (T) at low temperature as resulting from pinning of the phase of the SDW to the Mn moments, which are frozen below T_N, causing frustration on the host Cr moments on the surfaces between the resultant SDW phase domains.

The osmotic pressure for a disordered two-dimensional foam or emulsion is evaluated by a realistic computer simulation over the whole range of the liquid fraction. The data are consistent with a quadratic approach to zero as the liquid fraction nears the limit of stability of the froth.

The thermodynamic and structural properties of a fluid of hard axially symmetric ellipsoids are studied on the basis of two approximations, namely that (a) the orientations of the molecules can be restricted to a discrete number of directions and (b) the direct correlation function can be written as a superposition of three geometrical functions with weights which depend on the density of the fluid. The variational principle of Anderson and Chandler (1972) is used to determine the weights. The equation of state of the fluid is derived for a range of values of the ratio of axes (elongation) from 0.35 to 5.0. For comparison new simulations have been performed on the same systems. Except at the greatest elongation, the theoretical pressure agrees very well with the simulated pressure of the restricted orientation model as well as with that of a fluid of freely rotating ellipsoids. When the elongation is 5.0, the pressure of the fluid of freely rotating ellipsoids lies somewhat above the pressure of the restricted orientation model, although for this model the theory and simulation are still in accord. Thus far the theory has not revealed a transition to a nematic state although there is some evidence from simulation that when the elongation is 5.0, the isotropic state of the restricted orientation model is not stable. The direct correlation function of the homogeneous phase is used in conjunction with a density functional in order to investigate the density profile of the fluid confined to a slit. Except in the neighbourhood of a wall there is very good agreement between the theory and simulation. The discrepancy near the wall is to be expected from the simple form of functional used. Overall it appears that the approximation developed is adequate for describing the thermodynamic properties of the homogeneous and inhomogeneous isotropic state, but that it needs some improvement for studying the properties of the nematic state.

Equations of motion for model colloidal particles are derived, which include a mean-field local density representation for the many-body-hydrodynamics (MBH) interactions. These are implemented as a modification of Ermak's 1975 Brownian dynamics free-draining algorithm, with a local density prescription for the friction coefficient. The computed long-time self-diffusion coefficients derived from this model agree well with experiment at all volume fractions. The hydrodynamics model relative and dynamic viscosities are larger than those of the free-draining algorithm, reflecting slower structural relaxation processes taking place in the new model.

Accurate measurements of the nuclear spin-lattice relaxation rate (1/T₁) of ⁸Li nuclei in liquid Li_xSi_1-x have been performed as a function of temperature (T) and magnetic field (B) for various compositions using the beta -NMR method. The results show clearly that the magnetic field dependence of T₁ follows a square root B law around the x=0.5 composition. We interpret this as an indication for anomalous diffusion of electrons due to strong disorder. From the coefficient of the square root B contribution to 1/T₁ we derive that the partial Li s density of states at the Fermi level is about 1/10 of the total density of states at the Fermi level. This is the first observation of such a magnetic field dependence of 1/T₁.

The static dielectric function epsilon (q,0) of the molten copper halides at two temperatures near melting are obtained in the hypernetted chain approximation using the effective pair potentials constructed by Stafford et al. (1990) and their behaviour related to the onset of freezing. The epsilon (q,0) results for the copper halides are found to mimic the behaviour of the classical one-component plasma with plasma parameter Gamma =90.

An exact method that analytically provides transfer matrices in finite networks of quasicrystalline approximants of any dimensionality is discussed. We use these matrices in two ways: (a) to exactly determine the band structure of an infinite approximant network in analytical form; (b) to determine, also analytically, the quantum resistance of a finite strip of a network under appropriate boundary conditions. As a result of a subtle interplay between topology and phase interferences, we find that a strip of phason defects along a special symmetry direction of a low 2D Penrose approximant, leads to the rigorous vanishing of the reflection coefficient for certain energies. A similar behaviour appears in a low 3D approximant. This type of 'resonance' is discussed in connection with the gap structure of the corresponding ordered (undefected) system.

The number of self-avoiding walks, u_n, in several zeolitic frameworks has been calculated up to distances n higher than 20 steps. This allows us to estimate the 'connective constant' or effective coordination number of these structures as the limit of the ratio u_n/u_n-1 for large n. This parameter is correlated with the density of tetrahedra in the zeolite networks, and is shown to be very useful for characterizing the network connectivity, as well as quantifying the influence of the framework topology on the Si, Al atom ordering in these materials.

The dynamic structure factor of very large percolating clusters (bond and site), in two dimensions and three dimensions are calculated using the spectral moments method. Interactions are represented by the scalar model. Numerical results are presented and interpreted in terms of scaling arguments given by Alexander, Courtens and Vacher (ACV) in 1993. Concerning the q lambda <<1 limit, our results confirm and supplement previous numerical work and are in agreement with the scaling behaviour theoretically deduced by ACV. Concerning the q lambda >>1 limit, we have shown that the dynamic structure factor complies with the very nice asymptotic behaviour g(q, omega )=q^yH(q lambda ( omega )) where the scaling function H(x) is of power-law form x^{- tau '} in agreement with the theory. However, our results indicate a scaling behaviour with exponents that differ from those deduced by theory. The values obtained for tau ' are 1.20 for the two-dimensional bond and site percolating lattices, and 1.00 for the three-dimensional percolating lattices. Comparisons with previous simulations are reported.

The heat capacity of crystalline tolane (diphenylacetylene) has been measured by adiabatic calorimetry between 9 and 303 K in order to see any phase transitions like those found in a similar compound, biphenyl. No thermal anomaly has been detected. The phonon dispersion relation of the compound is calculated using a simple model and compared with that of biphenyl. It is shown that the absence of a phase transition in crystalline tolane is consistent with the calculated dispersion relation. A certain height of the potential barrier at the planar conformation in the intramolecular potential for twisting motion is essential for the lattice instability of crystalline biphenyl. Some smoothed thermodynamic quantities are tabulated.

Measurements of the thermal conductivities of rare-earth element dodecaborides REB₁₂ (RE=Tb, Dy, Ho, Er, Tm or Lu) were conducted in the temperature range 4.2-300 K. The temperature dependences of the electronic and phonon thermal conductivities in the paramagnetic-to-antiferromagnetic transition range were determined. For TmB₁₂ and HoB₁₂ the Lorenz function exhibits a distinct variation relative to L₀ (L₀ is the theoretical value of the Lorenz function) in the temperature range 50-90 K. By comparing the electronic and phonon thermal conductivities of the magnetic compounds (with Tb, Dy, Ho, Er and Tm) with the same quantities for nonmagnetic LuB₁₂, the influence of the interaction of electrons and phonons with 4f electrons on the heat transport was calculated. By the same token, we elucidated the influence of crystal electric field on the thermal conductivity of the REB₁₂ compounds.

The paper is concerned with two-dimensional classical fluids and solids composed of charged particles that interact only through Coulomb forces. An example of this type of system, and one with which the paper is particularly concerned, is a circular pool of either positive or negative ions trapped just below the free surface of superfluid helium. A calculation is presented of the modes of response of such a system in its own plane when exposed to an oscillating electric potential, especially in the long-wavelength limit and in the presence of a steady magnetic field applied in a direction normal to the plane of the system. The calculations are relevant to the experimental detection and study of these modes, which include shear modes in the crystal phase and viscous modes in the fluid phase.

Bulk granular magnetic solids have been prepared by the high-energy mechanical alloying of fine cobalt and silver powders, with cobalt concentrations of 5, 15 and 30 at.%. The threshold for giant magnetoresistance in the parent samples was found to lie between 5 and 15 at.% Co, with a maximum effect of 12% at 4 K in the 30 at.% Co sample. The effect of the mechanical alloying process parameters, such as the milling energy, milling time and annealing, on the magnitude of the GMR in this system was also investigated. The sub-micron scale homogeneity of the samples was found to vary with the milling energy, with completely homogeneous samples produced after milling at high energy, and only 60% homogeneity levels achieved in the sample milled at a lower energy. The isotropic and anisotropic contributions to the total magnetoresistance were separated experimentally, and the linear relationship between the two (derived for an ideal superparamagnetic system) was observed experimentally.

Field ionization properties of shallow carbon acceptors in MBE GaAs, where thermal lattice vibrations have an effect on the hole emission rate, are presented. The excited energy levels of the carbon atom are relatively far from the ground level; therefore, it appeared possible to observe pure phonon-assisted tunnelling of a hole from a single (ground) level over a wide temperature range, from 4.2 K to 22 K. Experimental results are interpreted using a multiphonon field ionization model that takes into account the interaction of acoustic phonons with a localized centre via deformation and piezoelectric potentials. In the temperature range considered, the influence of the deformation potential is found to predominate over the piezoelectric one.

The dark conductivity and transient photoconductivity measurements on thin films of a-Se_80-xTe_xGa₂₀ (0<or=x<or=20) have been reported in the temperature range from 148 to 318 K. The results indicate that, at higher temperatures, hopping conduction takes place in the tail states. At lower temperatures the conduction is due to variable-range hopping, which is in fair agreement with the Mott condition of variable-range hopping. The transient photoconductivity measurements show non-exponential decay, and recombination in these glasses may be considered to take place through the valence and conduction bands.

We compute the energies and transition probabilities for low excitations in the one-dimensional antiferromagnetic spin- 1/2 Heisenberg model by means of the recursion method. We analyse finite-size effects in the Euclidean time ( tau -) representation and compare the resulting estimate for the thermodynamical limit with two parametrizations for the dynamical structure factors in the spectral ( omega -) representation.

Spin chirality has generated great interest recently both from possible applications to flux phases and intrinsically, as an example of a several-site magnetic order parameter that can be long ranged even where simpler order parameters are not. Previous work (motivated by the flux phases) has focused on antiferromagnetic chiral order; we construct a model in which the chirality orders ferromagnetically and investigate the model's behaviour as a function of spin. Enlisting the aid of exact diagonalization, spin waves, perturbation theory, and mean fields, we conclude that the model probably has long-ranged chiral order for spin 1 and greater and no non-trivial chiral order for spin 1/2 .

The coupled-cluster method is applied to the spin- 1/2 triangular-lattice anisotropic antiferromagnet, a noncollinear and frustrated magnet with an anisotropy parameter first introduced by Singh and Huse (1989). Various physical quantities including the ground-state energy, the anisotropy susceptibility, the sublattice magnetization, and their critical exponents associated with the magnetic order-disorder quantum phase transition, are calculated by the systematic inclusion of two-spin correlations. These results are then compared and contrasted with those obtained from other methods such as series expansions and variational calculations. The feasibility of carrying out quantum Monte Carlo simulations using the results presented here is also discussed by drawing parallels with the coupled-cluster calculations on the square-lattice antiferromagnets.

A d¹ Ti³⁺ centre formed by X-irradiation of a synthetic zircon single crystal at 77 K has been studied by electron paramagnetic resonance (EPR) at 10 K. The Ti³⁺ ion occupies a substitutional site of point group symmetry 42m (D_2d). Absence of both site splitting in general crystal orientations and of superhyperfine structure from a +1 ion indicated that a nearby charge compensator was not involved. Comparison with EPR measurements on Ti³⁺/ alpha quartz centres, point charge calculations and ²⁹Si hyperfine splittings indicated that the Ti³⁺ centre results from electron capture by Ti⁴⁺ occupying a Zr⁴⁺ site in the crystal; the designation (TiO₈)/sup /s suggested. Precise spin-Hamiltonian parameters, including ⁴⁷Ti (I=5/2, 7.4%) and ⁴⁹Ti (I=7/2, 5.4%) hyperfine, nuclear electric quadrupole and nuclear Zeeman parameter matrices, together with high-spin nuclear terms of dimension I⁴, I³S, I⁵S, BI³ and BI⁵ were determined. The set of terms BI, BI³ and BI⁵ were found to be crucial in obtaining a good fit to the many 'forbidden' hyperfine lines observed. This resulted in determination of nuclear quadrupole and nuclear Zeeman parameters with precisions not hitherto attained in conventional single-crystal EPR measurements.

Ho centres in mixed-polytype ZnS crystals codoped with Ag are studied by measuring polarized site-selective photoluminescence and photoluminescence excitation spectra as well as by EPR spectroscopy. The optical data give evidence of Ho in the charge state 3+ (4f¹⁰), while in the EPR measurements, which are performed under illumination in the interband range, centres in the 2+ state (4d¹¹) are detected. From a crystal-field analysis we conclude that the dominant centres seen in the two kinds of experiment are identical with respect to the atomic configuration, apart from small nearest-neighbour relaxation effects which accompany the change of the charge state. The crystal field is found to have approximately cubic symmetry, with a small trigonal field component, whose axis is parallel to the (111)_w direction. Our data indicate that the Ho occupies zincblende-type interstitial sites with four metallic ions as nearest neighbours, most probably Ag substituting for Zn ions. The Ho²⁺ ground state involved in the charge transfer process is thought to lie close to the bottom of the conduction band; this explains the optical behaviour observed in our experiments.

PbTiO₃ ferroelectric thin films have been prepared on Si(001) by metal-organic chemical vapour deposition. The as-grown films were characterized by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. It is shown that the films were highly (001) oriented and had essentially the same lattice constants as the bulk single crystal. However, the as-grown films were subject to internal stress as shown by a downshift in the Raman modes when compared with a bulk single crystal.

Spark-processed Si was characterized by temperature-dependent cathodoluminescence (CL) measurements. The CL spectra were recorded between 6 and 300 K. Two separated CL maxima were observed. The high-energy peak centred around 480 nm and the low-energy peak near 650 nm. Whereas the intensity of the red CL increased at lower temperatures, the intensity of the blue CL had a maximum near 230 K and then decreased continuously with decreasing temperatures. The integrated intensity for both CL bands remained essentially constant at temperatures below 270 K. This behaviour is interpreted in terms of a competitive radiative mechanism. Furthermore, the localized distribution of cathodoluminescing centres across the surface of a sample was studied by spectrally resolved CL micrographs. The results are discussed in comparison with the photoluminescing characteristics of spark-processed Si and in the light of the luminescing properties of porous Si.

The mean positron lifetime tau , positron diffusion length L, and the positron and electron work functions ( phi ₊ and phi _-) for polycrystalline TiC have been experimentally determined. The results were tau =160(2) ps, L₊=138(27) nm and phi _-=3.96(0.08) eV; phi ₊ was shown to be almost certainly positive. These results strongly support the suggestion from recent first-principles electronic structure and positron state calculations that positions are trapped by and annihilate in metal vacancies in this material. XPS measurements indicate that the trapping sites may be predominantly in thin carbon-rich layers between grains, a picture which may also explain the long near-surface diffusion length.