Table of contents

Thermodynamic quantities of a hydrogen-bonded chain model are calculated by means of the effective potential method assuming a non-linear double-Morse interaction. The relevance of the quantum effect is pointed out, especially through the modification of the intensity and position of the non-linear contribution to the specific heat.

Measurements of the thermal conductivity (k) of solid oxygen doped with nonmagnetic impurities of N₂ (0.34 and 0.94%) and Ar (1%) were made in the temperature range 1-35 K. It was shown that in the antiferromagnetic alpha phase doping with nonmagnetic impurities has a drastic effect on low-temperature thermal conductivity which supposedly is a result of suppression of the magnon contribution to the heat transport by the impurities. At the same time, the jump in the thermal conductivity at the alpha - beta transition and the thermal conductivity in the beta phase are practically insensitive to the doping effect which we attribute to strong magnetostriction effects characteristic of the beta phase.

Atomistic simulation techniques are applied to the study of crystalline B₂O₃. New interatomic potentials are derived by fitting to ab initio energy surfaces derived from both periodic boundary conditions and molecular calculations, to which we add the restraints produced by the observed crystal structures. A novel approach based on linear programming (LP) methodology is used in these fitting procedures. We find that the best potentials have terms which are coordination number dependent. Our potentials are able to reproduce the crystal structures of both polymorphs of B₂O₃. Moreover, we predict the stability of new polymorphs with high boroxol ring contents which may be related to the structures of vitreous phases.

We use our recently developed interatomic potential, which can reproduce the crystal structures of both phases of B₂O₃, in a molecular dynamics study of the structures of vitreous B₂O₃. Our calculated results suggest that the use of a partial charge model, and the inclusion of a B-O-B bond-bending, three-body term are important in reproducing the structure of this vitreous material. They also demonstrate the presence of boroxol (six-membered) rings in the simulated structures. Improved results are obtained when the potentials are refined by the inclusion of coordination-dependent terms which result in a higher proportion of boroxol rings in accordance with experimental data.

Inelastic neutron scattering experiments have been performed to study the librational motion of the NH₄⁺ ion in the perovskite family NH₄MF₃ (M=Zn, Mn and Cd). The first excited librational band appears as a broad peak in the high-temperature cubic phase. Below the structural phase transition, to a lower symmetry, the band splits into three narrow features. The overtone bands are also detected in the Cd compound. The frequencies of the fundamentals increase for increasing cell size (Zn>Mn>Cd), the same trend as the activation energies. The INS spectra are analysed quantitatively by expanding the potential in symmetry adapted rotational wavefunctions.

The proton mobility and transport of energy in hydrogen-bonded chains is an interesting problem because they play a crucial role in the interpretation of certain biological processes. We propose a physical model which consists basically of two one-dimensional harmonically coupled sublattices corresponding to the oxygens and protons, respectively. Our study starts with an improved form of the Antonchenko-Davydov-Zolotaryuk (ADZ) model in which the dipole-dipole interactions due to the proton motion are introduced. These interactions may affect the response of the nonlinear excitations propagating along the chain. We look for a solution for which the motion of oxygen ions can be neglected. We find a Phi ⁵-type equation which admits nonlinear excitations of solitary-wave-type. In our mathematical approximation we find different classes of solution for the analytical description of the proton motion. The expressions and the corresponding necessary conditions for the existence of these solutions are given. We present numerical simulations for certain cases and examine the potential involved in the system. The introduction of the dipole interaction produces an influence on the electric field of the system which means that the proton motion is also affected, making the proton conductivity much easier. Finally, possible further extensions of the work are discussed.

We present a comprehensive theory of the electrical conductance G of phase-coherent, multi-channel, resonant structures in the presence of superconductivity. When voltages of the order of the level spacing are applied, particle-hole symmetry is broken and our results differ significantly from earlier descriptions. After deriving generalizations of the well-known Breit-Wigner formula, valid in the presence of superconductivity, results for resonant transport in three classes of structure are obtained. First, for a superconducting dot (SDOT) connected to normal contacts (N), we examine the change in conductance as the magnitude of the superconducting order parameter increases from zero. The change is typically negative, except near a normal-state resonance, where large positive changes can occur. Secondly, for a structure comprising a normal (N) contact, a normal dot (NDOT) and a superconducting (S) contact, we predict that finite-voltage, differential conductance resonances are strongly suppressed by the switching on of superconductivity in the S contact. In the weak-coupling limit, resonances which survive have a double-peaked line-shape. Thirdly, analytic results are presented for superconductivity-enhanced, quasi-particle interferometers (SEQUINs), which demonstrate that resonant SEQUINs can provide galvanometric magnetic flux detectors, with a sensitivity in excess of the flux quantum.

In this work, we have attempted to improve an effective-medium approximation previously developed by us to study the effective response of a class of strongly nonlinear composite media, which obey a current-field (J-E) relation of the form J= chi mod E mod ²E. The improved calculations are compared with numerical simulations and with the recently established bounds for strongly nonlinear composites. We find very good improvement over the previous work especially at high contrast between the components.

Within the linear response theory we derive an expression for the alternating current (AC) magnetoconductance (MC) of a quasi-one-dimensional mesoscopic system in terms of its pertinent transfer matrices. It is then employed in numerical simulations of the AC MC for some specific systems. At low frequencies and for weak magnetic fields, the AC MC of a clean system is expressible as a sum of independent contributions from the different one-dimensional subbands (which is trivially valid for DC conductance). For disordered systems, it is conjectured that neither the conductance nor its logarithm is self-averaging. Moreover, the fundamental phenomenon of positive MC no longer holds at finite frequencies.

'Resonating-valence-bond' (RVB) variational wavefunctions are numerically investigated in order to describe the ground state and the singlet low-lying excited states of the spin-1/2, J₁-J₂ Heisenberg antiferromagnet with 1/8<or approximately=J₂/J₁<or approximately=1 on triangular lattices of N=4p sites compatible with the classical Neel orders. Comparison with previous exact diagonalization results indicates that the singlet eigenstates associated with collinear Neel order, among which the ground state is found, can be accurately described with appropriate superpositions of two-sublattice RVB trial states, while the other singlet low-lying eigenstates associated with four-sublattice Neel order can be well approximated from the four-sublattice RVB trial states.

The disordered ground-state properties of a double-layer quantum antiferromagnet are investigated in the bond operator representation. The intralayer and interlayer couplings are J and J_{perpendicular to} , respectively. In a mean-field approximation, we obtain a spin gap which decreases with increasing ratio lambda =J/J_{perpendicular to} and disappears at the value lambda _c=0.292. In the region 0<or= lambda <or= lambda _c, we calculate the disordered ground-state energy, the intralayer and interlayer nearest-neighbour singlet strengths, and the correlation length.

Ce₃Al₁₁ is an orthorhombic compound in which the cerium atoms occupy two different sites. Due to the crystal structure, the presence of twins cannot be avoided. Kondo effects are present, leading to unusual magnetic properties. Below the ordering temperature, 6.2 K, neutron diffraction experiments performed on powder, and more recently on single crystals, evidenced two different magnetic phases: the compound is ferromagnetic between 6.2 K and 3.2 K, and becomes modulated with an incommensurate propagation vector k=(0,0,kz) at lower temperatures. In both phases, the cerium moments are very different on the two sites, one of them being strongly reduced. However, from the integrated magnetic intensities alone, one cannot determine on which of the two sites the moment is reduced. Polarized neutron measurements were undertaken to remove this ambiguity and to obtain the magnetization density maps. The easy magnetization direction being along the b-direction, the experiment was performed with b parallel to the applied field. However, in the projection onto the plane perpendicular to b, the two cerium sites are superimposed and cannot be separated. It was then necessary to measure the magnetic density in three dimensions. To overcome the twin problem, measurements were performed for two orientations of the crystal and a new procedure for data treatment was used. The magnetization density maps obtained from Fourier inversion or by the three-dimensional maximum entropy method clearly show that the reduced cerium moment is on the less symmetric crystallographic site.

Samples with low-energy C₆₀ ions implanted into porous silicon were fabricated with the ionized cluster beam deposition approach for improving the light emission of C₆₀. Depth analysis by secondary-ion mass spectroscopy showed that C₆₀ had been incorporated into porous silicon. The photoluminescence spectrum measured under excitation by an Ar⁺ laser (514.4 nm) at room temperature showed a large number of intense and well resolved fine-structure peaks. These features indicated the strong coupling of vibrational progressions with the electron states of C₆₀, induced by the interaction between the C₆₀ molecule and the nanometre-sized silicon particles.