Table of contents

A new method for measuring the spin of the electrically charged ground-state excitations in the quantum Hall effect is proposed and demonstrated for the first time for GaAs/AlGaAs multiquantum wells. The method is based on the nuclear spin-orientation dependence of the two-dimensional direct-current conductivity in the quantum Hall regime due to the nuclear hyperfine interaction. We use this method to determine the spin of the electrically charged excitations of the ground state at filling factor = 1.

The current knowledge about fluctuation-induced long-ranged forces is summarized. Reference is made in particular to fluids near critical points, for which some new insight has been obtained recently. Where appropriate, results of analytic theory are compared with computer simulations and experiments.

A neutron diffraction experiment was carried out on the glass-forming aqueous system of ammonium calcium nitrate tetrahydrate, ACN (NH₄NO₃·Ca(NO₃)₂·4H₂O). The method of isotopic substitution was used to determine the local structure round the NO^-₃ ion, the hydrogen-hydrogen (g_HH(r)), the hydrogen-oxygen (g_OH(r)), and the nitrogen-hydrogen (g_N2H(r)) radial pair distribution functions, in both the glassy (153 K) and liquid (303 K) states. The results show that significant changes occur on glassification. In particular, the nitrate ion exhibits an increase in the number of close hydrogen contacts and enhancement of its local structure. A microstructural model is proposed to explain the strong glass-forming ability of ACN.

The partial structure factors and pair distribution functions for liquid GeSe at 727(2) °C were measured by using the method of isotopic substitution in neutron diffraction. The results show that the liquid retains little memory of the high-temperature crystalline phase of GeSe. On melting, there is a reduction in the Ge-Se coordination number from 6 to 3.2(2) and both Ge-Ge and Se-Se homopolar bonds become features of the molten state with contact distances of 2.36(2) and 2.34(2) Å and coordination numbers of 0.8(1) and 0.22(3) respectively. The results are discussed by reference to the structures of molten CuSe and CuBr, which contain electronegative species of the same or similar size. The structure is also compared with that of the glass-forming network melt GeSe₂ and it is found that, unlike the latter, there is no strong indication of intermediate-range atomic ordering as manifest by a prominent first sharp diffraction peak in one of the measured partial structure factors.

The dielectric spectroscopy of the ferroelectric liquid crystal mixture FLC-6980 doped with a low concentration (~2% wt/wt) of anthraquinone dye has been studied. The complex permittivity has been measured in the frequency range 100 Hz to 1 MHz in SmC^* and SmA phases. Dielectric increments and the relaxation frequencies at various temperatures have been evaluated. Our results indicate that the relaxation frequency and the SmC^*-SmA transition temperature T(C^*A) decreases whereas the dielectric increment increases about 2.5-fold on dye addition.

We investigate the light-scattering intensity on three-dimensional cluster-cluster aggregations. The effect of reversible aggregation on the light-scattering intensity is clarified in terms of numerical simulations. For the case of weak interaction between particles, there is no peak as a function of the scattering wavenumber q in the static light-scattering intensity. We find that the profile of the light-scattering intensity on reversible cluster-cluster aggregations is apparently different from that for diffusion-limited cluster-cluster aggregations.

The Neutron Brillouin Scattering technique has been used to measure longitudinal excitations in a magnesium-zinc glass at momentum transfers, Q, within the first pseudo-Brillouin zone. The measurements were performed at room temperature and at constant momentum transfer, down to Q = 6.2 nm^-1, and an energy transfer of E = 16 meV. The experimental one-phonon spectral functions for the glass have been extracted and compared directly to theoretical predictions. At the first Brillouin zone boundary (Q_P/2 = 13 nm^-1) experiment and theory show some agreement, but at lower momentum transfers there is a shift of several meV in the peak position of the neutron intensity, towards lower energies. Evidence for the two observed modes is presented and the possibility of mode mixing is discussed. In conclusion, the need for new simulations is highlighted.

The phase transformation between diamond and graphite in preparation of diamond by pulsed-laser-induced liquid - solid interface reaction (PLIIR) was studied by calculating the probability of phase transition of the carbon atoms over a potential barrier in the pressure-temperature (P - T) phase diagram of carbon. It is found that the probability of phase transition from graphite to diamond is as high as 10^-3-10^-4 in the C pressure - temperature region where the pressure and temperature are in the range of 10 GPa to 15 GPa and 4000 K to 5000 K, respectively, in the pressure - temperature phase diagram. The distribution of the probability of the phase transformation from graphite to diamond was obtained in the corresponding pressure-temperature region, in which diamonds are prepared by PLIIR. In addition, the dependence of the probability for the transformation of graphite to diamond on temperature was investigated and found to be in agreement with the Arrhenius rule.

High-resolution kinetic neutron powder diffraction has been used to study the kinetic properties of the -Mn--Mn phase transition. This study reveals that the -Mn--Mn phase transition is homogeneous. The rearrangement of the Mn atomic positions in the - and -phases is discussed in terms of their influence on the low-temperature magnetic properties of elemental Mn.

By means of numerical simulations, we demonstrate that an alternating-current (ac) field can support stably moving collective nonlinear excitations in the form of dislocations (topological solitons, or kinks) in the Frenkel - Kontorova (FK) lattice with weak friction, as was qualitatively predicted by Bonilla and Malomed (Bonilla L L and Malomed B A 1991 Phys. Rev. B 43 11539). Direct generation of the moving dislocations turns out to be virtually impossible; however, they can be generated initially in the lattice subject to an auxiliary spatial modulation of the on-site potential strength. Gradually relaxing the modulation, we are able to get stable moving dislocations in the uniform FK lattice with periodic boundary conditions, provided that the driving frequency is close to the gap frequency of the linear excitations in the uniform lattice. The excitations that can be generated in this way have a large and noninteger index of commensurability with the lattice (so suggesting that the actual value of the commensurability index is irrational). The simulations reveal two different types of moving dislocation: broad ones, that extend, roughly, to half the full length of the periodic lattice (in that sense, they cannot be called solitons); and localized soliton-like dislocations, that can be found in an excited state, demonstrating strong persistent internal vibrations. The minimum (threshold) amplitude of the driving force necessary to support the travelling excitation is found as a function of the friction coefficient. Its extrapolation suggests that the threshold does not vanish at zero friction, which may be explained by radiation losses. The moving dislocation can be observed experimentally in an array of coupled small Josephson junctions in the form of an inverse Josephson effect, i.e., a direct-current-voltage response to the uniformly applied ac bias current.

We report magnetization, neutron diffraction and elastic constant measurements of the single-crystal rare earth alloy Gd₆₄Sc₃₆ as a function of magnetic field. For an a-axis field the material exhibits transitions from helix to fan to basal plane ferromagnet. At low temperatures the looked-in magnetic structure is very sensitive to the application of small fields (~5 mT).

With the formal similarity between three-dimensional layered systems and one-dimensional chains of potential wells in mind, we discuss the wavefunction formulation of the Korringa - Kohn - Rostoker (KKR) formalism in one spatial dimension. We show that screening allows the construction of wavefunctions for such interesting non-periodic chains as two semi-infinite bulks separated by a spacer segment. We also discover a powerful analogue of the Friedel sum usually associated with scattering of electrons by point defects.

The density of states of a tight-binding Hamiltonian on a two-dimensional quasiperiodic lattice (the Penrose tiling) is studied in terms of the first spectral moments. This approach shows that there is a tendency for a depletion of the local density of states at certain sites of the lattice, and a progressive localization of states at the centre of the spectrum. The effect of different kinds of phason flips in the first moments is also studied.

A model of the crystal field generated by a periodic array of charged tapes is developed to analyse the crystal-field interaction in RBa₂Cu₃O_x (R = Er, Ho) high-T_c copper oxides observed by the inelastic neutron scattering technique. The explicit calculation of the parameters of the Stevens Hamiltonian describing crystalline electric field effects in solids is performed for a specific charge density distribution uniformly extended in a certain direction of the crystal lattice. The model accounts for the x-dependence of the crystal-field parameters and allows us to determine the hole concentration in the CuO₂ planes as a function of oxygen stoichiometry. The model of the periodic array of charged tapes suggests a charge order induced in the CuO₂ planes by doping.

Measurements of spin-lattice relaxation time T₁ have been performed for polycrystalline ammonium bromide as a function of hydrostatic pressure and temperature. Activation parameters for NH₄⁺ ion reorientation as functions of pressure were obtained. A two-well potential model is proposed for description of ammonium reorientation motion. On the basis of the proposed model and obtained activation data the libration frequency of the ammonium ion as a function of pressure was calculated and good agreement between experiment and calculation was obtained.

The O K-edge x-ray absorption near-edge-structure (XANES) spectra of UO₂ and CeO₂ are presented and interpreted. Using different-size clusters around the excited atom in the full multiple-scattering (MS) simulation, we are able to link the features present in the spectra of each oxide to its specific atomic arrangement and electronic structure. The structures at the edge originate from oxygen 2p states hybridized with f and d orbitals of the cation split by the cubic crystal field. All of the other features come from MS with the neighbouring shells of the central oxygen atoms.