Table of contents

The spontaneous Aharonov-Casher (AC) effect associated with partially spin-polarized states in one-dimensional mesoscopic rings is investigated for the first time. The results obtained seem to be novel and interesting. Particularly, for an even number of electrons, the spontaneously introduced AC flux does not depend on the polarization parameter N_p; while for an odd number of electrons, it increases with a specific set of N_p and decreases with the other set. Moreover, the polarization magnetic field is calculated analytically.

Neutron powder diffraction studies as a function of temperature on MnPS₃ have revealed temperature-dependent features inconsistent with the previously quoted magnetic ordering. The features vanish at approximately 12 K, a temperature well above the accepted Neel temperature of 78 K. The features are consistent with a true Ising-like two-dimensional ordering with the moments being antiferromagnetically coupled and pointing along the (010) direction.

Extensive molecular-dynamics (MD) simulations are performed for a rigid-ion model of molten metal trichlorides (MCl₃), in different regimes of metal radius, density and temperature. The cases of molten YCl₃ and AlCl₃ are examined in more detail in order to ascertain the presence of medium-range order (MRO) in these materials. For the YCl₃ case, the experimental neutron structure factor, signalling the presence of MRO in the system, is fairly well reproduced by MD. The overall structural information available from the simulation indicates that order at intermediate distances basically consists of local 'layers' of corner-sharing (YCl₆)^3- octahedra. These roughly planar formations may represent a remnant of the real YCl₃ crystalline structure, constituted by layers of edge-sharing (YCl₆)^3- octahedra; the internal topology of the 'planes' is discussed in relation to the assumed model potential and to global charge ordering. The effect of metal size on the MRO features is then investigated by adjusting the model parameters to the case of molten AlCl₃, in which the Al³⁺ effective radius is significantly smaller than that of Y³⁺. In this case well defined tetrahedral (AlCl₄)^- units tend to be formed, and these link to each other in a highly connected network responsible for MRO. The presence in the melt of bound pairs of tetrahedra, interpretable as Al₂Cl₃ 'dimers', is also established, in essential agreement with the prediction of recent theoretical work by other authors.

An effective interionic pair potential in liquid copper at 1393 K was extracted from the neutron-diffraction data using an iterative fitting procedure based on the modified hypernetted-chain approach. The procedure involved extensive molecular-dynamics simulation using a system of 16384 particles. The potential, constructed within the framework of the nearly-free-electron approximation, is based on the Dagens-Rasolt-Taylor model, and it is repulsive and positive around the first-neighbour distance. The proposed potential when used in molecular-dynamics calculations reproduces, within the limit of experimental accuracy, the structure factor measured by neutron diffraction, including the long-wavelength region of the structure factor and the isothermal compressibility limit, as well as the experimental value of the self-diffusion coefficient. The bridge function of liquid copper derived from the simulation data is compared with a hard-sphere fluid bridge function obtained in the context of the modified hypernetted-chain approximation.

The tracer diffusion coefficient D₃₊ of the trivalent lanthanide cation ¹⁵³Gd³⁺ is studied in mixtures of isobutyric acid, water and 5*10^-4 M Gd(NO₃)₃ using an open-ended capillary tube. These data are given over the complete composition range at T-T_c=0.171 K. The coefficients D₃₊ are combined with the shear viscosity eta of the electrolyte. The results are discussed in terms of the Einstein-Stokes law; our analysis suggests three different behaviours of the diffusing entity in the mixtures and it is based on the preferential solvation phenomenon.

The local structures of zincblende-type solid solutions A_1-xB_xC can be described with five distorted unit cells based on a phenomenological valence force-field approach. Bond lengths and strain energies in these solid solutions can be obtained. The distorted feature of the local structure is well described by the five distorted unit cells. This structural model has been applied to 18 A_1-xB_xC systems. Results are compared with those from extended X-ray absorption fine structure and demonstrate that all the first- and second-neighbour bond lengths are in agreement with experimental values, with the largest divergence being about 0.01 AA. In addition, this model is also compared with the five special tetrahedra model.

Using a bond orbital model, we calculate the bond length and polarity of semiconductors, the symmetric relaxations around isovalent impurities in semiconductor-impurity systems, and the bond-length variations in solid solutions A_1-xB_xC of semiconductor alloys. We find that small impurities have a large relaxation, but the variations in bond energy are smaller than those of large impurities. The results are compared with other theoretical and experimental results and are found to be in excellent agreement with experiments.

Infrared and optical spectra were measured for synthetic quartz in Y-bar form, as it received progressive dosage of gamma -irradiation. The gamma -induced coloration varied in intensity over different growth regions of the Y-bar, in proportion to the density of aluminium impurities, the latter being dependent on the growth speed of the region in question as well as the polarization effect of the x-axis. From the variations in absorbance within the conventionally designated S₄, e₂ and A₁ absorption bands, we deduce that, over a broad range of dosage, Al-OH defects increased continuously while as-grown OH sites hardly decreased in concentration. This indicates the presence of another kind of as-grown A centre that does not show up in IR absorption. It is most noticeable in the -x region, which, like air-swept quartz, has a low radiation susceptibility.

We study one-dimensional harmonic chains in which clusters of two or three defect atoms are embedded randomly. The disorder in the systems appears in the masses of the atoms. Reflectionless modes are obtained by studying different kinds of correlation among the masses. The localization behaviour of the modes around these special frequencies is examined analytically as well as numerically. To discern the nature of the modes at and around those frequencies, density of states, bandwidth scaling and site Green functions are studied. If the special frequencies lie within the common band of the constituent atoms and at zero the modes are extended at and around them. However, the modes are critical when the special frequency appears at the upper band edge of the host system. The number of non-scattered modes is estimated for all cases. It is approximately square root N for the dimer problem. For the trimer problem with degenerate resonances appearing inside the constituent band it is approximately N³4 /. If the degenerate resonances of the trimer appear at zero frequency the number of non-scattered modes is approximately N⁵6/.

Quantum oscillations have been investigated far into the mixed state of the type II superconductor 2H-NbSe₂. The de Haas-van Alphen effect is found to persist to fields as low as approximately 0.3B_C2 and to temperatures of approximately 0.003T_C. A self-consistent band structure calculation, performed for 2H-NbSe₂ in the normal non-charge-density-wave state using a full potential linearized augmented plane wave (FLAPW) method, accounts well for the experimental angular dependence of the quantum oscillations in terms of a small flat hole Fermi surface around the Gamma point. This feature derives mainly from chalcogen p bands and has a mass-enhancement factor much smaller than expected from the specific heat. Quantum oscillations in the presence of the vortex lattice are observed to experience an additional damping, from which the order parameter is estimated to be Delta (0)=0.6+or-0.1 meV.

We have reported a comparative study of the relativistic and non-relativistic Landauer resistances (LR) of Thue-Morse (TM) lattices. The TM lattice treated by us consists of rectangular potential barriers, with their centres distributed according to TM sequence. The purpose behind our study is to examine how the relativistic impact on the LR of TM lattice depends on the energy of the electrons and various parameters related to the TM lattice. Among other things, we find that the relativistic impact increases with increase in the number of barriers in the TM lattice and reduction in the width of barriers.

The effective mass of the Frohlich polaron is calculated with the two-mode squeezed states in the intermediate-coupling region and found to be in close agreement, with the results of other models. Within the same approach, the induced charge density around an electron in polar solids is calculated. Our result for this charge density contains an additional term which shows a feature in the spatial dependence as a manifestation of the squeezing effect.

The composition of electronic levels as well as optical transitions associated with AgCl₆^4- and AgF₆^4- complexes have been studied through MS-X alpha and SCCEH calculations performed as a function of equatorial (R_eq) and axial (R_ax) metal-ligand distances. The scheme and composition of levels for AgCl₆^4- is rather different from that for AgF₆^4- and other more ionic systems. The first transition for KCl:Ag²⁺ (observed at 12 500 cm^-1) is assigned to a jump involving the 5a1_g orbital which is mainly built (about 70%) from 3p orbitals of axial chlorine atoms. Aside from explaining reasonably the five optical bands experimentally observed for KCl:Ag²⁺, the present work indicates that the first allowed charge-transfer transition of AgF₆^4- would lie in the ultraviolet region and confirms that the unpaired electron in AgCl₆^4$spends a little more time on equatorial ligands than on the central ion. All these results are consistent with a high value ( chi =2.8) for the optical electronegativity of Ag²⁺. The dependence of electronic transitions (and also of unpaired spin densities f_sigma and f_s) on R_eq and R_ax is found to be rather similar for both AgF₆^4- and AgCl₆^4- complexes. The relation between such a dependence and the band widths of optical transitions is outlined.

This work investigates the polarized charge-transfer (CT) spectra of the Jahn-Teller distorted CuCl₆^4- and CuBr₆^4- complexes formed in Cu²⁺-doped [N(CH₃)₄]CdCl₃ and [N(CH₃)₄]CdBr₃ crystals. The transition energies as well as the dominant polarization of the CT bands along the hexagonal c direction are explained in terms of rhombic D_2h distortions. In the bromide complexes, the strong spin-orbit interaction of the Br^- ligands leads to additional bands in the low-temperature spectra. Evidence of vibronic couplings to totally symmetric vibrations is found from analysis of the temperature dependence of the band width. The influence of Cu²⁺ impurities in the lattice dynamics is studied through the variation undergone by different spectroscopic parameters in the 10-300 K range. Throughout this work, the high sensitivity of CT transitions for detecting the structural phase transitions exhibited by these crystals is demonstrated. In particular, the enhancement of the first-order character of the ferroelectric phase transition in [N(CH₃)₄]CdBr₃:Cu²⁺ at T_F=157 K, which has been associated with the presence of Cu²⁺ impurities is worth noting.

We calculate the heat capacity of electrons as a function of the electron density and temperature in two-direction double-barrier resonant-tunnelling structures. The strength of the barrier potential increases in one direction, so that the system becomes 2D; the heat capacity as a function of the electron density goes to a step-like shape, which is similar to the one in the DOS. From to to 1D, the heat capacity reflects the peaks in the DOS with increasing electron density, and becomes a sawtooth-like shape in 1D. However, an asymmetric peak in the DOS makes two peaks in the heat capacity because the available DOS in thermal excitations becomes smaller as the chemical potential approaches the peak in the DOS. The heat capacity shows a linear dependence on the temperature in 3D and 2D, but not in 1D even at low temperatures. It exhibits a square root T dependence when the chemical potential is located near a pole in the DOS.

A detailed and comprehensive study of the one-impurity multichannel Kondo model is presented. In the limit of a large number of conduction electron channels k>>1, the low-energy fixed point is accessible to a renormalization group improved perturbative expansion in 1/k. This straightforward approach enables us to examine the scaling, thermodynamics and dynamical response functions in great detail and make clear the following features: (i) the criticality of the fixed point, (ii) the universal non-integer degeneracy, and (iii) that the compensating spin cloud has a spatial extent of the order of one lattice spacing.

The anisotropic susceptibility of the layered antiferromagnet NiPS₃(T_N=155 K) has been measured between 45 K and 650 K. The system may be described by the effective spin Hamiltonian H=DS_iz²- Sigma _ij J_ijS_i.S_j, with the quadratic single-ion anisotropy terms introducing anisotropy in an otherwise isotropic situation. The exchange J and single-ion anisotropy parameter D were determined from an analysis of the anisotropic susceptibility data for two different models: (i) the Oguchi model, in which a pair of spins chosen at random is treated exactly while its interactions with the rest of the crystal are approximated by a mean field and (ii) the correlated effective field (CEF) approximation developed by Lines, which reduces the many-body problem to a single-particle, non-interacting ensemble form, by the introduction of static temperature-dependent correlation parameters, which are evaluated by forcing consistency with the fluctuation-dissipation theorem. It is found that the CEF approximation is superior to the Oguchi model in describing the susceptibility of NiPS₃. The exchange and crystal held parameters for the CEF approximation are J/k=-58.0 K; D/k=16.1 K; g/sub ///=2.05 and g_{perpendicular to} =2.13.

We have measured time-resolved optical absorption spectra following the excitation of the self-trapped excitons (STEs) in NaCl from the lowest triplet state to the next higher electron-excited state. In addition to the rapid reduction of the band due to the electron transition of the STE (STE band), a new absorption band peaked between the STE band and the F band is found to be temporarily generated. The heights of the new band and that of the STE band show anticorrelated oscillations with a period of 1 ps and moreover the new band disappears and the STE band recovers within 7 ps after the excitation. It is shown that the new band is due to the transition from the adiabatic potential energy surface on which the lowest STE is located and the oscillation is interpreted to be a motion of a wave packet on the adiabatic potential energy surface. We find also that the F band grows in the relaxation process.

Using the embedded-atom method, the variation in the total energies and lattice parameters with the degree of long-range order has been calculated for the parent and 18R₁ martensite phases for several Cu-Zn-Al and Cu-Al-Ni shape memory alloys. It was found that the bond angle phi , i.e. the angle between lines connecting the nearest neighbours in the basal plane of the martensite, increases with increase in degree S_B2 of B2-type order for both Cu-Zn-Al and Cu-Al-Ni alloys. When the degree S_L2 of L2₁-type order increases, the bond angle phi increases for the Cu-Zn-Al shape memory alloys, but it decreases for the Cu-Al-Ni shape memory alloys. This result agrees well with experiment.