Table of contents

We examine the change delta G in the two-probe electrical conductance G of a mesoscopic sample due to the switching on of superconductivity and prove that when conductance of the normal mesoscopic host is sufficiently high, the onset of superconductivity always produces a decrease in G. If the superconducting order parameter is of magnitude Delta , we focus attention on the susceptibility chi _Delta =lim_{Delta to 0} delta G( Delta )/ delta ( Delta ²). For weakly disordered, (i.e. ballistic) normal hosts, the average value of this quantity is negative. For diffusive hosts, the mean of chi _Delta increases linearly with (N-G(0)), where N is the number of open channels in the external. current carrying probes, while the fluctuations about the mean are found to be independent of G(0). With increasing normal disorder, chi _Delta undergoes a transition to a region of large fluctuations, before approaching a strongly localized regime, where typical values of chi _Delta are of order (ln²G(0))^-1.

We study the topological decoherence, arising from environmental spins, which acts to suppress the quantum coherent behaviour of grain magnetization. It is found that this decoherence mechanism is so effective that even under the most favourable conditions, it will be very difficult to see quantum coherent behaviour of grains having a quantum number S greater than approximately O(10). This conclusion does not affect the possibility of seeing tunnelling behaviour for larger grains.

The magnetism of rare-earth intermetallic compounds is investigated with a model Hamiltonian containing crystal field and exchange terms. An algebraic analysis of the characteristic polynomial associated with the model is made and it is shown that, for Pr³⁺ (J=4) and Nd³⁺ (J=9/2), in the case of cubic symmetry, the ninth- and tenth-degree polynomials are decomposed into simple factors. From these factors, analytical expressions for the crystal field eigenvalues, the magnetic moments and the critical temperatures are derived. The results are applied to PrAl₂ and NdAl₂; exchange parameters and effective g factors are easily obtained from crystal field, T_c and low-temperature magnetic moments data.

The low-temperature behaviour of the thermodynamic quantities in the one-dimensional 1D antiferromagnet with a Haldane ground state was analysed using a concept of non-interacting quasiparticles as a starting point. The proposed description appeared to be in good agreement with the results of the Monte Carlo study of a 128-site spin chain and experimental data obtained in NENP (Ni(C₂H₈N₂)₂NO₂ClO₄) provided that the elementary excitations constituting the Haldane triplet obey Fermi statistics.

The structure and intrinsic magnetic properties of Sm₂Fe_17-xGa_x (0 < or = x < or = 6) compounds were studied. These compounds crystallize in the rhombohedral Th₂Zn₁₇-type structure and are single phase except for Sm₂Fe₁₁Ga₆ which contains some alpha -Fe. The Curie temperature T_c is found first to increase and then to decrease with increasing x, attaining a maximum value of 595 K at about x = 3, which is 208 K higher than that of Sm₂Fe₁₇. X-ray diffraction measurements on magnetically aligned powder samples show that samples with x < 1 and x > 5 exhibit easy-plane anisotropy while samples with 1 < x < 5 exhibit easy c-axis anisotropy. The samples with x = 1 and 5 show a mixture of easy-plane and easy-axis behaviour.

New complex perovskite-type dielectric materials, (Pb_1-xBa_x)(In_0.5Ta_0.5)O₃ (PBIT) and (Pb_1-xB_x) (In_0.5Nb_0.5)O₃ (PBIN) have been successfully synthesized using a rapid thermal sintering process. These new complex perovskite materials show relaxer-type dielectric properties, and temperature at which the real part of the permittivity shows a broad maximum is shifted towards lower temperature by the substitution of Ba ions for A-site Pb ions.

Structure factors obtained from neutron diffraction measurements on liquid K-Tl and Cs-Tl alloys exhibit large prepeaks at approximately 0.77 AA^-1 and 0.70 AA^-1. respectively. It is concluded that the liquid contains large units of thallium atoms, possibly bearing some resemblance to those found in crystalline K₈Tl₁₁.

We present ab initio calculations for the electronic density distribution, and electron-ion and ion-ion correlation functions for the liquid alkali metals at conditions near the melting point. Our calculations are based on the use of the neutral pseudoatom method to derive the interionic pair potential and on the variational modified hypernetted chain integral equation theory of liquids. The resulting theory is free of adjustable parameters, using the atomic numbers as the only input data. The interionic pair potentials obtained follow the expected trends, whereas the theoretical structural functions show a good agreement with experiment.

The feasibility of using the wavelength-dependent neutron depolarization method for bulk domain structure studies is demonstrated using pulsed polarized neutrons. Information on the mean magnetization, static magnetization fluctuations, and magnetic anisotropy was obtained for an amorphous ferromagnetic ribbon of Fe₇₈Co_8.1B_13.9 taken as an example. Several domain models were tested. A simple model of penetrating domains was found to be reasonable for as-cast ribbon under favourable circumstances.

The cationic distribution and some structure parameters of mixed spinels with a general formula Zn_xCo_3-x0₄, 0.5<x<1, prepared by thermal decomposition of corresponding zinc-cobalt nitrate precursors, have been studied by means of neutron powder diffraction technique. A partially inverse structure has been determined in all cases with Zn²⁺ cations occupying preferentially the tetrahedral positions.

Randomly quenched impurities pin the modulation wave of incommensurate (I) structures and prevent its free motion. However, rapid thermal fluctuations reduce the pinning frequency (or, in the case of structurally incommensurate insulators (SIIS) the phason gap Delta _psi ), and they have a significant effect on the spin-lattice relaxation time T₁ and the NMR and NQR lineshapes. A T₁ and lineshape model is developed for the I phase of SIIS. This model, which includes impurity pinning and thermal fluctuations, is used to explain ³⁵Cl(1) T₁ and experimental lineshape data for a pure Rb₂ZnCl₄ crystal. The results indicate that, while the impurities are of a strong-pinning symmetry-breaking type very close to the paraelectric-to-incommensurate transition temperature T_I, thermal fluctuations significantly reduced the impurity-induced pinning.

The first-principles self-consistent cluster method has been used to study the electronic structures and magnetic properties of Fe₂P and the effect of doping with Ni. We find that, for Fe₂P, the local magnetic moments of Fe_I atoms at the tetrahedral sites are smaller than those of Fe_II atoms at the pyramidal sites owing to the greater hybridization of Fe_I with P; when Ni atoms replace Fe_I atoms, the Fe_II magnetic moment shows a slight increase and the ferromagnetism of the (Fe_1-xNi_x)₂P compounds is strengthened. However, the substitutions of Ni in Fe_II sites reduce the Fe_I atom magnetic moment substantially. These results are in good agreement with experiments.

It is found that, below 80 K, orthorhombic TaS₃ demonstrates spontaneous resistance fluctuations. The spectral density of the fluctuations grows as the temperature decreases. In the temperature range between 50 and 70 K the spectra reveal a mean relaxation time which decreases as the temperature decreases with an activation energy about 1300 K. We conclude that the main source of the resistance fluctuations are thermally assisted jumps of dislocations of the charge-density waves.

Some general relationships have been derived in terms of the unrestricted Hartree-Fock method that define the conditions leading to the breaking of symmetry of one-electron orbitals at oxygen vacancies in oxides of the perovskite family. Numerical calculations of the critical values of relevant parameters have been carried out. Experimental data are discussed in the light of the results obtained.

The reflectivity and photoconductivity spectra for mixed crystals Cd_1-xFe_xTe (0.00<or=x<or=0.05) have been investigated. The reflectivity measurements have been carried out in a wide energy range-1.0-30.0 eV at 300 and 77 K. The photoconductivity spectra have been taken in the 1.0-3.0 eV range at 300 K. The analysis of the energy positions of maxima in reflectivity and photoconductivity spectra as a function of the Fe concentration allowed us to confirm the existence of the deep donor-like state predicted by the band-structure model of these compounds. Using the photoconductivity data the value of 0.08 eV above the top of the valence band for the energy position of this deep donor-like state was found. The influence of Fe²⁺(3d⁶) states on the changes in the energy positions of the reflectivity maxima E₀, E₁, E₁ + Delta ₁, E₂, E₁', E₁'+ Delta , c, d, e, f, h and i has been analysed and the results obtained are consistent with the models existing in the literature.

We calculate the density of states and band conductivity of a two-dimensional electron gas with a periodically modulated magnetic field. We consider the case of strips where the magnetic field is alternately going up and down with zero mean. We find characteristic features in the density of states and the conductivity at low energies as the electrons are only able to drift parallel to the strips of magnetic field. As the energy is increased, and the electrons start to be able to drift perpendicular to the strips there are pronounced band-structure effects in the conductivity.

We formulate microscopic expressions for capacitances, admittances and the rectification properties for small phase-coherent samples consisting of a number of metallic layers separated by insulators. The electric potential in such a structure is discussed with the help of characteristic functions which determine the variation of the microscopic potential inside the sample in response to an increase of the electro-chemical potential at a contact. An electrochemical capacitance matrix is derived which allows for field penetration into the conductor. We discuss the admittance matrix for conductors with nearby capacitors (gates) and analyse its magnetic field symmetry. We use the characteristic potentials to discuss the rectification properties of a conduction channel in the presence of nearby capacitors.

The L_2,3 edges of compounds containing tetrahedrally coordinated, isoelectronic d⁰ transition-metal oxyanions, TiO₄^4-, VO₄^3-, CrO₄^2- and MnO₄^- have been measured using electron energy-loss spectroscopy (EELS). The general shape of the electron energy-loss near-edge fine structure (ELNES) is found to be remarkably similar for these oxyanions and arises from the atomic multiplet spectrum of the d⁰ transition-metal ion modified by the tetrahedral field due to the oxygen ligands. The observed structure is also discussed within the framework of molecular-orbital (MO) theory. The possibilities of using these spectra as fingerprints for d⁰ transition-metal ions in tetrahedral coordination is discussed. The structure observed at the O K edges is also commented upon in the light of these findings.

The dependence of the magnetic moment of Fe₁₆N₂ on the distortion of the Fe lattice has been calculated using the discrete variational X alpha cluster method. The magnetic moments of Fe₁₆X₂, where X is B, C, N or a vacancy (VAC), have been also calculated. From the calculated results for Fe₁₆X₂ (X=N or VAC) and the results for Fe₁₆N₂ calculated with varying distortions of the Fe lattice, we have found that (i) the interaction between the 3d orbital of the first-nearest-neighbour Fe from N (Fe I) extending toward N and the N 2p orbital and (ii) the interaction between the 3d orbital of the second-nearest-neighbour Fe (Fe II) extending toward N and the N 2p orbitals are both weak. We have also found that the presence of N reduces the moments of Fe I and Fe II. The average calculated magnetic moment of Fe₁₆N₂ is 2.25 mu _B, which is far smaller than the experimental values obtained by Komuro and co-workers. but is in good agreement with experimental results obtained more recently by Takahashi and co-workers.

Orientational dependences of the quadrupole spin-lattice relaxation rates are calculated for a system of spin-3/2 nuclei in a single crystal subject to a random two-site exchange in a high magnetic field. A technique with which the transition probabilities per unit time w₁ ( Delta m=1) and w₂ ( Delta m=2) can be measured with approximately equal accuracies is proposed. The possibility of extracting the elements of the fluctuating electric-field-gradient tensor in a crystal-fixed coordinate system together with the exchange rate from the orientational dependences of the spin-lattice relaxation rates w₁ and w₂ is discussed. Two examples-a thermal exchange between two equilibrium sites in a temperature-independent double-potential well and an exchange between two sites above and below an order-disorder phase transition-are briefly discussed.

A method for directly calculating the Huang-Rhys S-factors and the effective phonon energy h(cross) omega as well as the linewidths of ⁴T₂ emission in chromium-doped oxides and fluorides is introduced. By use of this method, the experimental data required are only the wavelengths of the absorption and the emission peaks of the transitions between ⁴T₂ and ⁴A₂. The calculated values agreed very well with the experimental data published in the literature. Comparing the calculated linewidth with the experimental linewidth, one can also obtain information about whether the spectra result from one or more than one active centre and the existence of disorder in the crystal field of the active centres.

The luminescence spectra of CaS and SrS (as well as their solid solutions Ca_1-xSr_xS) doped with cerium have been measured under excitation by X-ray tube and X-ray synchrotron radiation. The comparison of these spectra with those of photoluminescence has revealed two types of radiation-induced Ce³⁺ emission centre. We suppose that the first type is an activator ion displaced from the regular site as a result of the Coulomb interaction with charged defects (trapped electrons and holes) created in its neighbourhood by the excitation in the fundamental absorption range, whereas the second type is probably due to the creation, by X-ray exciting radiation, of sulphur vacancies in the first coordination shell of Ce³⁺.

The role of multiple-scattering contributions in the formation of the X-ray absorption fine structure (XAFS) above tungsten L edges is discussed for the non-stoichiometric tungsten oxides WO_3-x. It is found that the presence of linear or near-linear atomic chains in the WO_3-x crystal structure leads, due to the focusing effect, to a strong increase in the XAFS amplitude. which results in an admixture of the L₂-edge XAFS With the XAFS above the L₁ edge.

We present a real-space renormalization group (RG) approach to study the electronic properties of the two-dimensional Penrose tilings. This approach is based on a recursive evaluation of the generating function which gives the density of states (DOS) of the system. The RG equations are derived in terms of the inflation rules of the Penrose tilings and the calculated DOS matches the results derived by purely numerical methods.