Table of contents

Experimental data on muonium states in HgO are presented. Muon spin rotation (µSR) experiments show that, at low temperatures, approximately 80% of muons implanted into a polycrystalline sample form paramagnetic centres with hyperfine interaction parameters A_iso = 14.93(5) MHz and D = 5.2(1) MHz. The remaining 20% form diamagnetic states. Diffusion of the paramagnetic state through equivalent sites is observed above 10 K, averaging the apparent anisotropy to a low value at and above 50 K. The variations of the amplitudes of the two signals with temperature indicate that the paramagnetic centres ionize in a temperature range centred around 160 K and that the associated donor levels lie considerably further from the conduction band edge than the corresponding states in ZnO and other II-VI compounds.

We calculate the correlation functions and the dc conductivity of Luttinger liquid superlattices, modelled by a repeated pattern of interacting and free Luttinger liquids. In a specific realization, where the interacting subsystem is a Hubbard chain, the system exhibits a rich phase diagram with four different phases: two metals and two compressible insulators. In general, we find that the effective low-energy description amalgamates features of both types of liquids in proportion to their spatial extent, suggesting the interesting possibility of `engineered' Luttinger liquids.

We observed de Haas-van Alphen (dHvA) oscillation in both the normal and superconducting mixed states of a heavy-fermion superconductor CeCoIn₅. The Fermi surfaces are found to consist of nearly cylindrical Fermi surfaces and small ellipsoidal ones, reflecting the unique tetragonal crystal structure. The detected cyclotron masses of 5-87 m₀ for these Fermi surfaces are extremely large, and correspond to a large electronic specific heat coefficient of about 1000 mJ K^-2 mol^-1. The cyclotron masses are also found to be field dependent in both the normal and mixed states.

The low-energy electronic structure of a metallic single-walled carbon nanotube (SWNT) in an external electric field perpendicular to the tube axis is investigated. Based on tight-binding approximation, a field-induced energy gap is found in all (n,n) SWNTs, and the gap shows strong dependence on the electric field and the size of the tubes. d a metal-insulator transition is predicted for all (n,n) tubes. We numerically find a universal scaling that the gap is a function of the electric field and the radius of SWNTs, and the results are testified by the second-order perturbation theory in weak field limit. the range Our calculation shows the field required to induce a 0.1 eV gap in metallic SWNTs can be easily reached under the current experimental conditions. It indicates the possibility of applying nanotubes to electric signal-controlled nanoscale switching devices.

We present ab initio calculations of a set of physical properties for the newly discovered MgB₂ superconductor. The zero pressure bulk modulus, the pressure derivative of the bulk modulus and their in- and out-of-plane linear values are evaluated. An analysis of the calculated parameters reveals the diversity in bonding interactions. The diboride is characterized by a moderately sizable anisotropy of compressibilites, which is smaller than for cuprates, but larger than for other related diborides. The anisotropic compression is expected to induce different pressure effects on different phonon modes and also to influence the electronic structure at the Fermi energy.

Isothermal and non-isothermal differential scanning calorimetry experiments were carried out to study rapidly solidified Al₉₀Fe₅Ce₅ alloy. Microstructural analysis suggests that icosahedral nanoparticles are homogeneously distributed in the matrix of annealed amorphous Al₉₀Fe₅Ce₅ alloy. The presence and homogeneous distribution of icosahedral structure units and icosahedral short-range domains appear to be critical for the formation and stability of the amorphous phase.

After a brief discussion of the Bogoliubov inequality and possible generalizations thereof, we present a complete review of results concerning the Mermin-Wagner theorem for various many-body systems, geometries and order parameters. We extend the method to cover magnetic phase transitions in the periodic Anderson model as well as certain superconducting pairing mechanisms for Hubbard films. The relevance of the Mermin-Wagner theorem to approximations in many-body physics is discussed on a conceptual level.

Mixed-spin systems composed of quantum spin chains with gapped magnetic excitations interacting through `auxiliary' magnetic ions show an effective separation between low-frequency classical and high-frequency quantum spin correlations. This phenomenon is realized in a family of rare-earth nickelates. Studies of these materials enable experimental measurements of some previously inaccessible fundamental properties of quantum spin chains to be made. The non-trivial intrinsic dynamics of the auxiliary spins gives rise to peculiar excitations of a mixed nature, and in certain cases may play a key role in long-range magnetic ordering.

The influence of swift-heavy-ion irradiation on the orientation of the hyperfine field in amorphous Fe₄₀Ni₃₅Si₁₀B₁₅ alloy is studied by Mössbauer spectroscopy. The change of the magnetic texture induced by 6 GeV Pb-ion irradiation is studied as a function of ion energy and the linear rate of electronic energy deposition for ion fluences ranging from 1×10¹¹ to 2.4×10¹³ ions cm^-2. The Mössbauer measurements revealed that in a substantial volume fraction of the irradiated samples the spins changed their orientation from the in-plane orientation to the perpendicular one. This effect was attributed to the formation of cylinders of `modified' amorphous structure along the ion path which may induce a stress as a result of which a substantial fraction of spins are aligned along the beam direction, i.e. perpendicular to the plane of the sample.

We introduce a Leonard-Jones (L-J) interaction into the two-dimensional (2D) Collins model, and consider the existence of the holes that are called the molecular fraction. The Gibbs free energy of solid, liquid and gas has been derived. From the Gibbs function we have obtained the whole diagram of the 2D monatomic L-J system, which includes the melting line, vaporization line, sublimation line and the triple point. Also, we have discussed a few properties of the critical point.

The heat capacity of three pure (n, p⩽2×10¹⁶ cm^-3) germanium crystals with different isotopic compositions was measured in the temperature range from 2.8 K to 100 K. These samples, one made of enriched ⁷⁰Ge (95.6%), Ge of natural isotopic composition and n, p < 10¹⁴ cm^-3, and one of the largest possible isotopic mass variance ^70/76Ge (43%/48%) with n, p<10¹⁴ cm^-3, show a change of the molar heat capacity (and corresponding Debye temperature, θ_D) as expected from the average mass variation, corresponding to θ_D∝M^-0.5 (M = molar mass) at low temperatures. The mass effect is best visible around 21.5 K, at the minimum of the corresponding Debye temperatures θ_D, and amounts to Δθ_D = 5.3 K for the difference between the Debye temperatures of ⁷⁰Ge and ^70/76Ge. The specific heat capacity of the natural Ge crystal agrees within 2% with the best data available in the literature taken on much larger masses of Ge.

A generalized form of Ginzburg-Landau theory is proposed which explains the non-hexagonal flux-line lattice found both in metallic and magnetic superconductors without invoking any anisotropic material-dependent properties. The Gibbs energy density postulated for magnetic superconductors (g) is of the form g(B,T) = α|ψ|² + ½β|ψ|⁴ + [1/(2m)]|(- i ℏ ∇-2eA)ψ|² + ∫(B/µ₀-M_ions)· d B-(B/µ₀-M_ions)·(µ₀M + µ₀H_ext) where M is the total local magnetization, M_ions is the local magnetization of the magnetic ions and H_ext is the externally applied field strength. The macroscopic Gibbs energy density and magnetization close to the upper critical field have been calculated for all possible periodic flux-line lattice structures, for high and low values of the Ginzburg-Landau constant (κ) in both metallic and magnetic superconductors.

The generalized theory is consistent with standard theory for high-κ metallic superconductors. However, for low-κ and/or strongly paramagnetic superconductors for which (1 + χ')/2<κ²<3.45(1 + χ')²/(1-χ')², where χ' is the differential susceptibility of the paramagnetic ions in the normal state, non-hexagonal flux-line lattice structures occur. When the flux-line lattice is non-hexagonal, (∂⟨M_SC⟩/∂⟨H_ext⟩)_Hext≈H_C2 = (1-χ')/(3 + χ'). Ferromagnetic and antiferromagnetic superconductivity occur when χ'>1. Furthermore, increasingly strong paramagnetism coexisting with superconductivity can produce a type I-type II phase transition. Experimental evidence for these phenomena and for a correlation between strong paramagnetism in magnetic superconductors and re-entrant superconductivity is discussed.

Nuclear magnetic resonance of ²D nuclei is measured in the paramagnetic phase of TbMn₂D₂ powder samples. Static and dynamic contributions of Mn and Tb moments to the nuclear probes are characterized on approach to the ordering temperature T_N = 272 K and the possibility of antiferromagnetic correlations between Mn and Tb magnetic moments is analysed. Field and temperature dependence of the magnetization in the ordered state is presented as well.

The nucleation and growth of Cd_1-xZn_xS and CdS_xSe_1-x nanocrystals in borosilicate glass are investigated through optical absorption and resonant Raman scattering measurements. The optical absorption spectra are interpreted with the help of a quantized-state effective-mass model applied for heterogeneous and homogeneous nucleation of nanocrystals. Stoichiometry changes of the particles during the growth stage are monitored by resonant Raman measurements. The findings indicate that the formation of Cd_1-xZn_xS particles involves a stage of homogeneous nucleation and then a rapid transition to ripening. No growth is observed. CdS_xSe_1-x particles undergo heterogeneous nucleation and growth simultaneously with a gradual transition into the ripening stage.

The reflection spectra, the excitation spectra of various emissions (2.4–4.5 eV) and the creation spectra of F centres have been measured in Na₆Al₆Si₆O₂₄(NaBr)_2x optical sodalite ceramics (x = 0.94 and x = 0.81) using VUV radiation of 6–35 eV. An analysis of the spectra allowed us to separate several groups of electronic excitations: the photons of 6.7–8.3 eV excite or ionize Br⁻ centres in the β-cages, 8.5–32 eV photons generate electronic excitations of the aluminosilicate carcass, while 33 eV photons excite Na⁺ ions up to the 2p⁵3s state. The creation mechanism of F centres, connected with the trapping of conduction electrons by pre-irradiation bromine vacancies and localization of holes at Br⁻ centres in β-cages, has been revealed in sodalites at 80 K. The efficiency of F centre creation triples in the multiplication region of electronic excitations of the aluminosilicate carcass (20–30 eV) in a sample with x = 0.94. Using methods of thermoactivation spectroscopy, the creation of thermally stable F centres (up to 480 K) has been detected in sodalites irradiated by x-rays or 27 eV photons. However, such F centres cannot be created by 7.6–11 eV photons, i.e. when there is no multiplication of excitations and an exciting photon forms only one electronic excitation.