Table of contents

A double Yukawa (DY) potential, fitted on the carbon - carbon (CC) van der Waals (vdW) Lennard-Jones 12,6 (LJ) potential and integrated on two facing spheres, yields another DY function, whose parameters are simply related to those of the original CC LJ. We exploit this fact to obtain a new analytic expression of the DY type that describes the high-temperature physical interactions between molecules in good agreement with the well-known Girifalco potential, which was obtained from the direct spherical integration of the LJ potential. The main advantage of having a DY pair potential comes from the fact that analytic expressions of thermodynamic functions can be obtained within the Percus - Yevick approximation of the Ornstein - Zernike equation. As an example, an analytic expression of the Helmholz free energy is derived, and the double tangent construction on its vdW loop allows the determination of the liquid - vapour coexistence line and an estimation of the critical parameters ( K, ). This is in good agreement with the results predicted from various other theories with the Girifalco potential.

Non-linear band dispersion in the Luttinger model of one-dimensional interacting electrons gives rise to collisions between the bosonic sound-like excitations. The decay rate is calculated beyond a plain perturbation theory both at T = 0 and . A self-consistent approach reveals a non-analytical dependence of the lifetime of excitations on the coupling constant, wave vector and temperature.

We have studied pattern formation under the quantum confined Stark effect and found different patterns with complex structure of the electron and hole wave functions which give rise to nonuniform dipolar patterns of the electric charge inside the quantum well layer. The results obtained indicate spontaneous breaking of the transversal invariance, which can lead to new optical effects and transport phenomena.

We present a general solution of the trapping model for positrons which diffuse inside a grain of arbitrary shape towards its surface. The consideration takes into account both the diffusion and transition regime. The explicit form relations for the mean positron lifetime and the positron lifetime spectrum were obtained. The theoretical relation for the mean positron lifetime was analysed thoroughly for a grain of ellipsoidal shape. We established that the shape of the grain influenced the positron annihilation characteristics.

Calcium niobium gallium garnet (CNGG), a new non-linear optical material, is studied by Raman and infrared reflectivity spectroscopy at low and at room temperature for various scattering configurations. No structural phase transition is observed in the temperature range of the experiments. The phonons are classified within their symmetry species in the space group assumption; their dynamical parameters such as wavenumber, damping and oscillator strength are obtained on the basis of the infrared reflectivity spectra analysis. The comparison of the infrared and Raman activity of the phonon modes is in favour of the aforementioned space group; however, Pockels electrooptic coefficient room temperature measurements did not allow us to confirm this space group.

After a crystal was intercalated with , the internal-friction peak due to the orientational relaxation of the molecules shifted to a lower temperature. Also, the activation energy decreased, by 26 meV. These results indicate that the depth of the orientational potential well is changed by the intercalation. This change is assumed to be due to the electrostatic interaction between and molecules. The decrease of the order-disorder phase transition temperature, which is by about 20 K, can also be explained using this model.

A double Ising spin model for uniaxially structurally modulated materials exhibits as a special feature phase transitions between phases with equal wavenumbers but different pseudo-spin configurations. The character of these `internal' transitions is investigated in the mean-field approximation, with the mean-field transfer-matrix method, and in Monte Carlo simulations. The structural changes at the transitions are characterized by different strengths of harmonics in a Fourier analysis of the spatial modulation. A dielectric anomaly in the phase diagram of betaine calcium chloride dihydrate (BCCD) and seemingly contradictory structure analyses are explained.

Structural properties of the (NaCl-type) and (CsCl-type) phases of alkaline-earth oxides and their phase transition have been investigated with the periodic ab initio linear combination of atomic orbitals method implemented in the CRYSTAL program. The geometries have been optimized and the bulk modulus evaluated. The calculations have been done at the Hartree-Fock (HF) and density functional theory (DFT) levels. In this last case, the exchange-correlation potential correcting the electronic density uses either one local or three non-local models. The comparison of the different approaches allows us to identify a trend, in order to obtain results in better agreement with experiment.

A theoretical and numerical investigation of the quantum tunnelling of the domain walls in ferromagnets and weak ferromagnets has been performed, taking into account the interaction between the walls and the thermal excitations of the crystal. A numerical method for making calculations of the probability of thermally stimulated quantum depinning as a function of temperature has been evolved.

thin films in the range have been synthesized by sulphuration/seleniation of Ni thin films thermally evaporated on glass substrates. Growth conditions of temperature, time and pressures have been established to obtain samples with different selenium content and a theoretical model of the process has been proposed. Structural properties of the films are studied by Rietveld refinement of x-ray diffraction data. Values of lattice constant, a, sulphur/selenium positional parameter, u, and Se content in solid solution have been obtained. Moreover, Ni-anion and anion-anion bond distances have been obtained and analysed in relation to the selenium in the samples. It has been found that the lattice parameter follows a Vegard law with the selenium content. Resistivity measurements as a function of temperature from 10 to 200 K have been obtained for samples with x = 0.22 and x = 0.40 respectively. A change in the resistivity behaviour is evident in the sample with x = 0.40 at K, which can be associated with a phase transition observed in single crystals with the same Se content.

We have investigated the attractive Hubbard model in the low-density limit for the 2D square lattice using the ladder approximation for the vertex function in a self-consistent, conserving formulation. In the parameter region where the on-site attraction is of the order of the bandwidth, we found no evidence of a pseudo-gap. Furthermore, we have observed that the suppression of the Fermi surface known to destroy superconductivity in one and two dimensions, when these systems are treated using a non-self-consistent theory (Schmitt-Rink, Varma C M and Ruckenstein A E 1989 Phys. Rev. Lett. 63 445), does not occur when pair-pair interactions are included. However, we do find a quasi-particle lifetime that varies linearly with temperature, which is similar to the findings from many experiments. Thus, although this system has a Fermi surface, it shows non-Fermi-liquid-type behaviour over a wide temperature range. We stress that our work uses thermal Green's functions along the real-time axis, and thus allows for a more accurate determination of the dynamical properties of a model than theories that require extrapolations from the imaginary-frequency axis.

Constrained density-functional theory (CDFT) has been used by several authors for determining model Hamiltonian parameters of high- superconductors and other transition metal compounds. These methods use the fact that the orbitals are well localized and can easily be split into atomic components. We generalize them to systems with relatively delocalized electrons, so that they can be used, for example, for determining parameters for a Hubbard model of conducting polymers (e.g., polyacetylene). We selected the molecule and a linear C chain as test cases. The results of the constrained density-functional calculations were fitted to Hartree-Fock model calculations of the Hubbard model. In the case of we also fitted the CDFT results to exact solutions of the Hubbard model. For we found a substantial discrepancy between the U-values from the exact and the Hartree-Fock model calculations, which shows that it is important to specify the approximations that are used in determining the model parameters. On the other hand, the CDFT calculations themselves lead to roughly uniquely defined model parameters.

The low temperature properties of polycrystalline samples of the filled skutterudites and , as well as the unfilled skutterudites and , have been investigated by means of electrical resistivity, specific heat and magnetic susceptibility measurements. The resistivity of exhibits a rather abrupt drop-off with decreasing temperature near 100 K; this drop-off temperature increases with increasing applied hydrostatic pressure, which is reminiscent of the onset of coherence in so-called Kondo lattice materials. The compounds and exhibit values of the electronic specific heat coefficient and Pauli susceptibility at low temperature which are enhanced over those of the lanthanum-filled and unfilled skutterudites. These quantities yield a Wilson ratio of order unity, which indicates that they both correspond to the properties of itinerant electrons. These transport, magnetic and thermodynamic properties suggest a moderately heavy fermion ground state in and .

The electronic structures and cohesive properties of the hydrogenated face-centred cubic (FCC) and hexagonal close-packed (HCP) phases of the CrNiFe alloy have been studied using the first-principles linear muffin-tin orbital method combined with an x-ray diffraction study. The cohesion energy of these crystalline phases is calculated as a function of the Wigner-Seitz radius. The predicted structure is in agreement with that observed experimentally and the calculated equilibrium lattice parameters are in good agreement with the experimental values. The electronic structure is used to determine the relative stability of different structures. It is shown that the nature of the transformation in CrNiFe alloy is due to the weakening of the interatomic bond.

Under the action of a high applied electric field, the non-equilibrium electron gas in a lateral superlattice exhibits the properties of a ferroelectric. That it is in principle possible for a transverse electric field to appear spontaneously as a result of a non-equilibrium phase transition is established. A theory of electron ferroelectricity taking into account the interaction of charge carriers with acoustic phonons is constructed.

The electronic states of the system, where , have been explored through measurements of the x-ray diffraction, magnetization, electrical resistivity, and thermoelectric power. Three structural phases appear as the composition and temperature are varied: the high-temperature orthorhombic (HO) phase with a pseudobrookite-type structure and the slightly deformed monoclinic (HM) one, and the low-temperature monoclinic (LM) one which is completely different from the former two phases. For , the HO-HM transition with a small magnetic anomaly and the HM-LM transition with significant spin-singlet Ti-Ti pairs successively take place, but for , no transition to the LM phase exists there. While the HO-phase is like a highly correlated metal, the HO-phase compounds with have a strongly localized state due to the disorder and the polaron formation, and exhibit a spin-glass-like phase at low temperatures.

(PrBCO) is unique in the ( R = rare earth) series because it is not superconducting. In fact, for , drops monotonically with Pr concentration, with going to zero at 55% Pr. There have been many studies of this material with the hope that an explanation for the lack of superconductivity in PrBCO might help explain why is superconducting. Unfortunately, to date, none of the proposed explanations has been totally satisfactory and without objection due to the considerable controversy of the results obtained by different experiments. In this paper, a series of samples of (YPrBCO) have been studied by x-ray diffraction, Raman spectroscopy and XPS. The results reveal that the Pr dopant does not change the orthorhombic symmetry of the samples. The peak positions at 502 and of the Raman spectra become shifted for different Pr concentration, which suggests a change in oxygen content upon substitution of Pr ion for Y ion. There are two valences of Pr ions in YPrBCO. The valence of Pr is changed with change in Pr content x. The relative content of and is closely related to the total Pr content x. The valence of Pr is close to 3+ for small x, and is increased as x increases. The valence change of the Pr ion would directly result in the increase of oxygen content. A ligand hole localized on the Pr-O(2)/O(3) band would remove a hole from the conduction band and yield a formal site. The strong hybridization between Pr 4f electrons and conduction holes in the planes induces a localization of mobile holes causing the suppression of superconductivity in the YPrBCO system. In addition, the existence of some extra oxygen atoms around Pr will also affect the distortion of the planes and possibly assist in the suppression of the superconductivity.

The magnetic properties of a diluted spin-2 and spin-5/2 ferrimagnetic Ising system are investigated on the basis of the effective-field theory with correlations. In particular, the effect of a positive single-ion anisotropy D on the compensation temperature in a pure system with D only on spin-5/2 atoms is investigated, in order to clarify the characteristic feature of the temperature dependence of the total magnetization M observed in a molecular-based magnetic material, . The influences of D and the concentrations of magnetic atoms on the properties of the system on a honeycomb lattice are examined. The results show that several (two or three) compensation points are possible in the diluted system with special values of D and concentrations of magnetic atoms.

A systematic study of the structural and intrinsic magnetic properties of the hydrides (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. Hydrogenation leads to a relative volume expansion of the unit cell and a decrease in x-ray density for each compound. Anisotropic expansions mainly along the a- and b-axes rather than along the c-axis for all of the compounds upon hydrogenation are observed. The lattice constants and the unit-cell volume of and decrease with increasing R atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Hydrogenation results in an increase in the Curie temperature and a corresponding increase in the saturation magnetization at room temperature for each compound. After hydrogenation a decrease of in the average Fe atomic magnetic moment and a slight increase in the anisotropy field for are achieved at 4.2 K. First-order magnetization processes (FOMP) occur in magnetic fields of around 1.5 T and 4.0 T at 4.2 K for and , and around 1.4 T at room temperature for . The abnormal crystallographic and magnetic properties of and suggest that the Ce ion is non-triply ionized.

Results of ageing experiments on the alloy system are reported. All alloys display a maximum in the temperature dependence of the maximal rate of the relaxation, . The temperature of this maximum, , is ( is the freezing temperature) for the investigated spin-glass and cluster-glass materials. depends on the concentration of the dopant, and may depend on the waiting time used in the experiment.

A polymer compound has been studied by optical response and electron spin resonance (EPR) using and as dopants and paramagnetic probes. The transition between low-spin (LS) and high-spin (HS) states of ions occurs with a broad hysteresis loop around room temperature and is accompanied by a colour change from bright pink in the LS state to chalky white in the HS state. With increasing doping level the hysteresis loop narrows and shifts to lower temperatures. In the HS state of the EPR spectra of and are severely broadened by spin-spin interactions with paramagnetic HS modulated by a rapid spin-lattice relaxation of the latter ions. The EPR data give evidence of the presence of domains of the LS and HS ions in the transition region.

The magnetic structure of the iron in melt-spun amorphous ribbons with the compositions and was investigated using high-field Mössbauer spectrometry. The low-temperature variation of the hyperfine parameters under fields ranging from 0 to 8 T is reported. A geometrical model was developed that accounts quantitatively for the experimental results. It describes the evolution of the spin structure in the framework of angular deformation of the cone formed by the iron magnetic moments. It is shown that exhibits a collinear spin structure of the iron from 2 T, while in , the iron spins remain distributed at random within a cone up to 8 T. At 8 T, the semi-angle of the cone apex amounts to about .

The various proton orderings occurring in crystals are investigated in the framework of a lattice-gas-type model with account taken of the interactions of the proton subsystem with the orientations of the ionic groups. In particular, it is shown that the orientational ordering of ions is strongly involved in the I-II-III-IV phase transitions which take place in and appears to be the basic mechanism determining the observed phase sequence. The phase diagram obtained together with the microscopic parameter values calculated for agree with the experimental data for the phase orderings.