Table of contents

The pinning force calculated from Larkin-Ovchinnikov collective pinning theory is used to determined the density profile (in one, two and three dimensions) of flux lines in the Bean critical state. In all cases one finds a result different from the conventional linear profile. The dependence of the corresponding magnetization on the external magnetic field is calculated. Strong single-particle pinning leads, in three dimensions, to a behaviour distinctly different from the collective pinning result.

Liquid water at 523 K is simulated assuming a flexible single-point-charged (SPC) potential. Both structural and dynamical properties at this temperature are compared with those at 298 K. The resulting structure and self-diffusion coefficient are in satisfactory agreement with the experimental data. The frequency peaks in the spectral densities are also in accord with the experimental infrared spectra. It has been shown that the changes of the properties are related to the degree of hydrogen bonding. When the temperature increases from 298 K to 523 K many hydrogen bonds vanish (40%) and the number of oxygen atoms with two or more direct hydrogen bonds is dramatically reduced (70%). Thus, the tridimensional hydrogen bond network, which is typical of liquid water at room and lower temperatures, is substantially destroyed at 523 K.

A survey of the published data on the type of anchoring (homeotropic, planar or tilted) of nematic liquid crystals on Langmuir-Blodgett (LB) films of charged and dipolar amphiphiles, as a function of surface density of molecules and number of molecular layers in the film, reveals a complex and seemingly controversial picture. A theoretical model of the anchoring, taking into account the non-homogeneous electric field originating from surface charges and dipoles, which dictates the liquid crystal orientation via a competition of dielectric, flexoelectric and ordoelectric torques, predicts a rich variety of anchoring states, depending on the sign of surface charges, the orientation of surface dipoles and the signs of dielectric anisotropy and total flexocoefficient. In specific situations, critical molecular densities and thicknesses for anchoring transitions (either first or second order) from homeotropic to tilted to planar or vice versa are predicted. "Reentrance" sequences, homeotropic-planar-homeotropic and planar-homeotropic-planar, are also possible, while surface density increases monotonically. Thus, the model resolves the apparent contradictions between various experiments and puts them in a general framework for the first time.

Phonon- and random-phason-type structural disorder was measured in Al-transition metal (TM) ternary alloys, prepared with Al excess by rapid quenching. The disorder parameters correlate with the calculations on the energy stability of Al₁₂TM icosahedra. Thus, substitution of "unstable" elements (Fe and Co) for "stable" elements (Cr and Mn) causes a strong increase in both types of disorder, leading finally to crystallization. This chemical trend is modulated around special compositions of the TM sublattice. The degree of structural order correlates with the thermal stability of icosahedral phases.

The peritectoid reaction of alpha -Pd+Pd₃Gd (or H-Pd₅RE (RE identical to Sm or Eu)) to Pd₇RE (RE identical to Gd, Sm or Eu) in Pd-Gd, Pd-Sm and Pd-Eu alloys is found to be suppressed after the slow cooling to room temperature of alloys in the presence of hydrogen (about 30 bar) which has been introduced above the peritectoid reaction temperature and then maintained during slow cooling. The suppression of the ordering to the Pd₇Gd ordered structure in Pd-Gd alloys is believed to be due to the retardation of nucleation and growth of the Pd₇Gd ordered domain during cooling as a consequence of a weakening of Pd-Gd bonding by the dissolved hydrogen in the alpha -Pd phase. In the cases of Pd-Sm and Pd-Eu alloys, the suppression is related to the transition of H-Pd₅Sm(Eu) phases which initially coexist with alpha -Pd phases to L1₂-type Pd₃Sm(Eu) phases under hydrogen exposure. The initially short-range-ordered Pd₇Gd (Sm, Eu) phases coexist with the alpha -Pd, Pd₃Gd or H-Pd₅Sm(Eu) phases below the peritectoid reaction temperatures, but relatively at high temperatures they also have a tendency to disorder upon hydrogen exposure. Similarly, for the low-temperature hydrogen treatments, the initially coexisting H-Pd₅Sm(Eu) phases with the Pd₇Sm(Eu) phases also transform to the L1₂-type Pd₃Sm(Eu) phases. The Pd₃Sm(Eu) phases which have been transformed by hydrogen are reverse transformed into the alpha -Pd and H-Pd₅Sm(Eu) phases by heating above the peritectoid reaction temperatures via the formation of Pd₇Sm(Eu) phases.

Mossbauer spectroscopy was used to study the early stages of precipitation in Cu-0.6 at.% Fe alloy. A set of data for the concentration of iron in solution was obtained for annealing times from 1 to 10⁵ s at 400 degrees C. The results were interpreted in terms of quenched monovacancies and divacancies. A proposed analytical expression describes the annealing fairly well. The application of the Mossbauer spectroscopy in short-time-scale annealing experiments seems to promise new possibilities for investigation of the diffusion properties.

Heat capacities of Ne adsorbed in the one- and three-dimensional channels in the high-silica zeolites ZSM-23 and ZSM-5 have been observed in the temperature range from 2 K to 23 K for various concentrations of Ne. For lower concentrations of Ne, the low-temperature heat capacities for both the one-dimensional and the three-dimensional channels, both about 5.5 AA in diameter, are described by the Einstein model, which implies that Ne atoms are localized independently or construct clusters of minute size in the channels. However, for the higher concentration where channels are nearly filled with Ne atoms, the heat capacities of Ne reflect the difference between the topological channel structures of the two zeolites. In the case of three-dimensional channels, the heat capacity is revealed to be well reproduced by the Debye model for a continuous solid with the characteristic temperature Theta _D=29 K, which is unexpectedly smaller than the values Theta _3D approximately=67 K for the bulk solid Ne and Theta _2D approximately=50 K for the two-dimensional solid Ne. This suggests the possibility of an unusual softening effect of phonons in solid of Ne in the three-dimensional network of channels. While the heat capacity of Ne in the one-dimensional channels cannot be understood in terms of the one-dimensional Debye model, it can be explained using the Einstein model. These results for the classical Ne particle are significant when compared with the thermal behaviour of He in the same zeolites, which reflects the characteristic quantum effect at low temperatures.

We have performed band structure calculations for PrBa₂Cu₃O₇ using the full-potential LAPW method, which is based on density functional theory. The Pr 4f electrons are treated as valence states and form narrow bands at the Fermi level. Furthermore we have simulated a Pr valency of 3+ (4+) by treating the Pr 4f states as localized states with a fixed occupation of 2 (1). We have determined partial charges, densities of states, difference densities, electric field gradients, and their asymmetry parameters for this system and discuss the differences between the "itinerant case" and the two model calculations representing the localized f states. The corresponding total energies yield the results that the itinerant f states are the most stable, followed by Pr with a 3+ valency, and the 4+ being the least stable. The overall electronic structure obtained by the itinerant calculation is rather close to the trivalent Pr case.

Optical characteristics of free and self-trapped excitons are investigated in simple cubic CsCl crystals of high purity. The reflection, photoluminescence, cathodoluminescence and X-ray luminescence spectra, as well as the excitation spectra for intrinsic, impurity and near-defect emissions and spectra of the anion Frenkel defect creation by radiation are measured in CsCl crystals at 5-80 K in the spectral region of 2 to 10 eV. A weak emission at 8.3 eV and pi -emission of self-trapped excitons (2.82 eV) are detected in CsCl at 5 K. The energy gap is estimated as E_g=8.4 eV at 5 K. Some peculiarities of the radiative and non-radiative decay of excitons in SC CsCl in comparison with those of FCC KCl and RbCl crystals are discussed.

The distance dependences of the intrinsic parameters B_k have been evaluated using the ab initio molecular-orbital MO theory for the Pr³⁺-Cl^- ion pair at different internuclear distances. The theoretical values for the corresponding power-law exponents are t₄=6.0 and t₆=6.9. The present result shows much better agreement for the theoretical variations of B₄(R) and B₆(R) with the experimental data than a previous theoretical calculation. The comparison of the present ion-pair system with the earlier theoretical and recent experimental results for the Pr³⁺-F/sup /on-pair system shows similar distance dependences of the intrinsic parameters.

The temperature dependence of the thermopower in amorphous alloy Ca_xAl_1-x is calculated in the framework of the dynamic concentration excitation (DCE) concept. The results are in good agreement with experimental data.

The dynamics of the ZnCl₄ anion in the paraelectric phases of (Rb_1-xCs_x)₂ZnCl₄ with x=0.005, 0.01, 0.03 and 0.05 are investigated through ³⁵Cl nuclear quadrupole frequency nu and spin-lattice relaxation time (T_1Q) studies. The data, collected as a function of temperature for different lines in these compounds, are analysed to obtain the hindering potential barrier heights and the relative mean square amplitudes for librational motions parallel and perpendicular to the a-axis of the crystal structure. A comparison of these results shows that the presence of impurity affects only perpendicular motions and the corresponding potential becomes anharmonic, probably developing a double-well structure, as the concentration of impurity increases. As the paraelectric-incommensurate transition temperature is suppressed by the presence of impurity, the above results suggest that the perpendicular motions have an important role to play in stabilizing the paraelectric phase in this system.

In the intermetallic compound YFe₂, the metallic hyperfine field at the Y nucleus, as measured by ⁸⁹Y NMR spectroscopy, is affected by the substitution of one or more neighbour Y atoms by different rare-earth impurities. In order to understand the underlying mechanisms for the transferred hyperfine interaction, we have performed first-principles calculations, within the local spin-density theory, for embedded clusters representing a Y atom and its vicinity in the compounds Y_1-xR_xFe₂ (R=Gd, Tb, Ho, Tm or Lu) and Y_1-xGd_xNi₂. The contact magnetic hyperfine field and the dipolar field were obtained; the resulting total held was found to increase with the spin of the rare-earth impurity. Orbital hyperfine fields were not considered. The contact transferred field was found to arise from direct polarization of the s electrons by the rare-earth spin.

The dielectric functions of composite materials consisting of free-electron-type metal particles (Ag or Cu) and a dielectric host material with epsilon _h=1.0, 2.99 or 6.0 are calculated using the Maxwell-Garnett model. These functions present an anomalous region when the interband transitions of the metal particles occur at frequencies above the resonance frequency (2 epsilon _h=- epsilon _m) In this case the Lorentz oscillator model is used to analyse the optical behaviour of the composite material. The Lorentz parameters are calculated and discussed.

Here we report Compton scattering studies in polycrystalline Cu₂O and Ag₂O with 59.54 keV gamma radiation. An interesting comparison has been made between the valence Compton profiles of these two compounds scaled to lattice momentum by normalizing them to equal electron density for outer valence electrons, and this comparison shows some difference between the bonding characters of Cu₂O and Ag₂O.

A single crystal of a rare-earth random-site alloy, Ho_0.5Er_0.5, has been studied with X-ray diffraction. We have used the technique of resonance-enhanced scattering at the Ho and Er L_III absorption edges to study element-specific effects in the temperature dependence of the magnetic structure below the Neel temperature. In the highest-temperature phase, a c-axis modulated structure in which the Ho and Er moments take different tilt angles from the basal plane, measurements at the two edges show identical satellite positions. The lowest temperature phase, a conical spiral which locks to a structure with wavevector ⁹/₄₀c^* below 20 K, is accompanied by a basal-plane lattice modulation that is related to the magnetic structure. The modulation changes the symmetry of the crystal lattice; as a consequence we suggest that the magnetic structure locks to one with equidistant spin-slip discommensurations. In addition, we find an unusual energy and polarization dependence in the satellite intensity in this phase that is not seen at higher temperatures. We present arguments to demonstrate that the unusual behaviour is not accounted for by multiple-scattering events.

The L_2,3MM Auger spectra of solid Ge have been measured using synchrotron radiation excitation. The most intense L_2,3M_4,5M_4,5 Auger spectrum has been recorded with several photon energies around the 2p thresholds. As opposed to the 3p thresholds, no core-exciton-induced resonant Auger structures are found. The relative strengths of the different Auger groups have been determined and compared with calculations performed for Ge atoms. The relative intensities of Auger transitions within each Auger group are discussed. Some previously unresolved Auger transitions have been found. The linewidths of Auger transitions are observed to grow when the final-state holes locate in deeper M levels. This final-state broadening implies the presence of Auger cascades.