Publications

We measured for the first time the fundamental and higher harmonic components of AC susceptibility of CuBa2Ca3Cu4Oy superconductor produced by the high-pressure technique, in various experimental conditions (temperature, DC magnetic field, AC held amplitude and frequency). The results are discussed qualitatively in the frame of the critical state and other models in the literature. By using a very simple frequency-dependent critical state model, we estimated the parameters of practical interest, critical current density and pinning potential at various DC fields and temperatures and the irreversibility field. We also extrapolated our estimations to both practical temperature of 77 K and longer time scale as of DC magnetization and transport measurements. The results appeared to be quite self-consistent. (C) 2001 Published by Elsevier Science B.V.

Femtosecond transmission spectra of highly polar CdTe are compared to more covalent GaAs contrasting semiclassical kinetics with two-time quantum kinetics based on the Dyson equation. Nonequilibrium heavy holes in CdTe show ultrafast energy redistribution via the Frohlich mechanism even if photoexcited below the LO phonon energy. This subthreshold relaxation is a genuine quantum kinetic effect. It gains importance if the polaron self-energy is comparable to the phonon energy. Conservation of the free-particle energies is not required under these conditions.

The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases, For SCB (oclylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the Isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.

A quantum kinetics of the Bose-Einstein condensation in the self-consistent (s.c.) Hartree-Fock-Bogoliubov (HFB) model of the interacting Bose gas is formulated and numerically solved for the example of excitons scattering with a thermal bath of acoustic phonons. The theory describes the condensation in real time starting from a nonequilibrium initial state towards the equilibrium HFB solution. The s.c. changes of the spectrum are automatically incorporated in the scattering terms.

Based on the general Lindhard theory describing the partitioning of particles' energy in materials between ionisation and displacements, approximate analytical solutions have been derived, for media containing one and more atomic species, for particles identical and different from the medium ones. Particular cases, and the limits of these equations at very high energies are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

Two new phases Th4Fe13Sn5 and ThFe0.22Sn2 have been studied by Fe-57 and Sn-119 Mossbauer spectroscopy. The Fe-57 Mossbauer spectra of Th4Fe13Sn5, measured at room temperature and 4.2 Ii without and with applied magnetic field up to 5.5 T, exhibit four sextets. Our measurements prove that all four Ft: sub-lattices are collinear. The Sn-119 Mossbauer spectra of Th4Fe13Sn5 show two sextets with different B-eff, reflecting the influence of ferromagnetic nn iron atoms. The results obtained from full-potential linear augmented plane wave (LAPW) calculations are in agreement with the experimental data and confirm the assignment of Mossbauer spectra, The Mossbauer data for the ThFe0.22Sn compound prove the existence of two different sites both for Fe and Sn, all having icosahedral co-ordination. (C) 2001 Elsevier Science B.V, All rights reserved.

A high-frequency (95 GHz) electron paramagnetic resonance (EPR) study is reported on single crystals of the planar tetranuclear complex Fe-4(OCH3)(6)(dpm)(6) (where Hdpm = dipivaloylmethane), which has been previously shown to present typical single-molecule magnet behavior. The spectra, all originating from the S = 5 ground state, possess quasi-axial symmetry along the normal to the plane defined by the four Fe(m) ions. The measured spectra are shown to belong to three different structural variations of the compound, resulting from disorder in the ligands around two of the Fe(III) ions. Accurate values could be obtained for the second- and fourth-order crystal field parameters related to the parallel EPR spectra, while the other parameters could be determined only for the dominant species. The separation between individual lines is decreasing and vanishing with increasing temperature. This effect is attributed to the contribution of fast relaxing excited states, whose population is varying with temperature.

A novel approach to model diffraction line shifts caused by elastic residual or applied stresses in textured polycrystals is proposed. The model yields the complete strain and stress tensors as a function of crystallite orientation, as well as the average values of the macroscopic strain and stress tensors. It is particularly suitable for implementation in Rietveld refinement programs. The requirements on refinable parameters for all crystal Laue classes are given. The effects of sample symmetry are also included and the conditions for strain invariance to both the sample symmetries (texture and stress/strain) are discussed.

The effects of the incorporation of small amounts of Sn, Mn and Sm (0.5 at.%) on the structure and transport properties of glassy As2Se3 have been investigated. The metals influence both the disordered network structure of the glass matrix and the transport properties in accordance with the chemical properties of the doping elements. On metal addition, the electrical conductivity increases while the photoconductivity decreases. For Sm-modified glass, the position of the characteristic photoconduction band shifts toward higher photon energy. For Sn, the band shift is almost negligible and, for the addition of Mn, a photoresponse was not detected.