Publications

DC magnetization and AC complex susceptibility measurements on (Bi,Pb) : 2223 high-temperature superconductors impurified with various amounts of BaZrO3 are presented. The results are discussed in the frame of the critical state model. and the values of the inter- and intragranular critical current density as well as of the field for full penetration are estimated. The values of the intergranular critical current density are consistent with those obtained from transport measurements. The intragranular critical current density and the field for full penetration have similar values from both DC magnetization and AC susceptibility measurements. It was shown that, in the (Bi,Pb) : 2223 system. BaZrO3 impurification changes only the properties of the intergrain matrix, while the superconducting properties of the grains are not modified.

The effect of rare earth (Dy and Sm) and transition metal (Mn) luminescent impurities on the optical properties of As2S3 glass is studied in a wide spectral region. The Raman, infrared, band-to-band and edge absorption spectroscopies are employed to obtain information about the incorporation of impurity ions in the host glass structure and the corresponding changes in intrinsic optical characteristics. The effects of light-soaking and thermal treatment on the doped As2S3 glasses were examined as well. In the fundamental absorption region a reflectivity maximum at 2.98 eV shows blue (Dy, Sm) or red (Mn) shift depending on the electronegativity of the impurity, in accordance with the corresponding variations of the glass structure. Near the edge absorption the impurity affects strongly the slope and the magnitude of the weak absorption tail. In the wide range of transparency the addition of impurity suppresses several absorption bands indicating the interaction of dopants with the host glass contaminations. Some variations of the characteristic Raman spectra under light exposure acid thermal ageing of doped glasses were registered. The observed effects of metal dopants on the As2S3 glass are discussed in connection with the expected behaviour of the impurities in the glass.

The medium range order in disordered chalcogenides has been explained by the existence of reminiscent crystal-like configurations with distorted arrangement of the atoms. Using the paracrystalline theory, two parameters have been defined for the characterization of the MRO in glass: paracrystallite thickness and paracrystallite distortion.

The luminescence and thermoluminescence have been measured in vitreous materials doped with Nd3+. The luminescence spectra show a maximum at 400 nm and a shoulder at about 440 nm. The measured global thermoluminescence shows a broad band with a maximum at 270 degreesC, which suggest a continuous distribution of electron traps induced by ionizing radiation. Thermoluminescence measured through two optical filters which transmit at about 400 nm and 440 nm wavelength shows a structured behavior with two glow peaks at 270 degreesC and 330 degreesC. This apparently contradictory behavior has been explained by the existence in this material of a continuous distribution of tr ap levels and a discrete distribution of recombination levels. The 270 degreesC and 330 degreesC glow peaks have been interpreted as being due to recombination of the electrons released from electron traps with O2-. and Nd3+ hole centers. Due to the high stability of the defects induced by the radiation observed from thermoluminescence measurements, this material could be used as a new radiation detector.

Amorphous thick films in the system Se-S have been prepared by splat-cooling Local irradiation of the films with laser pulses of various intensities and wavelengths were carried out. The effects induced by laser pulses with special emphasis to the eutectic composition Se42S58 were investigated by optical microscopy and before and after annealing of the films. For low density of power incident on the film, a local photo-darkening effect is observed. For mean power density the photo-darkened region increases. For high power density a considerable destructive effect appears, which determine the appearance of clusters of bubbles after annealing the film at the softening temperature.

We have doped the YBa2Cu3O7-delta,S superconducting ceramics with BaZrO3 up to 75 wt.% and studied the changes of some physical properties. The most important finding is the enhancement of the critical current density, which has a maximum at around 5 wt.% doping level. Compared with the undoped samples, the critical current density is four times higher if the doping compound is introduced prior to the calcination treatment, or two times higher if BaZrO3 is introduced prior to the sintering treatment, The dependence of the critical current density on the doping level is consistent with the dependence of the Seebeck coefficient. We also observed a very small decrease of the critical temperature. The real density of the samples decreases with increasing doping level as expected from the ratio between the theoretical densities of YBa2Cu3O7-delta and BaZrO3, the effect being important for doping levels higher than 25 wt.%.

The confinement of a nematic liquid crystal (LC), 4-octyl-4'-cyanobiphenyl, in the molecular sieves of AIMCM-41 and cloverite type is investigated by dielectric spectroscopy, thermal analysis and FTIR spectroscopy. We focus herein on the samples containing the LC not only in the pores and cavities, but also on the external surface of the molecular sieve grains. A bulk-like dynamical behaviour was found for these samples. An interaction of the LC molecules with the surface took place by coordination to Lewis sites and by hydrogen bonding with OH groups. (C) 2001 Elsevier Science B.V. All rights reserved.

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.