National Institute Of Materials Physics - Romania

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5840 articles found

4631. Improvement of the superconducting transport properties of YBa2Cu3O7-delta by BaZrO3 doping

Authors: Aldica, G; Bradea, I; Jaklovszky, J; Crisan, A

Published: JUN 2001, JOURNAL OF SUPERCONDUCTIVITY, 14, 409, DOI: 10.1023/A:1011186724681

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.%.

4632. Study of nematic liquid crystals confined in molecular sieves. 4-Octyl-4 '-cyanobipbenyl (8CB) in AlMCM-41 and cloverite

Authors: Frunza, L; Frunza, S; Schonhals, A; Zubowa, HL; Kosslick, H; Fricke, R

Published: MAY 28 2001, JOURNAL OF MOLECULAR STRUCTURE, 563, 495, DOI: 10.1016/S0022-2860(00)00894-2

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.

4633. Superconducting properties from AC susceptibility and harmonic generation in CuBa2Ca3Cu4Oy bulk superconductors

Authors: Crisan, A; Iyo, A; Tanaka, Y; Hirai, M; Tokumoto, M; Ihara, H

Published: MAY 15 2001, PHYSICA C, 353, 240, DOI: 10.1016/S0921-4534(00)01744-5

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.

4634. Subthreshold carrier-LO phonon dynamics in semiconductors with intermediate polaron coupling: A purely quantum kinetic relaxation channel

Authors: Betz, M; Goger, G; Laubereau, A; Gartner, P; Banyai, L; Haug, H; Ortner, K; Becker, CR; Leitenstorfer, A

Published: MAY 14 2001, PHYSICAL REVIEW LETTERS, 86, 4687, DOI: 10.1103/PhysRevLett.86.4684

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.

4635. Surface tension of some liquid crystals in the cyanobiphenyl series

Authors: Tintaru, M; Moldovan, R; Belca, T; Frunza, S

Published: MAY 2001, LIQUID CRYSTALS, 28, 797, DOI: 10.1080/02678290010025459

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.

4636. The influence of BaZrO3 on the transport properties of the (Bi,Pb)2223 superconducting system

Authors: Mihalache, V; Aldica, G; Crisan, A

Published: MAY 2001, JOURNAL OF MATERIALS SCIENCE LETTERS, 20, 891, DOI: 10.1023/A:1010912429361

4637. Bose-Einstein condensation quantum kinetics for a gas of interacting excitons

Authors: Schmitt, OM; Thoai, DBT; Gartner, LBP; Haug, H

Published: APR 23 2001, PHYSICAL REVIEW LETTERS, 86, 3842, DOI: 10.1103/PhysRevLett.86.3839

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.

4638. Analytical approximations of the Lindhard equations describing radiation effects

Authors: Lazanu, S; Lazanu, I

Published: APR 21 2001, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 462, 535, DOI: 10.1016/S0168-9002(00)01309-7

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.

4639. A Mossbauer study of the new phases Th4Fe13Sn5 and ThFe0.22Sn2

Authors: Principi, G; Spataru, T; Maddalena, A; Palenzona, A; Manfrinetti, P; Blaha, P; Schwarz, K; Kuncser, V; Filoti, G

Published: APR 12 2001, JOURNAL OF ALLOYS AND COMPOUNDS, 317, 572, DOI: 10.1016/S0925-8388(00)01389-X

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.

4640. Single-crystal high-frequency electron paramagnetic resonance investigation of a tetranuclear iron(III) single-molecule magnet

Authors: Bouwen, A; Caneschi, A; Gatteschi, D; Goovaerts, E; Schoemaker, D; Sorace, L; Stefan, M

Published: APR 5 2001, JOURNAL OF PHYSICAL CHEMISTRY B, 105, 2663, DOI: 10.1021/jp003441d

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.