National Institute Of Materials Physics - Romania

Scanning Hall probe microscopy has been used to study flux Structures and dynamics in 5 mu m x 5 mu m YBCO thin film squares, which are mesoscopic with respect to the magnetic penetration depth, lambda(T), at temperatures close to T-C. A number of unusual vortex phenomena are observed in these microstructures which differ qualitatively from the expected behaviour of more macroscopic pieces of film. In field-cooled (FC) experiments to similar to 65 K a full Meissner state is generated for cooling fields less than similar to 6 Oe, reflecting the relatively small demagnetization factors in our samples. Cooling in higher fields, however, results in only a very weak diamagnetic response at low temperatures whose magnitude is almost independent of the cooling field. In contrast we observe considerable trapped flux upon field-removal whose magnitude grows monotonically with cooling field. Remarkably, all FC flux distributions exhibit almost perfect rotational symmetry, and can be nearly completely cancelled in a reversible fashion by tuning the field applied to the initially FC state. Our field-cooled and zero-field-cooled results have been analysed in terms of a Bean-like critical state model containing constant edge and bulk current densities, and most of the observed phenomena can be explained by considering the relative weight of these two components. Not all flux profiles call be described by our simple model, however, and under certain circumstances symmetry-breaking 'dipole'-like flux structures can form in several adjacent YBCO squares. We speculate that these are related to the unidirectional At-ion milling process which was used to pattern the squares and could have broken the expected four-fold symmetry. We note that our results could have important implications for the miniaturization of thin film HTS devices.

While time-dependent perturbation theory shows inefficient carrier-phonon scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic description of carrier-phonon interaction predicts fast carrier capture and relaxation. The considered processes do not fulfill energy conservation in terms of free-carrier energies because polar coupling of localized quantum-dot states strongly modifies this picture.

Recently we have found superconductivity in a cubic antiperovskite-like compound Li2BPd3. A new pseudo-binary complete solid solution Li2B(Pd1-xPtx)(3), x = 0-1 with similar structure has been synthesized and observation of superconductivity in the entire x-range is reported. Our results strongly suggest that superconductivity is of bulk type. Critical temperature T, is decreasing approximately linearly with amount (x) of Pt from 7.2-8 K for Li2BPd3 to 2.2-2.8 K for Li2BPt3. From isothermal magnetization (M-H) measurements, lower critical fields H-cl (138Oe/(x=0), 38Oe/(x=1)), upper critical fields H-c2(WHH) (3.4 T/(x=0), 1 T/(x=1)), coherence length xi(0) (9.8 nm/(x=0), 17.9 nm/(x=1)) and penetration depth lambda(0) (190nm/(x=0), 364nm(x=1)) were estimated and shown to follow approximately linear dependencies with.x, either. Structure and superconducting similarities with MgCNi3, viewed as a bridge between low and high T-C superconductors are increasing the expectations that Li2B(Pd1-xPtx)(3), x = 0-1 superconductor can be included in the same class of `intermediate' superconductors. For x = 0-1 a weak fishtail effect was observed at low and intermediate fields. Apart from this effect, some samples for x = 1 have shown magnetization jumps at fields close to H-c2.

A detailed description of the tunneling processes, within Aharonov-Bohm (AB) rings containing two-dimensional quantum dots is presented. We show that the electronic propagation through the interferometer is controlled by the spectral properties of the embedded dots and by their coupling with the ring. The transmittance of the interferometer is computed by the Landauer-Buttiker formula. Numerical results are presented for an AB interferometer containing two coupled dots. The charging diagrams for a double-dot interferometer and the Aharonov-Bohm oscillations are obtained, in agreement with the recent experimental results of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)] We identify conditions in which the system shows Fano line shapes. The direction of the asymetric tail depends on the capacitive coupling and on the magnetic field. We discuss our results in connection with the experiments of Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)] in the case of a single dot.

The Rayleigh scattering of the Mossbauer radiation is a less common technique used in the study of the properties of condensed matter. The detection of the scattered radiation via the nuclear resonance absorption, with an energy resolution of the order of 10(-8) eV makes this technique very sensitive to dynamics changes. The results of our studies by the Rayleigh scattering of the Mossbauer radiation on pyrolytic graphite (C) and rubidium tetrachlorozincate (Rb2ZnCl4) are reported. The contribution of the coherent inelastic scattering to the total intensity in C(002) is presented like a consequence of the coupling motions of C atoms in the hexagonal plane. The normal-incommensurate phase transition in Rb2ZnCl4 is discussed in connection with the photon-phonon interaction.

We study a non-adiabatic excitation of an electron system in a 1D quantum ring radiated by a short terahertz pulse. The response of two models, a continuous and a discrete, is explored. By introducing a spatial asymmetry in the external perturbation, a net current can be generated in the ring at a zero magnetic field. Effect of impurities and ratchets are investigated in combination with symmetric and asymmetric external excitation. (c) 2005 Elsevier B.V. All rights reserved.

Metal-doped benzil crystals have been grown by thermal gradient solidification in a vertical transparent growth configuration to investigate the effect of metallic guest on the ordered organic host. We have identified the conditions for growing homogeneous, optically good crystals of benzil doped with sodium and silver, limiting the effect of supercooling, low thermal conductivity and anisotropy of the growth speed (temperature gradient at the liquid-solid interface: 10-25 degrees C, moving speed of the growth interface 2.0 mm/h). The nature and concentration of the dopant are parameters affecting, through the growth process, the crystalline perfection and the optical properties of the organic matrix. Bulk optical characterisation, by spectrophotometrical methods, has offered details on some intrinsic properties of the system metal particles/benzil crystalline matrix. Analytical processing of the experimental data emphasised that benzil is a wide optical band gap organic semiconductor E-g = 2: 65 eV: We also have investigated the effect of sodium and silver on the properties of benzil crystal as potential transparent semiconductor matrix for (nano) composite metal/molecular organic material. With the increase of sodium concentration from c = 1 to 6 wt%, a small narrowing of the band gap has been remarked. The same behaviour has been found for benzil doped with silver (c = 2wt%) compared to pure benzil. (C) 2004 Elsevier B.V. All rights reserved.

This work concerns the preparation and characterisation of homogeneous bulk Co2Mn (Si, Ge, Ga, Sn Sb0.8Sn0.2) to be used as targets for pulsed laser deposition of magnetic contacts for spintronic devices. The samples have been prepared by direct synthesis from elemental powders followed by vertical gradient freeze. Characterisation of their structural properties and compositional homogeneity was performed by X-ray diffraction and energy-dispersive X-ray spectroscopy combined with scanning electron microscopy. The results show one-phase for the Heusler alloys. The Curie temperatures of the samples have been measured by both thermogravimetric analysis and differential scanning calorimetry and are in a good agreement with the previously published values. The saturation magnetisation values are close to the values for single crystals. (C) 2004 Elsevier B.V. All rights reserved.

A model for the structure of copper-tin liquid alloy has been developed using the standard reverse Monte Carlo method. The interstitial void structure (size distribution) was analysed. The effects of various kinds of voids (small size and large size) on the interference function and radial distribution function were investigated. Predictions related to the formation of some ternary alloys by filling the interstices of the basic alloy were advanced.

The temperature variation of the fine and hyperfine parameters of Mn2+ in single crystals of PbWO4 in the low temperature range reveals the presence of a resonant mode of frequency omega = 8.8 x 10(12) rad s(-1). Moreover, above 60 K, where the temperature induced broadening becomes dominant, a T-2 variation of the relaxation time was inferred from the analysis of the temperature dependence of the Mn2+ linewidth. This variation is attributed to a Raman relaxation process due to the coupling with the same local resonant mode.