Publications

Systematic excitation-power-density dependent and time-resolved single-dot photoluminescence studies have been performed on type-I InP/Ga0.51In0.49P quantum dots. These dots are rather flat and therefore exhibit larger than normal single-dot ground-state transition energies ranging from 1.791 to 1.873 eV. As a result of their low height, the dots have a very high aspect ratio (ratio of width to height) of approximately 27:1. In general, even at high excitation power densities, the dots with ground-state transition energies above 1.82 eV exhibit only s-shell emission, while the larger dots exhibiting ground-state emission below 1.82 eV tend to exhibit emission from several (in some cases up to eight) shells. Calculations indicate that this change is due to the smaller dots having only one confined election level while the larger dots have two or more. Time-resolved investigations indicate the presence of fast carrier relaxation and recombination processes for both dot types, however, only the larger dots display clear interlevel relaxation effects as expected. The temporal behavior has been qualitatively simulated using a rate equation model. Also, in a more detailed analysis, the fast carrier relaxation is described on the basis of a quantum kinetic treatment of the carrier-phonon interaction. Finally, the dots display a clear single-photon emission signature in photon statistics measurements.

We have measured the AC susceptibility response in field-cooling conditions of (Cu,C)Ba2Ca2Cu3Oy with aligned crystallites. In a certain but large range of the applied DC magnetic field, the out-of-phase susceptibility has two dissipation peaks. Due to the characteristics of the sample, the shape of the second peak and its position's field-dependence, we ruled out the possibility that the anomalous second dissipation peak could be related to intergrain dissipation. Because this multilayered cuprate is known to be a two-gap superconductor, we believe that our results are an experimental indication of a two-component vortex matter in high-critical temperature superconductors.

We present magnetization data on superconducting strongly Pr-doped YBa2Cu3O7-6 showing the existence of irreversibility at temperatures much higher than superconducting critical temperature and Neel temperatures for both planar Cu and Pr systems. A model in terms of cluster glass is proposed.

Cobalt ferrite and magnetite nanoparticles were obtained by "the complexation method" belonging to soft chemistry, in which cholic acid sodium salt acts as complexation agent. The precursors were characterized by IR, UV-vis spectra and magnetic measurements. X-ray diffraction spectra and transmission electron inicroscopy proved the nanocrystalline structure of the spinelic ferrites. (c) 2006 Elsevier B.V. All rights reserved.

The effective nonlinear coefficient and temperature acceptance bandwidth of three Lu and Sc co-doped GdCa4O(BO3) 3 type nonlinear crystals were measured. NCPM for SHG in to the blue-UV spectral region can be obtained by controlling the co-dopant concentration. Measurements were based on intra-cavity SHG of a CW Ti: Sapphire laser, and the effective nonlinear coefficients were found to be in the range of 0.5 to 0.6 pm/V for the three crystals used. The FWHM temperature acceptance bandwidth was measured to be more than 35 degrees C using a 6 mm long Gd0.871Lu0.129Ca4O(BO3)(3) crystal. A maximum of 115 mW at 407.3 nm in a single direction was measured using a 6.5 mm long Gd0.96Sc0.04Ca4O(BO3)(3) crystal. (c) 2007 Optical Society of America.

For the fabrication of the rectifying contact, respectively the Schottky diode structure on semi-insulating GaAs (100) oriented, Cr-doped, with a resistivity of rho similar to 10(6) - 10(7) Omega cm we have used Au-Ti contact. The Ti contact was deposited on a plasma etched GaAs surface in high vacuum (10(-8) torr), Au was deposited in medium vacuum (10(-6) torr) and the Au-Ti/GaAs interface was formed by a rapid thermal annealing procedure at T = 320 - 360 degrees C in low vacuum (10(-1) torr). The representative I-V characteristics in dark and under illumination at different power levels are presented. It is worth to mention that we estimate a barrier height of Phi(B) similar to 0.84 V (in the frame of thermionic emission model). The spectral response at two operating points and at different temperatures is presented. The signal has a maximum in the Delta lambda range (850-950) nm, which corresponds to the generation of photo-carriers due to a band-to-band direct transition. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Sr2FeMoO6 was synthesizes by soft chemistry route using citric and/or oxalic acid as complexing agents. This method provides good compositional control with promising results as a base for further improvements. The material shows complex temperature- and field(magnitude and orientation) dependent electro-magnetic behavior as probed by magnetic and transport measurements. The zero-field cooling curves of magnetization versus temperature taken at different magnetic fields, show two temperature regions separated by a transition temperature T-cups(ZFC). When H increases, T-cups(ZFC) values decrease. At 5 K and 5 T the saturation moment is 3.55 JIB per formula unit, and the coercitive force is 0.021 T. The relative value of the magnetoresistive effect (i.e. 100 (rho(H)-rho(0))/rho(H), %, with rho being resistivity) at 5 K is 68% and 36% when applied field H is parallel and perpendicular the measuring current I, respectively. The results are discussed in relation to magnetic sublattices and the interaction that produces anti-site defects and spin-dependent electron anisotropy. (C) 2006 Elsevier B.V. All rights reserved.

In this paper a study of some microwave microstrip bandpass filters (BPF) using defected ground structures (DGS) is presented. This technique allows designs of tight couplings without the necessity of using very narrow coupling gaps. Based on the results of the study, four-pole cross-coupled planar microwave bandpass filters with a single and with two ground slots are designed, fabricated and measured. The performances of these novel structures indicate some technological advantages, compared to classical microstrip filters, without defected ground.

The thermal behaviour of five polynuclear coordination compounds containing tartaric anion as ligand, namely (NH(4))(3)[LnFe(C(4)O(6)H(4))(3)(OH)(3)] (Ln=La and Eu), (NH(4))(2)[PrFe(C(4)O(6)H(4))(3)(OH)(2)] and (NH(4))[LnFe(C(4)O(6)H(4))(3)(OH)]center dot 3H(2)O (Ln=Nd and Gd) was investigated. The reaction progress was studied by TG/DTA and FTIR measurements. Oxalates and oxocarbonates were identified as intermediates. In the case of Ln=La, Nd, Pr, Eu and Gd, pure LnFeO(3) was obtained as final decomposition product. The thermal decomposition of Eu-Fe compound, leads to a mixture of mixed (ortho-ferrite (EuFeO(3)) and garnet (Eu(3)Fe(5)O(12))) and simple oxides (Eu(2)O(3) and alpha-Fe(2)O(3)).

The critical temperature, T-c, for (Bi,Pb)Sr(Ca2-x, Y-x)CuO/2223, with x between 0 and 0.45, polycrystalline samples have been investigated. T-c of (Bi,Pb)(2)Sr2Ca2-xYxCu3O10+delta phase, 2223, found in samples with x 0.05, displays a parabolic dependence with the increase of Y content. The change of T-c for 2223 originates mainly from the increase of x. The variation of both x and oxygen content, delta, influences the T-c of 2212. The doping state of 2212 and 2223 in Y substituted samples suggests that, generally, in the samples consisting of both superconducting phases, 2212 is in over-doped state whereas 2223 is in under-doped state. The last affirmation suggests that 2212 could bring a significant contribution in the transport parameters of (Bi,Pb)SrCaCuO/2223 samples.