Publications

We analyze the electronic transport through a quantum dot that contains a magnetic impurity. The coherent transport of electrons is governed by the quantum confinement inside the dot, but is also influenced by the exchange interaction with the impurity. The interplay between the two gives raise to the singlet-triplet splitting of the energy levels available for the tunneling electron. In this paper, we focus on the charge fluctuations and, more precisely, the height of the conductance peaks. We show that the conductance peaks corresponding to the triplet levels are three times higher than those corresponding to singlet levels, if electronic correlations are neglected (for non-interacting dots, when an exact solution can be obtained). Next, we consider the Coulomb repulsion and the many-body correlations. In this case, the singlet/triplet peak height ratio has a complex behavior. Usually the highest peak corresponds to the state that is lowest in energy (ground state), regardless if it is singlet or triplet. In the end, we get an insight on the Kondo regime for such a system, and show the formation of three Kondo peaks. We use the equation of motion method with appropriate decoupling.

High-resolution spectroscopy of the rare earth ions in transparent ceramics of garnets and sesquioxides produced by solid-state synthesis indicate that the variety, nature and structure of the centers formed by the doping ions, their quantum states (energy levels, transition probabilities) and interionic interactions as well as the distribution of the doping ions at the available lattice sites is similar to those of the corresponding single crystals. Moreover, the increased compositional versatility of ceramics enables tailoring of new or improved laser materials. It is inferred that from spectroscopic point of view, the transparent polycrystalline ceramics can substitute the single crystal and extend their capabilities. (c) 2008 Elsevier B.V. All rights reserved.

A comparative analysis of the high-resolution spectra of the Nd3+ and Pr3+ in strontium lanthanum hexa-aluminates, charge compensated with magnesium Sr1-x(Nd,Pr)(y)Lax-yMgxAl12O19, on a large composition range is presented. The spectra are mainly dependent on the composition parameter x and show two families of centers for x = 0.8, with distinct spectral characteristics. Based on spectral data, composition dependence and crystal structure, tentative structural models for the these centers are proposed. It is inferred that this complex picture results from the interplay between the effects determined by the anionic coordination of the centers and the perturbative effects caused by the multiple occupation of specific sites by cations of different valence. (c) 2007 Elsevier B.V. All rights reserved.

We report an anomalous temperature T variation of the field H(on) for the onset of the second magnetization peak in La(1.81)Sr(0.19)CuO(4) single crystals with the external magnetic field H oriented parallel to the c axis. While the peak field H(p) has a continuous decrease with increasing T, H(on) exhibits a sudden decrease for T similar to 11-15 K. This behavior appears to be related to the particular T dependence of the superfluid density in the case of two-band superconductivity affecting the T variation of the elastic energy of the vortex system at low H.

Four Fe-Mn-Si alloys, Fe(62)Mn(32)Si(6), Fe(62)Mn(20)Si(5)Cr(8)Ni(5), Fe(62)Mn(16)Si(5)Cr(12)Ni(5) and Fe(65)Mn(9)Si(7)Cr(10)Ni(9), were obtained by the melt-spinning method. The samples were structurally, magnetic and shape memory effect (SME) investigated, both ''as quenched'' and thermally treated. The Mn-rich compositions show different phase, magnetic behavior and SME in comparison with Mn-poor compositions. The thermal treatments generate transformation between the two existing majority phases (alpha and gamma), related magnetization and SME behavior. The features are derived from the corroboration of structural, magnetic interaction and magnitude of SME data. (c) 2008 Elsevier B.V. All rights reserved.

Four series of cobalt ferrites were prepared by conventional ceramic method. Calcined powders were analyzed by X-ray diffraction and Mossbauer spectroscopy to characterize the crystal structure. Green bodies were sinterized at temperatures between 1250 degrees-1300 degrees C, in air, for 5 hours. Chemical composition of samples and sintering parameters have a strong influence on the magnetic and magnetostrictive properties of the sintering bodies.

CoMnxFe2-xO4 with x = 0; 0.10; 0.25; 0.40; 0.60 and 2.0 were prepared by low temperature combustion method. The samples were investigated using X-ray diffraction analysis at room temperature. All the synthesized samples exhibited XRD patterns specific for cubic spinel phase with a space group Fd3m. Infrared spectroscopic analysis, using KBr pellets in the range 400 - 4000 cm(-1) was carried out using a FTIR 600Plus Fourier transform infrared spectrometer. The magnetic properties of samples have been characterized at room temperature using a vibrating sample magnetometer, the magnetic properties measurements demonstrate an increase of the saturation magnetization with the increase of Mn content.

Structural, magnetic and transport properties of CeAg1-xNixSb2 compounds are investigated on polycrystalline samples. The experimental results show the formation of ferromagnetic compounds with sharp transitions decreasing from 9.5 K in the 100% Ag compound to 8.1 K for 33% Ni substitution, 7 K for 67% Ni substitution to 6 K for the 100% Ni compound. The characteristic temperatures, considered as the temperature of the resistivity maximum To increases from 26 K to 44 K, 54 K and 65 K, respectively. These results indicate Ce(AgNi)Sb2 compounds as a ferromagnetic Kondo system.

Carbon Nanotubes are exotic low-dimensional systems, and due to their remarkable properties, are promising components for technological applications. The electronic and vibrational signatures raising from their existence as individual entities or associated in bundles, is still a controversed subject. A careful analysis of the G (similar to 1600 cm(-1)) and RBM (100-200 cm(-1)) band features as: profile, peak position and intensity in the Raman spectra may be a valuable indication about their dispersion state. The UV-VIS-NIR absorption spectrum comes to support this assumption. Observing the absorption spectra associated to resonant excitation of semiconducting and metallic nanotubes, may reveal complementary informations concerning the individualization procedure efficiency.

In the present work an X-band electron magnetic resonance (EMR) investigation in the paramagnetic regime of colossal magnetoresistive manganite La2/3Ca1/3 MnO3 (LCMO) nanowires was carried out. The temperature dependence of the EMR line width has been analyzed in terms of the small polaron hopping scenario. From this analysis, the polaron activation energy E-a in LCMO nanowires was evaluated. A discussion is given concerning the factors which could explain the smaller value of E-a as compared with the corresponding ones for LCMO nanoparticles.