Publications

Semiconducting single-walled carbon nanotubes resonantly excited by the interband E-22(S) electronic transitions (at 1064 nm) display for the two components of the radial Raman band-one associated with the isolated tubes and the other associated with the bundled tubes-an anti-Stokes/Stokes Raman intensity ratio (I-aS/I-S) which deviates oppositely from the predictions of the Maxwell-Boltzmann formula. A cooling and heating vibration process, evidenced by an enhancement and diminishment of (I-aS/I-S), appears in the isolated and bundled nanotubes, respectively. Here we confirm a cooling process, observed only for semiconducting nanotubes, which emerges from the relaxation of the E-22(S) excited state by the electronic relaxation from E-22(S) to E-11(S) that precedes the spontaneous luminescence emission at E-11(S). Metallic nanotubes do not exhibit luminescence and no cooling effect is observed. Both semiconducting and metallic nanotubes show for the bundled component of the radial Raman band an enhancement of (I-aS/I-S) such as is frequently observed in a coherent anti-Stokes Raman scattering process.

Re-Cr-Ni composite metallic films were prepared using an original plasma deposition method developed at INFLPR, Bucharest, called thermionic vacuum arc ( TVA). The method is based on the evaporation of a metal followed by ignition of a plasma in the vapours. These three-component films/alloy films were deposited using three simultaneous TVA plasma sources in the same vacuum chamber. Surface corrosion at temperatures up to 1000 degrees C was found not to take place in these Re-Cr-Ni alloy films as shown by thermogravimetric analysis. The current results demonstrate that the TVA method is a promising candidate tool for the synthesis of multiple compound films. Films of uniform and controlled composition can be simultaneously obtained using this method. Moreover, high melting point metals can be involved in these superalloy films, thus leading to applications in extremely hot conditions such as turbine blades and aircraft parts.

Exchange bias and spin valve systems with Fe-Mn antiferromagnetic layers of different Fe concentrations were obtained by the thermo-ionic vacuum arc method. The adherence of the multilayer system to the Si substrate depends on the Fe-Mn composition. The overall roughness depends on the growing order of the antiferromagnetic/ferromagnetic layers. Very low blocking temperatures for exchange bias were observed for the considered compositions of the Fe-Mn layer. The coercive forces of both the pinned and the free layers of spin valve structures can be considerably modified along a large set of samples simultaneously prepared. (C) 2008 Elsevier B.V. All rights reserved.

In this paper we present a comparative optical study of praseodymium and praseodymium/ytterbium-doped lutetium double phosphates in the wide spectral range (VUV-IR) and for various concentrations of the activators. The absorption and emission spectra of Na3Lu(PO4)(2):Pr3+ and Na3Lu(PO4)(2):Pr3+, Yb3+ were measured at room and low temperatures. It was shown that spectroscopic properties of investigated systems strongly depend on the concentration ratio of the Pr3+ and Yb3+ ions. (c) 2007 Published by Elsevier B.V.

Magnetotransport data on a single crystal of the superconducting Mo3Sb7 are reported for temperatures down to 0.4 K and in magnetic fields up to 12 T. The magnetoresistance data are analyzed in terms of the Ginzburg-Landau fluctuation theory. From the experimental data the upper critical field H-c2(0) and Ginzburg-Landau coherent length xi(O) are determined to be similar to 2.3 T and 12 nm, respectively. The H,2(0) value agrees with that inferred from the Werthamer, Helfand, Hohenberg, and Maki theories for conventional type-II superconductors.

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.