Publications

Abnormal anti-Stokes Raman emission (AASRE) is defined by an anti-Stokes/Stokes intensity ratio much greater than expected on the basis of the equilibrium population of excited vibration states provided by the Boltzmann law. AASRE can be observed both in materials with intrinsic nonlinear optical properties as LiNbO3 or materials in which the nonlinear optical properties are induced by resonant optical excitation, as carbon nanotubes. Using a surface enhanced Raman scattering (SERS) device, consisting from single wall carbon nanotubes deposited on a rough Au or Ag support and LiNbO3 in form of powder, we demonstrate that under continuous single beam excitation they present AASRE whose properties are similar to a single beam pumped Coherent Anti-Stokes Raman Scattering. AASRE is usually accompanied by a coherent backscattering, it can be considered as Anderson localization of light that results from a wave-mixing mechanism of the incident laser light with a Stokes shifted Raman light produced by a spontaneous Raman light scattering process. The transport of the light inside of sample, giving rise to AASRE and coherent backscattering, is done by an elastic light scattering mechanism.

The tunnel-diode resonator (TDR) technique for accurate measurements of the magnetic penetration depth is used to measure the London and Campbell penetration depths of polycrystalline SiC doped (10wt.%) MgB2. The Campbell length was used to investigate the field and temperature dependence of the critical current density. The as determined critical current density provides values as high as 6x10(6) A/cm(2) at 4.2K, 1T, which is higher than values estimated by Bean method.

Ba(Zn1/3Ta2/3)O-3 (BZT) perovskite material has stimulated a large interest due to its excellent dielectric properties in microwave frequency range, enabling the miniaturization of communications devices. In this work, BZT ceramics were prepared by the conventional solid-state reaction method, being doped with Nb5+, Ce4+ or Yb3+. BZT resonators were compositional, structural, morphological and electrical characterized. The influence of the dopants and sintering temperature on the microwave dielectric properties of BZT samples was investigated. The resonator doped with 0.5 mol% Nb2O5 and sintered at 1600 degrees C / 2 h exhibits the best dielectric behaviour: e(r) similar to 25.9 and Qxf similar to 159 THz.

In the present contribution, the peculiarities of the electronic and nuclear energy loss of some "exotic" particles, as well as the transient phenomena produced in materials during their stopping are investigated, for their potential use in detectors. In some cases (e. g. strangelets or fullerene ions), these phenomena are produced directly by the incoming particles, and also by their secondary products, while in others (e. g. weakly interacting massive particles) only by the recoils resulting from their elastic scattering on the nuclei of the target. The analysis is made in the frame of a thermal spike model, as time and space dependencies of the electronic and atomic temperatures in the neighbourhood of the interaction point or of the trajectory of the particle.

In this paper we continue the previous investigations on nanostructured GexSi1-x films. The films were deposited by magnetron sputtering and annealed in N-2 atmosphere at 700 degrees C. Their structure was investigated and correlated with the electrical behavior. For this, conventional and high-resolution transmission electron microscopy together with selected area electron diffraction was used. Electrical measurements of current-voltage and current-temperature curves were made. The majority of crystallites that forms the films have the composition Ge50Si50 and 15 - 30 nm size. The I - T characteristics have Arrhenius dependence, with two activation energies interpreted as transitions between quantum confinement levels.

The paper presents the synthesis and optical/structural characterization of 1% CdSe-doped Li2O-Al2O3-P2O5 glasses as promising materials for temperature sensors based on luminescence. The doped glasses were synthesized by a conventional dry route, embedding the dopant into the molten glass under continuous stirring. UV-Vis absorption revealed characteristic features for CdSe semiconductor with a cut-off correlated with the band gap of the dopant. Structural information was provided by FTIR and Raman spectroscopy. FTIR and Raman maxima are assigned to vibration modes characteristic for the phosphate vitreous network. Fluorescence spectroscopy by 365 nm excitation put in evidence the luminescent emission of the doped glass in the visible range. SEM images show CdSe particles which are responsible for the yellow color of the doped samples, being detected 12-24 nm quantum dots and the smaller particles are under the detection limit.

Pure triglycine sulphate crystals (TGS) show peculiar dielectric behavior in the ferroelectric phase, after crossing down the Curie point. Dielectric dispersion was investigated on the frequency range 1 Hz divided by 10 MHz. Temperature was changed at a sliding rate of 0.6 degrees C/minute, crossing up and down, several times, the Curie point between 65 degrees C and -120 degrees C. The second component of permittivity, drawn versus the frequency, shows two types of relaxation in the ferroelectric phase, with the characteristic time of tau(L) similar to 10(-3) sec and tau(H) similar to 10(-7) seconds. The first is recognized to be related with ferroelectric domain wall relaxation (domain cropping), although not clearly understood yet. The second one is ascribed to the "critical slowing down" effect, similar to the relaxation in the paraelectric phase, above T-C. A third, middle relaxation time, seems to be a sort of interaction of the two previously mentioned relaxation mechanisms. Papers from the literature, analyzing molecular dynamics and structural changes, could not correlate the flipping time similar to 10(-11) sec of the NH3 group of the glycine GI in the structure (the main responsible of the ferroelectric transition), with the mentioned relaxation times. Several aspects of the dielectric parameters evolution, related to the two relaxation processes on the large temperature range shall be analyzed and discussed.

Caesalpinia pulcherrima (Linn) popularly known as Peacock flower in India belongs to family Caesalpiniace. In this work, we describe environment friendly technique for green synthesis of gold nanoparticles from AuCl4 solution using the Caesalpinia pulcherrima (Peacock flower) flower extract as reducing agent. The gold nanoparticles were characterized using UV-visible spectroscopy and transmission electron microscopy (TEM). The UV-visible spectra indicate a strong Plasmon resonance that is located at similar to 450 nm. The TEM analysis shows that products have spherical morphology with size ranging between 10-50 nm. The study also indicates that gold nanoparticles show good antimicrobial activities when compared to the standard antibiotics.

We use a generalized Master equation (GME) formalism to describe the non-equilibrium time-dependent transport through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire are modulated by out-of-phase time-dependent functions which simulate a turnstile device. One lead is fixed at one end of the sample whereas the other lead has a variable placement. The system is described by a lattice model. We find that the currents in both leads depend on the placement of the second lead. In the rather small bias regime we obtain transient currents flowing against the bias for short time intervals. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method.

A mathematical approach for describing the influence of the ions dissolved in a liquid crystal on the impedance measurements is presented. New independent variables allow re-formulating the basic coupled differential equations of continuity and Poisson equation as well. The obtained forms are simpler than those used up to now. Illustrative examples are analyzed.