Publications

The effects of the electron-phonon interaction in zinc oxide depending on the free conduction band electron concentration are invetigated. In n-doped polar semiconductors, as the free carrier concentration is increased, the plasmonic mode associated to charge density fluctuations gradually approaches, its frequently to the optical phonon ones. As a consequences, their interaction is materialized in coupled mode oscillations. Using a dielectric cotinuum-model, applying the equation of motion technique for the operator associated to charge density fluctuations in ZnO, and taking in to account the supplementary constraints introduced by its uniaxial symmetry, the response function of the system is obtained. In polar semiconductors with cubic crystalline structure there are two plasmons-optical phonon coupled modes. It is shown that in ZnO, three such coupled modes appear. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We study the differential conductance in the Kondo regime of a quantum dot coupled to multiple leads. When the bias is applied symmetrically on two of the leads (V and -V, as usual in experiments) while the others are grounded, the conductance through the biased leads always shows the expected enhancement at zero bias. However, under asymmetrically applied bias (V and lambda V, with lambda>0), a suppression-dip-appears in the differential conductance if the asymmetry coefficient lambda is beyond a given threshold lambda(0)=3 root 1+r determined by the ratio r of the dot-lead couplings. This is a recipe to determine experimentally this ratio which is important for the quantum-dot devices. This finding is a direct result of the Keldysh transport formalism. For the illustration we use a many-lead Anderson Hamiltonian, the Green's functions being calculated in the Lacroix approximation, which is generalized to the case of nonequilibrium.

Iron local environment was investigated by EXAFS in Fe- and (Fe, Eu)-doped TiO(2) photocatalysts, prepared by hydrothermal and high-energy ball milling (HEBM) routes. In the case of the hydrothermal samples, the substitution of Ti(4+) by Fe(3+) ions was evidenced. For the samples prepared by HEBM, the iron environment corresponds to mixed metallic and oxidized (FeO, alpha-Fe(2)O(3)) configurations, without a clear evidence of iron incorporation into the TiO(2) lattice. This could be related to the catalyst contamination by iron microparticles detached from the balls during milling process.

We study the coherent transport in a one-dimensional lead with two side-coupled quantum dots using the Keldysh's Green function formalism.The effect of the interdot Coulomb interaction is taken into account by computing the firstand second order contributions to the self-energy.We show that the Fano interference due to the resonance of one dotis strongly affected by the fixed parameters that characterize the second dot. If the second dot is tuned close to resonance an additionalpeak develops between the peak and dip of the Fano line shape of the current. In contrast, when the second dotis off-resonance and its occupation number is close to unity the interdot Coulomb interaction merely shifts the Fano line and no other maxima appear.The system we consider is more general than the single-dot interferometer studied experimentally by Kobayashi et al. [Phys. Rev. B 70, 035319 (2004)] and may be used for controlling quantum interference and studying decoherence effects in mesoscopic transport.

Thin films of phosphate glasses were prepared by pulsed laser deposition in oxygen atmosphere on ITO-coated glass and silicon substrates respectively, using a Nd:YAG laser (266 nm). The targets were Li(2)O-Al(2)O(3)-P(2)O(5) bulk glasses doped with Pr and Nd. Infrared transmission-, energy dispersive X-ray- and X-ray photoemission spectroscopy investigations carried out on targets as well as on the deposited films proved a satisfactory compositions transfer on both substrates. Scanning electron microscopy and atomic force microscopy show continuous films with glassy spheres. Processes where substrates were positioned perpendicularly to targets produced discontinuous drops free films.

Thin films of (001) Bi2Sr2Ca2Cu3O10 With high zero-resistance critical temperature T(c)0 = 75-95.1 K and low roughness up to three half-c-axis unit cells were grown by metal-organic chemical vapor deposition (MOCVD) on substrates with large film-substrate lattice mismatch and different film-substrate mismatch anisotropy. Comparative analysis of the films on (001) and (110) MgO and NdGaO3 suggests that Bi-2223 films can easily accommodate large mismatch film-substrate differences, while mismatch on different directions, that is, mismatch anisotropy, has a strong influence on the quality of the film. The highest quality (low roughness, high uniformity, and high T-c0(R=O)) is obtained when mismatch anisotropy, taken as the mismatch ratio (r), is given only by compressive or only by tensile mismatch stress, and it is around 1, that is, it is as for the (001) MgO substrate. Mismatch anisotropy (value and sign) is an important parameter and can be used to tune superconducting properties of the film.

Four-cation nanograined strontium and magnesium doped lanthanum gallate (La0.8Sr0.2) (Ga0.9Mg0.1)O3-delta (LSGM) and its composite with 2 wt% of ceria (LSGM-Ce) were prepared. Morphologcally homogeneous nanoreactors, i.e., complex intermediate metastable aggregates of desired composition were assembled by spray atomization technique, and subsequently loaded with nanoparticles of highly energetic C3H6N6O6. Rapid nanoblast calcination technique was applied and the final composition was synthesized within the preliminary localized volumes of each single nanoreactor on the first step of spark plasma treatment. Subsequent SPS consolidations of nanostructured extremely active LSGM and LSGM-Ce powders were achieved by rapid treatment under pressures of 90-110 MPa. This technique provided the heredity of the final structure of nanosize multimetal oxide, allowed the prevention of the uncontrolled agglomeration during multicomponent aggregates assembling, subsequent nanoblast calcination, and final ultra-rapid low-temperature SPS consolidation of nanostructured ceramics. LaSrGaMgCeO3-delta nanocrystalline powder consisting of similar to 11 nm crystallites was consolidated to LSGM-Ce nanoceramic with average grain size of similar to 14 nm by low-temperature SPS at 1250 degrees C. Our preliminary results indicate that nanostructured samples of (La0.8Sr0.2)(Ga0.9Mg0.1)O3-delta with 2 wt% of ceria composed of similar to 14 nm grains can exhibit giant magnetoresistive effect in contrast to the usual paramagnetic properties measured on the samples with larger grain size.

Thin films and crystalline powder of PbI2 intercalated with ammonia or pyridine have been studied by optical absorption, Raman scattering, photoluminescence, FTIR spectroscopy and thermo-gravimetric analysis. Ammonia intercalated PbI2 shows an increased optical band gap, of about 0.6 eV, with an intense broad emission band peaking at about 2.3 eV as signature in the photoluminescence spectrum. The intercalation of PbI2 with ammonia is noticed in the Raman spectrum by the appearance of new lines situated in low frequency range. The D-3d coordination geometry of Pb2+ in the PbI2 crystal is reduced by compression to orthorhombic one, the distribution of the electronic states in valence band is also changed. Thus, the top of the valence band undergo a deformation inducing a weakness of the interaction between the lead and iodine ions within a layer. Also the PbI2 intercalated with pyridine is featured by optical and vibration properties different that of pure crystalline powder. Finally, the intercalation of PbI2 with different molecules changes the basic semiconducting properties of the crystal. Thermal analyses and infrared absorption spectra have been used to study the desorption of guests species from PbI2 in order to evaluate the stability range and morphological changes with temperature.

Fe(III)-doped TiO2 aerogels are prepared by acid catalyzed sol-gel method followed by supercritical drying, and then heat treatment. Raman spectra together with X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns of the iron-doped TiO2 aerogel samples revealed the existence of both anatase and brookite crystalline phases. It was found that the brookite phase formation is favored by the increase of the iron content in the dried samples. XRD measurements show that the lattice constant c of anatase phase decreases with the dopant addition, while the value of a remains essentially unchanged. The microstructure of the investigated samples is relatively compact with small mesopores as revealed from transmission electron microscopy (TEM). The most enhanced photocatalytic activity was exhibited by the TiO2 aerogel sample with 1.8 at.% Fe(III) whose apparent rate constant of the salicylic acid photodegradation was found to be of almost six times higher than that of Degussa P25.

Commercial wires of Nb3Sn and NIgB2 were inspected through X-ray cone-beam micro-tomography (mu CT). Details of the architecture of the wires of different topologies were visualized. In the case of MgB2 wires this allowed to compare the geometrical perfection of the component elements between the wires with 18 and 7 superconducting sub-elements heat treated for different conditions. Defects such as 3D voids and interruption of the diffusion barriers were also visualized. High resolution 3D-mu CT observations of the Nb3Sn wire architecture allowed also direct and non-invasive determination of the twist-pitch parameter. 3D tomography shows important advantages over 2D traditional microscopy methods.