National Institute Of Materials Physics - Romania

On subjecting a bulk 2H-PbI2 crystal to vacuum annealing at 500 K followed by a sudden cooling at liquid nitrogen temperature stacking faults are generated that separate distinct layers of nanometric thickness in which different numbers of I-Pb-I atomic layers are bundled together. Such structures, containing two, three, four, five etc I-Pb-I atomic layers, behave as quantum wells of different widths. The signature of such a transformation is given by a shift towards higher energies of the fundamental absorption edge, which is experimentally revealed by specific anisotropies in the photoluminescence and Raman spectra. The quantum confining effect is made visible by specific variations of a wide extra-excitonic band (G) at 2.06 eV that originates in the radiative recombination of carriers (electrons and holes), trapped on the surface defects. The excitation spectrum of the G band, with p polarized exciting light, reveals a fine structure comprised of narrow bands at 2.75, 2.64, 2.59 and 2.56 eV, which are associated with the PbI2 quantum wells formed from two, three, four and five I-Pb-I atomic layers of 0.7 nm thickness. Regardless of the polarization state of the laser exciting light of 514.5 nm (2.41 eV), which is close to the band gap energy of PbI2 (2.52 eV), the Raman scattering on bulk as-grown PbI2 crystals has the character of a resonant process. For p polarized exciting light, the Raman scattering process on vacuum annealed PbI2 becomes non-resonant. This originates from the quantum well structures generated inside the crystal, whose band gap energies are higher than the energy of the exciting light.

Spectral emission of a Xenon discharge in the presence of NaCl powder is strongly modified and dominated by the XeCl excimer band at 308 nm. Materials introduced in gas electrical discharges interact with the reactive species produced in the plasma: electrons, ions and metastable atoms. In this paper, the influence of ions and metastable atoms of a capacitive RF Xenon discharge plasma (20W, 30MHz) 5 Torr on the structure of a NaCl powder introduced in the discharge has been investigated.

We describe the output performance of the 1.34-mu m F-4(3/2)-> I-4(13/2) transition in Nd-doped vanadates under in-band pumping with diode lasers at the 0.88-mu m wavelength, directly into the F-4(3/2) emitting level. An end-pumped Nd:YVO4 crystal yielded 3.4 W of continuous-wave (CW) output power for 9.3 W of absorbed pump power. The slope efficiency with respect to the absorbed pump power was 0.43. To the best of our knowledge this is the first domonstration of such a laser system. Comparative results obtained for the pump with diode laser at 0.81 mu m, into the highly-absorbing F-4(5/2) level, are given in order to prove the advantages of the in-band pumping.

The corrosion and inhibition behavior of carbon steel in 1M HCl in the presence of Bis (2-Benzothiazolyl) disulphide (BTD) was investigated using weight loss measurements, electrochemical measurements and UV-VIS spectrophotometry. The morphology of carbon steel surface was investigated by using microscopic analysis and Mossbauer spectrometry. The corrosion current was determined by using Tafel polarization. The inhibition efficiency increased with BTD concentration and the experimental results suggest that the presence of BTD in the solution increase the surface coverage (0) and therefore, indicate the adsorption of BTD. The adsorption of this compound on the metal surface obeys Langmuir's adsorption isotherm. Mossbauer spectroscopy shown at this stage the main product of corrosion consists in a mixture of alpha, beta and gamma-FeOOH, where gamma-FeOOH is the main phase. BTD inhibitor acts as an incipient "rust transformer" and favors the formation of a "superficial closed layer". The UV-VIS spectrophotometry shown a decrease of the concentration of BTD in 1M HCl solution after corrosion, indicating that an adsorption process between organic compound from aqueous phase and the electrode surface.

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.