National Institute Of Materials Physics - Romania

Statistics of level spacing and magnetization are studied for the phase diagram of the integer quantum Hall effect in a two-dimensional finite lattice model with Anderson disorder.

We investigated the relaxation of the irreversible magnetization of YBa2Cu3O7-delta, Pb2Sr2Y0.53Ca0.47Cu3O8, and Bi2Sr2CaCu2O8+delta single crystals with significant quenched disorder in the region of the second magnetization peak. It was found that for an applied magnetic field between the onset field and the peak field the relevant current-density dependence of the activation energy exhibits a sudden change, which can be interpreted as a crossover from elastic to plastic vortex creep. The evolution of this change with magnetic field illustrates the increase of the collective pinning barrier between the onset field and the peak field. The observed increase of the collective pinning barrier is Limited by the plastic barrier at the peak field. This appears to be a general behavior, and may have important consequences on the interpretation of the thermally induced vortex phase transition at high magnetic fields.

A general fermion system interacting with a time-periodic external held is discussed in the second quantization formalism transposed in the Fock-Floquet space. The method simulates the evolution of the system with the help of an auxiliary parameter which is analog to the time and defines the vectors associated to the states of the system, such that the matrix elements in the Hilbert space have a physical correspondence. Assuming that only the principal quasi-energies have physical relevance, the ground state of the free system is constructed in the occupation-numbers representation and an analog of Wick's theorem is derived. The perturbation series of the Green's functions is subsequently obtained, and the linear response of a given observable is expressed in terms of the correlation functions. The results have formal similarities to those of the standard theory of the many-particle conservative systems.

The effects of aluminium doping on Mn site in the magnetoresistive La-2/3 Ca-1/3 Mn1-x, AlxO3 (x less than or equal to 0.05) powders have been investigated by EPR. The temperature and x dependence of the resonance linewidth has been investigated and discussed in terms of exchange narrowing and spin-lattice relaxation. In the paramagnetic regime, the exchange coupling integral, J, between Mn3+ and Mn4+ ions shows a decrease with increasing x. it could arises from weakening of the double-exchange interaction by aluminium doping which reduces the exchange field at the Mn sites.

The changes produced by anodic and natural oxidation upon the trap parameters in nanocrystalline porous silicon were compared. To put them in evidence, we used optical charging spectroscopy The same trapping It levels (with the same activation energies) were observed after both oxidation processes. In comparison with flesh samples, a new trapping level (the deepest one) appears and the concentrations of the surface traps strongly diminish.

A new method for the preparation of BaFCl:Eu2+ has been developed. A coprecipitation chemical reaction between BaCl2 and NaF acidified aqueous solution has been used. Doping with Eu2+ was carried out by adding EuF3 during preparation time. A thermal treatment in vacuum similar to those used in the sintering process of supraconductive ceramics was used in order to accomplish the chemical reaction and to improve the homogeneity of europium ion distribution. Finally, a fine powder consisting of microcrystalline, 4-5 mu m grains was obtained. The product has been checked by X-ray diffractometry and characterised by optical methods. Photoluminescence (PL) measurements attest the europium impurification and oxygen contamination during preparation, which has a great importance for the photostimulability properties of the compound. Photostimulated luminescence (PSL) of the X-irradiated samples shows good performance as X-ray storage phosphor. (C) 2000 Elsevier Science B.V. All rights reserved.

The paper presents the study of paramagnetic centres formed by gamma -rays irradiation of polycristalline (NHa)(2)SeO4 and (HH4)(2)SeO3 at 77 K and at room temperature. From the e.s.r. spectra and from the behaviour on heating NH3+ radical as well as radicals from selenium: SeO4-, SeO3-, SeO2- were put into evidence. The spin density values on oxygen and selenium orbitals were calculated. A mechanism of formation of these species is suggested.

Changes in the ambient humidity induce significant, long-termed variations both in conductance and in the calibration characteristics of SnO2 gas sensors for different reducing gases. These gas-dependent variations depend both on humidity changes (initial and final humidity) and on the time spent in each humidity. A previous knowledge of the conductance and calibration transients, together with a permanent information about humidity and about the time spent in each humidity, should result in significant improvements in the operation of electronic noses equipped with SnO2 gas sensors. (C) 2000 Elsevier Science S.A. All rights reserved.

Graded ferroelectric (GF) structures were obtained by successive deposition of Pb1-3y/2Lay(Zr0.4Ti0.6)O-3 films with different La contents on Pt/TiO2Si substrates. The method used for deposition was sol gel. The final structure consists of four layers, starting from the Pt electrode to the top gold electrode: (1) no La; (2) 4% La; (3) 8% La; and (4) 12% La. Ferroelectric properties of the GF structure were investigated, and an anomalous shift of the hysteresis loop along the polarization axis was found. The sense of this shift depends on the initial direction of the ferroelectric polarization. The presence of hysteresis displacement "up" or "down" can be explained if the presence of a built-in charge in the GF structure is admitted. The built-in potential produced by this charge is computed using a simple theoretical model. (C) 2000 American Institute of Physics. [S0003-6951(00)01040-8].

Surface-enhanced Raman scattering (SERS) is a process with origins, electromagnetic and chemical. The electromagnetic enhancement consists of the excitation of surface plasmons in the metallic support of the thin film. With only the electromagnetic enhancement mechanism, the surface spectra should not differ from volume Raman spectra. However, between SERS and volume Raman spectra, there are differences resulting from the chemical reactions taking place at the polymer/metal interface, intermediated by solvent molecules, that finally depend on the types of polymers and metallic supports. Polyaniline (PAN) is an excellent material to emphasize the chemical component of SERS. This is due to its particular structure with a repeating unit that contains two entities at different weights-a reduced state and an oxidized state-that, in turn, react differently with a metallic substrate. SERS spectra depend on the oxidizing properties of the metal surface, which involves an intermediate compound of the types Ag2O and Au2O3 when N-methyl-2-pyrrolidinone is used as the solvent. This article presents new results concerning the surface chemical effects that produce variations of the PAN SERS spectra. The SERS spectra of the PAN emeraldine base (PAN-EB) layered on Au support are characterized by a semiquinoid structure that we believe is induced on the intermediate compound Au2O3. In the presence of H2SO4, the SERS spectra change gradually as the degree of acid protonation doping increases. The SERS spectra of the fully protonated PAN-EB are identical to those obtained on PAN emeraldine salt (PAN-ES) synthesized by cyclic voltammetry in an acid medium and are invariable with the type of metallic support. The SERS spectra show that the emeraldine salt can be partially or totally deprotonated with water or NH4OH. The deprotonation is complete for the Ag support and partial for the Au support. The SERS spectra of the fully protonated PAN-EB are characterized by a double band with maxima at about 1330 and 1370 cm(-1). Although the generation process of positive charge on the macromolecular chain of PAN-EB doped in the presence of (C4H9)(4)NBF4 is similar to that due to protonic acid doping, involving cation addition (C4H9+ or H+ ions, respectively) in SERS spectra, the complex band situated at about 1330-1370 cm(-1) no longer appears. The doping of PAN-EB with FeCl3 produces two polymer cm forms: a salt type characterized by a protonated structure similar to that found for PAN-ES and a base type similar to the leucoemeraldine form. (C) 2000 John Wiley & Sons, Inc.