Publications

A detailed formula for the equivalent pyroelectric coefficient of a bimorph structure acting as a pyroelectric detector was deduced. It is shown that electric fields occur in the structure when it is subjected to a temperature variation, due to the unequal charges generated by the two components. The field dependence of the ferroelectric polarization has to be considered in this case and the (partial derivativeP/partial derivativeE) derivative appears explicitly in the deduced formula. The effect is that the equivalent pyroelectric coefficient has different values for heating and cooling. This could lead to a redressing effect on the pyroelectric signal if the temperature difference varies periodically. The equivalent pyroelectric coefficient is, also, thickness dependent. This fact offers the possibility of designing structures with high values of pyroelectric coefficient compared with the values of the component phases. (C) 2000 American Institute of Physics. [S0021- 8979(00)04701-6].

The formation of icosahedral phases in Ag-substituted Ti-Zr-Ni alloys was investigated by in situ annealing diffraction experiments using synchrotron radiation. The as-quenched (Ti1-xAgx)-Zr-Ni alloys (x=2, 5, 7 and 10 at.%) are amorphous and/or in a mixed nanostructured state. Upon heating, the Ti-Zr-Ni-Ag alloys transform to either a crystalline or a quasicrystalline state, depending on the Ag content. Differential scanning calorimetry (DSC) studies were performed to identify the nature and sequence of the phase transitions. (C) 2000 Elsevier Science B.V. All rights reserved.

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.