We present a Nuclear Quadrupole Resonance (NQR) study of Cu(2) (planes) and Cu(1) (chains) sites in two YBa2Cu3Oy≈7 powder samples with carbon concentration equal to 500 ppm and 3000 ppm, and in a free-carbon sample. By comparison with standard spectra in YBa2Cu3Oy , we show that a new NQR line appears at 30.90 MHz, the intensity of which is strongly correlated with the amount of carbon in the sample. Moreover, according to the spin-lattice relaxation rate 1/T 1 measurements at different frequencies of the spectrum, we attribute this anomalous line to the formation of clusters in which the superconductivity is destroyed by the presence of carbonate groups, which substitute preferentially for copper chain site. Those results are in good agreement with those of Wang et al. [1], obtained by high resolution electron microscopy.

Long range ordering between chromium and thallium has been evidenced for the first time in copper oxychromate Tl3(CrO4)Sr8Cu4O16 using HREM, XRD and ND. This compound, which crystallizes in an orthorhombic cell with a = 3.7803(1) Å, b = 15.2573(2) Å and c = 17.6737(3) Å derives from the "1201" TlSr2CuO5 structure by replacing one thallium oxygen polyhedra out of four by one CrO4 group in an ordered way. The Mayn features that characterize this original structure deal with the waving character of the [CuO2]∞ layers, and the absolute planar character of the [(Tl,Cr)O]∞ layers. In the [(Tl,Cr)O]∞ layers, the thallium and oxygen atoms form triple ribbons of edge-sharing TlO4 tetrahedra and TlO6 octahedra running along a, whereas the CrO4 tetrahedra are displayed in rows running along a, but are disconnected from each other and from the other polyhedra in the layer. Two kinds of CuO6 octahedra are observed, whose distortions originate from their connection with different polyhedra along c. This superconductor, whose T c can be raised to 34 K after argon-hydrogen annealing, exhibits a rather high superconducting volume fraction (> 40%) compared to other "1201" thallium cuprates.

Industrial applications of the bulk superconducting YBa2Cu3O7 material imply to control the growth of large oriented monodomains in samples of big size (several centimeters). The laboratory EPM-Matformag is committed to produce such materials according to three different methods (zone melting, solidification controlled by a magnetic field, crystal growth from a seed). The results obtained show that it is possible by such methods to elaborate a material with high performances at the centimeter scale and to produce it in series. The availability of such materials allows the measure of physical properties on a large scale and the testing of prototypes for cryo-electrotechnical applications (magnetic bearing, flywheel, coupling device, current lead...).

SrTiO3 had been often tentatively used as an insulating barrier for HT superconductor/insulator heterostructures. Unfortunately, the deposition of SrTiO3 on the YBa2Cu3O7 inverse interface results in a poor epitaxial regrowth producing a high roughness dislocated titanate layer. Taking into account the good matching with YBa2Cu3O7 and LaAlO3,CeO2 and Ce1−x Mx O2 (M = La, Zr), epitaxial layers were grown by pulsed laser deposition on LaAlO3 substrates and introduced into YBa2Cu3O7 based heterostructures as insulating barrier. After adjusting the growth parameters from RHEED oscillations, epitaxial growth is achieved, the oxide crystal axes being rotated by 45° from those of the substrate. The surface roughness of 250 nm thick films is very low with a rms value lower than 0.5 nm over 1 μm2. The YBa2Cu3O7 layers of a YBa2Cu3O7/CeO2/YBa2Cu3O7 heterostructures grown using these optimized parameters show an independent resistive transition, when the thickness is larger than 25 nm, respectively at T c1 = 89.6K and T c2 = 91.4K.

We have studied the effect of the deposition conditions on the hydrogen incorporation modes and content and their effects on the optoelectronic properties of three different series of a-Si:H films prepared by R.F. magnetron sputtering at high substrate temperature (250 °C) and high deposition rates (~ 10 Å/s). We have correlated infrared absorption measurements with optical transmission and Photothermal Deflection Spectroscopy (PDS) experiments. The samples were characterized successively in their as-deposited state and after annealing at a temperature around 180 °C. The results indicate that the modes of H incorporation as well as the hydrogen content in the three series are completely different from those observed for the samples prepared by Plasma Enhanced Chemical Vapour Decomposition of pure silane (P.E.C.V.D.) at the same substrate temperature. The microstructure of the films is also different. The density of deep defects measured in the as-deposited is slightly higher in the former case. This density decreases significantly after annealing at 180 °C and becomes comparable to that obtained for a-Si:H samples prepared by P.E.C.V.D. at 250 °C at low deposition rates (~ 1 Å/s), with however a higher disorder in the R.F. sputtered films.

A finite element method is presented to compute time harmonic microwave fields in three dimensional configurations. Nodal-based finite elements have been coupled with an absorbing boundary condition to solve open boundary problems. This paper describes how the modeling of large devices has been made possible using parallel computation. New algorithms are then proposed to implement this formulation on a cluster of workstations (10 DEC ALPHA 300X) and on a CRAY C98. Analysis of the computation efficiency is performed using simple problems. The electromagnetic scattering of a plane wave by a perfect electric conducting airplane is finally given as example.

We have performed experiments consisting to probe a multilayer material with a pulsed laser light flux and to measure the time dependent temperature at interfaces. A real time and high gray level imagery technique gives the backscattered flux at the entrance face of the composite and the incident heat flux entering in the slab. We further present an analytical model for studying transient diffusive phenomena in a one dimensional multilayer medium submitted to periodic external conditions. This approach uses a matricial formalism and the development of the incident thermal flux on an orthogonal complete base of rectangular functions called "Walsh functions". The use of the transfer matrix formulation then leads to a eigenvalues transcendental equation. The experimental method for steady or periodic heating of the slab together with the model predictions yields the thermo-physical parameters of the multilayer medium.

This paper describes a numerical method to determine the losses ina solid superconductor plunged in transverse magnetic field or in transport current.The model which is based on the Bean critical state, shows that thehysteretic phenomena are taken in account.

This paper proposes a methodology for the sizing of static converters which the structure is given. The sizing problem is translated into a constraint optimisation problem, and the constrained model of the converter is formulated symbolically in order to proceed the optimisation of its parameters. In the paper, the necessary symbolical treatments are presented to obtain a generic formulation of the sizing constraints from the definition of the converter structure and an analysis of its operating cycle.

We present, in this paper, a study of the thermomechanical behaviour of a single crystal in transformation plasticity using Taylor's approach, and we neglect the internal stress associated to the interactions between martensitic variants. We represent those interactions by a matrix deduced from a thermodynamical potential's optimization associated to the transformation. We also point out the contribution of the variant's reorientation mechanism to the thermomechanical behaviour.

En s'appuyant sur les résultats analytiques obtenus dans le casélastique, nous établissons les formules permettant de calculer le champdes contraintes thermiques induit par une irradiation en balayage par unfaisceau laser ayant une répartition spatiale d'énergie quelconque. Lasurface du matériau est considérée semi infinie et la température induitequasi stationnaire. Nous introduisons ensuite une notion de sourcevirtuelle pour que les résultats théoriques puissent simuler un caspratique.

An optical fibre sensor to measure weak partial pressures of carbon dioxide has been designed and realized. This sensor is based on direct molecular absorption in the near infrared at 4.3 μm.It is an extrinsic and amplitude modulation type sensor. In the linear region between 0and 2000 μbar, the calibration curve that represents the transmitted power versusthe carbon dioxide partial pressure in air shows a sensitivity of 20 nW/mbar with a minimum detectable pressure of 25 μbar.This article describes a temperature sensitivity analysis for all optoelectronic components and proposes a calculation method to distinguish two successive levels of partial pressure in a surrounding atmosphere where the temperature can vary. The experimental results show that without using a reference signal, the maximum variation of temperature is ±1.4 °C. The use of a reference signal combined with thermoelectric cooling of the photodetector tolerates a temperature variation of ±25 °C.

Organic field-effect transistors (OFETs) usually operate in the accumulation mode, where the biasing of the gate induces the charging of the insulator-semiconductor interface; the bias is negative in the most common case of p-type semiconductors. We show here that the application of a positive gate bias leads to the formation of a depletion layer, and a subsequent decrease of the drain current. We develop an analytical model for this depletion mode of the OFET. It is shown that measurements in the depletion mode give access to parameters such as the doping level and the density of trap levels. The model is applied to data obtained on sexithiophene (6T) film and single crystal OFETs. A substantial amount of traps is found in unsubstituted 6T, whereas the dihexyl substituted 6T is practically trap free. 6T single crystals are characterized by a very low doping level, which can be related to their very high purity. The possible use of the depletion mode to increase the on-off current ratio of OFETs is discussed.

We present experimental results on low frequency noise in amorphous PIN diodes under forward bias by pulses and by direct current. We have developed a new method using a differential amplifier to measure the noise in pulse regime. The study separates the white noise and the 1/f noise depending on current levels. The white noise can be evaluated by the shot noise through the various interfaces damped by their dynamic impedance. The excess noise shows a behaviour in f −1/2, due to defects in the intrinsic zone and a current dependence in I 3/2 , current limited by the NN+ junction.