National Institute Of Materials Physics - Romania

The resistive transition R(T) and the current-voltage characteristics (I-V) in the range 10(-9)-10(-6) V and close to the critical temperature (T-c) were measured in a single-grain of melt-textured YBa2Cu3O7-delta (YBCO) and in a detwinned YBCO single-crystal, with the transport current parallel to the (a,b) planes and in zero external magnetic field using a SQUID picovoltmeter. In the case of the melt-textured sample, we measured the I-V curves also at higher levels of dissipation. For T < T-c, we interpreted the lower part of the I-V curves in the framework of the Jensen-Minnhagen model of current-induced unbinding of thermally-created vortex-antivortex pairs, which leads to V proportional to I(I-I-c1)(a-1). Our results confirm a recently proposed current-temperature (I-T) phase diagram by S.W. Pierson. In addition, we determined for the first time the dependence of the characteristic (unbinding) current I-c1 with t = 1 - T/T-c for temperatures very close to T-c and low transport currents. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

The relaxation process of accumulated carriers on the anode interface of a metal-insulator-semiconductor-insulator-metal structure is investigated. It is studied by measuring the dependence of the transferred charge on the time interval between the voltage trapezoidal pulses. The presence of two independent relaxation channels is evidenced. This aspect of the relaxation process is incorporated in a model that describes the entire charge transfer process. A good fit between the computed data and the experimental ones is found for different maximum voltage and rising slope values. A possible mechanism for the relaxation of the interface accumulated carriers is discussed. (C) 1998 American Institute of Physics. [S0021-8979(98)10423-1].

The effects of addition to YBa2Cu3O7-x Of lithium halides (YBa2Cu3O7-x (LIF)(gamma) and YBa2Cu3O7-x (LiCl)(gamma)) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to gamma=0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48 K for gamma=0.02 in YBa2Cu3O7-x (LiF)(gamma) and 91.30 K in YBa2Cu3O7-x (LiCl)(gamma)), but for higher concentration of Li it rapidly decreases (81.68 K for gamma=0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law: j(c)proportional to B-alpha, with a lithium-concentration-dependent a. The voltage-current characteristics follow a law typical for granular superconductors, V proportional to(I-I-c(B, T))(n(B,T)). The dependence of the intergranular critical current, I-c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.

Bismuth titanate ferroelectric thin films were deposited on platinum coated silicon using chemical solution deposition. Photoconductive and photovoltaic effects were observed both in continuous and modulated light. With the increasing of the annealing temperature the maximum of the photoconductive signal spectral distribution is moving toward longer wavelengths, from 340 nm to 370 nm. The existence of the photovoltaic effect could be due to some potential barriers existing at the grain boundaries and me enlargement of its spectral distribution toward longer wavelengths could be due to the presence of some traps at the grain boundaries.

In the present paper the electroluminescence of PIN diodes with either strained SiGe/Si or Ge islands in the i-region has been investigated experimentally and by quantitative modelling. The modelling helped to improve the diode structure. Consequently, diodes with strained Si(0.80)Ge(0.20) could be improved so as to reveal emission up to room temperature, if the thickness was high enough. To overcome the thickness limitation due to plastic relaxation, we used selective epitaxy on small areas. We also present results for diodes with Ge islands in the active region. The internal quantum efficiency of light emitting diodes with strained SiGe was at room temperature similar to 10(-4), while diodes with islands emitted ten times less light. (C) 1999 Elsevier Science B.V. AII rights reserved.

We calculate the quantum states corresponding to the drifting and channeled classical orbits in a two-dimensional electron gas (2DEG) with strong magnetic and electric modulations along one spatial direction, x. The channeled states carry high, concentrated currents along the y-axis, and are confined in an effective potential well. The quantum and the classical states are compared. (C) 1998 Elsevier Science B.V. All rights reserved.

A hydrothermal method has been used to obtain polycrystalline substituted magnetites, Fe3-xMxO4, (M = Cr, Mn, Co, Ni, Cu; 0 less than or equal to x less than or equal to 0.375) starting with ferrous-hi hydroxide mixtures at temperatures up to 300 degrees C. Well-defined magnetite crystallites, 0.2-0.3 mu m in size have been obtained. The appearance of the intermediate phase alpha-FeOOH during hydrothermal transformation into magnetite has been evidenced and discussed. By means of Fe-57 Mossbauer spectroscopy, the distribution of different incorporated cations in the magnetite lattice has been analysed. (C) 1998 Elsevier Science B.V. All rights reserved.

The theoretical treatment of one-phonon thermal diffuse scattering (TDS) in single-crystal neutron diffraction at fixed incident wavelength is reanalysed in the light of the analysis given by Popa & Willis [Acta Cryst. (1994), A50, 57-63; Acta Cryst. (1997), A53, 537-545] for the time-of-flight method. Isotropic propagation of sound with different velocities for the longitudinal and transverse modes is assumed. As in time-of-flight diffraction. there exists, for certain scanning variables, a forbidden range in the one-phonon TDS of slower-than-sound neutrons, and this permits the determination of the sound velocity in the crystal. A fast algorithm is given for the TDS correction of neutron diffraction data collected at a fixed wavelength: this algorithm is similar to that reported earlier for the time-of-flight case.

The L alpha and L beta spectra of tungsten were measured using a high-resolution single-crystal x-ray spectrometer. The observed spectra were fitted into Lorentzians. The fit residuals of the L alpha and L beta spectra indicate the presence of satellites in the vicinity of each spectrum, which are originated from Coster-Kronig transitions. Linewidths, energies, and intensities were estimated for each diagram line and compared with previous data. The L beta(2) satellite line observed by Salgueiro er al. [J. Phys. (Paris), Colloq 48, C9-609 (1987)] was confirmed on the high energy side of the L beta(2) line. The origin of this satellite is discussed experimentally and theoretically. [S1050-2947(98)02011-3].

We measure the azimuthal anchoring energy of the nematic 5CB on sinusoidal holographic unidimensional and bidimensional gratings. We find that the anchoring strength is almost the same for all of the gratings and up to one order of magnitude stronger than expected by the topographic mechanism proposed by Berreman. We observe strong memory effects which dominate both the elastic anchoring and the easy axis gliding. We show that these memory effects limit from below the anchoring energy and are the main obstacle in the realization of substrates with weak anchoring conditions.