National Institute Of Materials Physics - Romania

The trends of temperature dependence of the surface order parameter and of the anchoring strength coefficient are theoretically evaluated in the Landau-de Gennes phenomenological theory for nematic liquid crystals. The cases of planar, homeotropic and tilted alignment at a planar limiting surface are considered. It is shown that the cases with surface order parameter greater than the bulk one or smaller than this can occur in function of the phenomenological coefficients in the surface free energy expression. The anchoring strength coefficient, W, diminishes when temperature approaches the nematic-isotropic point, after passing through a maximum in most of cases. The possibility of a nonzero anchoring strength at temperatures above the transition temperature is revealed. The obtained trends of W(T) describe well a large variety of experimental results from literature.

High power laser diodes are of interest due to their potential use in medicine and military applications. This paper presents a systematic study of rapid degradation on our AlGaAs/GaAs large optical cavity devices. The increase of normalized threshold current vs. time was experimentally studied. An evaluation for normalized threshold current vs. absorption coefficient for different reflectivities is presented. For the optical output vs. time curve an abnormal increase of light characteristics was experimentally observed. The variation of the optical power was correlated to material parameters during operation and this behavior has been proposed as a practical criterion to select devices that are on their route to rapid degradation. The output optical power vs. electron fluence curve was measured for irradiated devices and a dislocation climb motion was assumed.

The concentration of primary radiation defects induced by charged pions, in the energy range 20 MeV-50 GeV, in the bulk of silicon, GaAs and diamond has been calculated in order to characterise the radiation resistance of these materials. The mechanisms by which pions impart energy to the lattice have been analysed and their contribution has been evaluated starting from the data on pion-nucleus interaction. The energy partition of the primary recoil nuclei between ionisation and displacements has been considered in the frame of the Lindhard theory. The energy dependence of the concentration of primary radiation defects presents two maxima: one in the region of the delta resonance, and another one around 1 GeV. The main conclusion is that diamond is hardner to pion irradiation than both silicon and GaAs in the whole energy range investigated. The pion-induced degradation in the inner detector system at LHC has been evaluated, starting from the simulated spectra, and considering different detector materials: diamond, silicon and GaAs. (C) 1998 Elsevier Science B.V. All rights reserved.

Raman measurements were carried out for sputtered amorphous SiNi thin films. Raman scattering in a-Si is not influenced by a low Ni content (less than or equal to 8.3 at.%) to any significant degree, the position of peaks and their widths are not changed. A shift to lower frequencies and a broadening of the high frequency band transverse optical (TO) (which results in a redistribution of phonon states to lower frequencies) are observed with an increase of the Ni content. These modifications are correlated with the presence of Ni in the coordination sphere of Si, as resulted from the calculated local structure in Ni-Si alloys on the basis of the diffraction patterns. (C) 1998 Elsevier Science S.A. All rights reserved.

Thin Au-C-60 films with global composition AuxC100-x (x between 0.35 and 4.50 at.%) were prepared by vacuum co-deposition and investigated by X-I ay diffraction, EDS, XTEM and electrical transport. Evidence indicated the formation of distorted nanocrystalline Au, presumably stabilized by interface electron transfer into C-60 LUMO. Electrical results are interpreted in the frame of the dominant current model, with a continuous density of localized states induced by Au in the C60 gap. There is also evidence for variable range hopping at low temperatures. (C) 1998 Elsevier Science S.A. All rights reserved.

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.