National Institute Of Materials Physics - Romania

The dephasing and relaxation kinetics of bosonic excitons interacting with a thermal bath of acoustic phonons is studied after coherent pulse excitation. The kinetics of the induced excitonic polarization is calculated within Markovian equations both for subcritical and supercritical excitation with respect to a Bose-Einstein condensation (BEC). For excited densities n below the critical density n,, an exponential polarization decay is obtained, which is characterized by a dephasing rate Gamma = 1/T2. This dephasing rate due to phonon scattering shows a pronounced exciton-density dependence in the vicinity of the phase transition. It is well described by the power law Gamma proportional to (n - n(c))(2) that can be understood by linearization of the equations around the equilibrium solution. Above the critical density we get a non-exponential relaxation to the final condensate value p(0) with \p(t)\ - \p(0)\ proportional to 1/t that holds for all densities. Furthermore we include the full self-consistent Hartree-Fock-Bogoliubov (HFB) terms due to the exciton-exciton interaction and the kinetics of the anomalous functions f(k) = (a(k)a-k). The collision terms are analyzed and an approximation is used which is consistent with the existence of BEG. The inclusion of the coherent exciton-exciton interaction does not change the dephasing laws. The anomalous function Fk exhibits a clear threshold behaviour at the critical density.

Electrical resistivity and calorimetric measurements on Dy3Co show that below the Neel temperature (T-N = 44 K) the non-collinear antiferromagnetic structure exhibits: field-induced magnetic phase transitions of a first-order type along all principal axes, accompanied by a strongly anisotropic giant magnetoresistance and by a change of the Sommerfeld coefficient of the specific heat. Quantum tunnelling of the magnetization appears to be possible for T < 0.6 K.

Using a newly developed multiple scattering scheme, we calculate band structure and transmission properties for acoustic waves propagating in bubbly water. We prove that the multiple scattering effects are responsible for the creation of wide gaps in the transmission even in the presence of strong positional and size disorder.

The presence and concentration of nitrogen and hydrogen impurities in thick diamond films grown by microwave plasma chemical vapor deposition at various H-2 gas flow rates, keeping a constant [CH4]:[H-2]=2.5% concentration ratio, have been determined by electron spin resonance and optical absorption spectroscopy. The relative concentration of both impurities, present as paramagnetic atomic species with different relaxation properties, has been found by ESR measurements to decrease exponentially with the increase in the H-2 gas flow rate. Moreover, the resulting values were proportional to the content of substitutional nitrogen and CHx groups obtained from infrared and ultraviolet-visible optical absorption measurements, respectively. The decrease in the concentration of both impurities with an increase in the quality of the studied diamond films, early observed from high resolution electron microscopy studies on the same samples, strongly suggests that the incorporation of both impurities, as paramagnetic atomic species, is directly related to the concentration of the extended lattice defects. (C) 2000 American Institute of Physics. [S0021- 8979(00)08611-4].

The analysis of the activation energy in the magnetization relaxation of Bi2Sr2CaCu2O8 + delta (BSCCO) single crystals reveals plastic vortex creep above the second magnetization peak (SMP). Above the peak field, the magnetic field dependence of the activation energy U(B) is very close to the form U(B) alpha B-1/2 and the intrinsic variation of U with the current density is weak. The existence of a crossover from elastic to plastic vortex creep across the SMP of high-temperature superconductors (HTSC) seems to be a general behavior, and may have important consequences on the nature of the thermally induced vortex solid-vortex fluid transition at high magnetic fields. (C) 2000 Elsevier Science B.V. All rights reserved.

The medium-range order in covalent glasses was analyzed in the frame of the paracrystalline theory. This theory, earlier applied to lamellar and fibrous structures, finds in the chalcogenide glasses an interesting working case.

Trapping levels in fresh (one month) and naturally aged (one year) nanocrystalline porous silicon have been investigated using the optical charging spectroscopy method. Four significant maxima and/or shoulders were observed for fresh samples and five for aged ones. They have been attributed to five and six trapping levels, respectively. The trapping centers corresponding to the most shallow four levels are situated at or nearby the internal surface of the porous silicon films. (C) 2000 American Institute of Physics. [S0003-6951(00)01621-1].

The a-Si:H p-i-n photovoltaic cells photosensitized with an organic layer of 5,10,15,20-tetra (4-pyrydil) 21H,23H porphine (TPyP) have been prepared. The action spectrum of these cells was extended by approximately 30 nm to longer wavelengths, with respect to non-sensitized cells. The presence of the organic layer gives some modifications in the electrical and photovoltaic behavior of the cells, that is why an analysis of dark current-voltage characteristics of the cells at room temperature, is also presented. (C) 2000 Elsevier Science B.V. All rights reserved.

In this article we study the electroluminescence of p-i-n diode structures with Ge dots consisting of coherent three-dimensional small (pyramids) and larger (dome) islands. The Ge dots are formed through strain-induced islanding. The diode structures, including one layer with Ge dots, were deposited on Si mesas with variable areas in order to study the influence of limited area deposition on self-assembling. It was observed that the reduction of deposited area improves island uniformity. The combined analysis of island distribution and electroluminescence spectra has lead to the conclusion that domes in small diodes have a smaller Si content or are less relaxed than domes in larger diodes. The diodes are found to emit up to room temperature near the optical communication wavelength of 1.3 microns. (C) 2000 American Institute of Physics. [S0021-8979(00)04610-7].

Magnetic compounds of the type R2Fe17-xVx (R=Y, Gd and x=1, 1.5, 2) were synthesized by arc melting (with or without interstitial carbon) and studied by x-ray diffraction (XRD), Mossbauer spectroscopy, and hysteresis loop measurements. Compounds with the structure Y2Fe17-xVx (x=1, 1.5, 2) were found to be hexagonal, whereas the intermetallics containing carbon introduced by melting and those of the type Gd2Fe17-xVx (x=1, 1.5, 2) were found to be rhombohedral. For each symmetry, the lattice parameters a/c were obtained from the XRD spectra and a considerable expansion of the unit cell volume was observed in the Y2Fe17-xVxC (x=1, 1.5, 2) systems as effect of carbon additions. Room-temperature transmission Mossbauer spectra were analyzed considering the various inequivalent iron sites and yielded the average hyperfine magnetic field. This field was studied as a function of vanadium content x for all classes of compounds under investigation. The maximum hyperfine field values were obtained for the carbides. The hysteresis loops recorded at 4.2 K in an applied field of 1.5 T provided complementary information on the saturation magnetization as a function of vanadium substitution for each group of intermetallics involved. (C) 2000 American Institute of Physics. [S0021-8979(00)24408-3].