Two-neutron elastic transfer 4He(6He,4He)6He at E = 151 MeV

Using the framework of the coupled reaction channels (CRC) the elastic scattering and the elastic transfer in the system ⁶He + ⁴He measured at E = 151 MeV have been analysed. It is shown that the structure observed in the backward range of the angular distributions is influenced by the interference of the elastic 2n-transfer with a two-step process passing through the 2⁺ excitation in ⁶He. The two-neutron transfer mechanism is studied in the microscopic approach and it is found that for the ground-state transition the one step dominates by a factor 10 over the two-step mechanism at this energy. Received: 29 October 2001 / Accepted: 4 December 2001     

Thermally stimulated current study of electron-irradiation induced defects in semi-insulating InP obtained by multiple-step wafer annealing

Electron-irradiation induced defects in semi-insulating (SI) InP wafers with Fe concentration ranging from 1.5×10¹⁵ to 2.5×10¹⁵ cm⁻³, which have been obtained by multiple-step wafer annealing (MWA) under phosphorus vapor pressure, were studied using a thermally stimulated current (TSC) method. New traps, e₁, e₂, e₃, e₄ and e₅, with activation energies of 0.22, 0.28, 0.37, 0.44 and 0.46 eV, respectively, were observed. Based upon the annealing behavior of traps and the calculated defect levels, traps e₁ and e₅ produced by the irradiation with electron doses above 1×10¹⁵ cm⁻² were linked to In_P and P_In antisite defects, respectively, that probably form complexes. Traps e₃ and e₄ produced by the irradiation with doses above 1×10¹⁴ cm⁻² were associated with In and P vacancy related defects, respectively.     

Photoemission study of the growth of the NdF3/Si(111) interface

Photoemission spectroscopy with synchrotron radiation was used to study the NdF₃/Si(111) interface as a function of annealing temperature for NdF₃ films. These films range in thickness from 1–20 monolayers and were deposited at room temperature. Without annealing, both F-Si and Nd-Si bonding is observed, indicating that the planar triangular NdF₃ molecules lie flat on the Si(111) substrate. At annealing temperatures between 400 and 500° C, the NdF₃/Si(111) interface is dominated by Nd-Si bonding as evidenced from a line-shape analysis of the Si 2 p and Nd 4 f core levels. By resonant excitation of the giant 4 d-4 f absorption resonance, the photoemission signal from the partially occupied 4 f orbitals is enhanced and can be distinguished from the photoemission signal of the overlapping F 2 p valence band. At higher temperatures F is completely lost due to the decomposition of NdF₃.     

Laser resonance external photoelectric effect in F 2 − color centers of LiF crystals

The laser-resonance two-step external photoelectric effect is revealed in the F ₂ ^– color centers of LiF crystals exposed to nanosecond laser pulses. The first photoionization step uses IR radiation tunable within the resonance absorption band of the color centers and the second step the second harmonic of a Nd:YAG laser. The photoionization cross sections of the F ₂ ^– centers are estimated.     

Observation of superluminal and slowdown light propagation in doped lithium niobate crystals

We investigate the group velocity of light in a one-dimensional volume grating inside lithium niobate crystals doped with different impurities. The superluminal and slowdown light propagations are both observed in the crystals. The relationships between the group refractive index and the grating amplitude and phase shift are presented and discussed.     

Bright thermal atomic beams by laser cooling: A 1400-fold gain in beam flux

Using a three-step transverse laser cooling scheme, a strongly diverging flow of metastable Ne(3s ³ P ₂] atoms is compressed into a well-collimated, small diameter atomic beam (e.g., 1.4 mrad HWHM divergence at 3.6 mm beam diameter) with an unmodified axial velocity distribution centered at 580 m/s. The maximum increase in beam flux 1.04 m downstream of the source is a factor 1400; the maximum increase in phase space density, i.e., brightness, is a factor 160. The laser power used is only 140 mW. The scheme is extendable to a large variety of atomic species and enables the application of bright atomic beams in many areas of physics.     

Alkali metal intercalation into SnS2

The intercalation of sodium and potassium into the layered semiconductor SnS₂ has been investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and ion scattering spectroscopy (ISS). After deposition of the alkali metals onto (0001) cleavage planes of SnS₂ in ultra high vacuum (UHV), semiconducting intercalation phases were formed. They seem to be homogeneous and disordered under the given experimental conditions. The valence electrons of the alkali metals are transferred into electronic states of the host lattice, whose valence band density of states changes significantly during intercalation. The underlying changes of the binding properties of the host lattice are discussed. The course of intercalation can be separated into three phases. During an induction period the concentration of the alkali metal on the surface remains very small, the electronic states of the substrate are shifted by band bending. During an intercalation period the topotactic reaction proceeds. After reaching saturation compositions of the intercalation phase at the surface, the alkali metal diffuses into the bulk. Crystal or surface defects seem to have a significant influence on the kinetics of intercalation and on the stoichiometry of the intercalation compounds.     

The structure of n-alkane binary mixtures adsorbed on graphite

The thermodynamics and structure of the surface adsorbed phase in binary C15-C16 and C15-C17 n-alkane mixtures confined in graphite pores have been studied by differential scanning calorimetry and small-angle X-ray scattering. The previously observed selective adsorption of the longer alkane for chain length differences greater than five carbon atoms is verified but reduced for chain length differences less than or equal to two. With a difference in chain length of one carbon atom, Vegard's law is followed for the melting points of the adsorbed mixture and the (0 2) d-spacing is a continuous function of the mole fraction x. With a two-carbon atom difference, samples aged for 1 week have a lamellar structure for which the entities A_1−xB_x try to be commensurate with the substrate. The same samples aged for 1 month show a continuous parabolic x-dependence for both the melting points and the d-spacings. An explanation in terms of selective probability of adsorption is proposed based on crystallographic considerations.     

Scaling in large Prandtl number turbulent thermal convection

We study the scaling properties of heat transfer Nu in turbulent thermal convection at large Prandtl number Pr using a quasi-linear theory. We show that two regimes arise, depending on the Reynolds number Re. At low Reynolds number, NuPr ^-1/2 and Re are a function of RaPr ^-3/2. At large Reynolds number NuPr ^1/3 and RePr are function only of RaPr ^2/3 (within logarithmic corrections). In practice, since Nu is always close to Ra ^1/3, this corresponds to a much weaker dependence of the heat transfer in the Prandtl number at low Reynolds number than at large Reynolds number. This difference may solve an existing controversy between measurements in SF₆ (large Re) and in alcohol/water (lower Re). We link these regimes with a possible global bifurcation in the turbulent mean flow. We further show how a scaling theory could be used to describe these two regimes through a single universal function. This function presents a bimodal character for intermediate range of Reynolds number. We explain this bimodality in term of two dissipation regimes, one in which fluctuation dominate, and one in which mean flow dominates. Altogether, our results provide a six parameters fit of the curve Nu(Ra, Pr) which may be used to describe all measurements at Pr≥0.7. Received 27 February 2002 / Received in final form 29 May 2002 Published online 31 July 2002     

Photoreflectance investigation of hydrogenated (InGa)(AsN)/GaAs heterostructures

The optical response of as grown and hydrogenated In_0.32Ga_0.68As_1-yN_y/GaAs single quantum wells (y = 0, 0.027) has been investigated from T = 80 K to room temperature by photoreflectance. Three excitonic spectral features detected in the N free sample shift to lower energy in the N containing sample and back to higher energy upon H irradiation of the N containing sample. In the hydrogenated sample, a progressive change with increasing temperature of the nature of the lowest energy transition from an excitonic to a band-to-band character has been explained in terms of an increasing release of carriers from traps formed by H and N clusters. A reduction in the oscillator strength of the lowest energy transition and an increase in the binding energy of the heavy-hole exciton have been explained in terms of an increase in the electron effective mass upon N introduction into the In_xGa_1-xAs lattice. Received 23 June 2002 Published online 19 November 2002