Crystallization process of Bi(Pb)-Sr-Ca-Cu-O glass

A crystallization study has been carried out for rapidly solidified Bi₂Pb_0.5Sr₂Ca₄Cu₅O_x glass. Glass transition temperature T _g, crystallized superconducting phases and microstructural changes were measured and analysed by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The crystallization mechanism of the three superconducting phases — (2201) 20 K phase, (2212) 80 K phase, and (2223) 110 K phase — has been discussed, and a time-temperature-transformation diagram for the glass has been constructed.     

Effect of Al content on the properties of Cr1−xAlxC films prepared by RF reactive magnetron sputtering

Cr_1−xAl_xC films were deposited on high-speed steel by RF reactive magnetron sputtering. In this study, we aimed to identify the effect of the Al content on the properties of Cr_1−xAl_xC films. We found that Cr_1−xAl_xC films exhibited a fine columnar grain microstructure with some special characteristics, such as high hardness of Hv 1426, a low friction coefficient of 0.29, and a large contact angle of 90° for x = 0.18. Furthermore, an increase in Al content resulted in a decrease in film hardness and an increase in contact angle. Moreover, on annealing at 923 K, the mechanical properties of the films improved and a dense protective film of complex Cr₂O₃ and Al₂O₃ oxides was formed on the surface for better wear resistance, which will ultimately increase the lifetime of the high-speed steel substrate.     

Design and Fabrication of 1.06 μm Resonant-Cavity Enhanced Reflective Modulator with GaInAs/GaAs Quantum Wells

A resonant-cavity enhanced reflective optical modulator is designed and fabricated, with three groups of three highly strained InGaAs/GaAs quantum wells in the cavity, for low voltage and high contrast ratio operation. The quantum wells are positioned in antinodes of the optical standing wave. The modulator is grown in a single growth step in an molecular beam epitaxy system, using GaAs/AlAs distributed Bragg reflectors as both the top and bottom mirrors. Results show that the reflection device has a modulation extinction of 3 dB at -4.5 V bias.     

Double neutron/proton ratio of nucleon emissions in isotopic reaction systems as a robust probe of nuclear symmetry energy

The double neutron/proton ratio of nucleon emissions taken from two reaction systems using four isotopes of the same element, namely, the neutron/proton ratio in the neutron-rich system over that in the more symmetric system, has the advantage of reducing systematically the influence of the Coulomb force and the normally poor efficiencies of detecting low energy neutrons. The double ratio thus suffers less systematic errors. Within the IBUU04 transport model the double neutron/proton ratio is shown to have about the same sensitivity to the density dependence of nuclear symmetry energy as the single neutron/proton ratio in the neutron-rich system involved. The double neutron/proton ratio is therefore more useful for further constraining the symmetry energy of neutron-rich matter.     

Possible candidates for discrete gamma rays linking superdeformed and yrast states in an odd nucleus:193Pb

The francium isotopes^200–202Fr were produced in the reaction³⁵Cl+¹⁷⁰Yb using bombarding energies of 4.9–5.3 MeV/nucleon. Fusion products were separated in-flight from the primary beam using a gas-filled recoil separator. An alpha line with the alpha particle energy and half-life of (7468±9) keV and (19 ₋₆ ⁺¹³ ) ms, respectively, was assigned to²⁰⁰Fr. Previously reported decay properties of^201,202Fr were confirmed. Communicated by V. Metag     

Production of fast evaporation residues by the reaction20Ne+208Pb at projectile energies of 8.6, 11.4 and 14.9 A.MeV

Velocity distributions and production cross sections of evaporation residues have been measured in the reaction²⁰Ne+²⁰⁸Pb at projectile energies of 8.6, 11.4, 14.9 A.MeV. Essential deviations from statistical model of deexcitation have been observed. Monte Carlo simulations involving emission of non-equilibrium particles have been used in order to reproduce experimental velocity, charge and mass distributions of evaporation residues and to estimate indirectly multiplicities of pre-equilibrium particles. Communicated by V. Metag     

Virtual photon scattering at subwavelength sized tips

Silicon AFM tips can profitably be used as optical sensors for near field optical microscopy. In particular they are able to convert evanescent waves (also called virtual photons) into propagating ones which are conveniently guided in the tip and transferred to the air at the back of the cantilever. In this paper it is shown that virtual photons scattered at the tip end are also converted into real ones which are observed in the far field. Contrary to the previous observations of similar conversion at STM metal tips the scattered emission at dielectric silicon tips does not have a dependence with distance of a full exponential decay but rather follows the more complex dependence of the dielectric capture. This observation is consistent with Mie's theory of scattering of real photons by small particles. Experiments are performed with boths andp polarizations showing a regular dependence of the scattered intensity with the incidence angle. The same experiment is also performed with metallized silicon tips showing a metal scattering behaviour. This results can help to achieve a better control of the tip position in the close range of distances in a Photon Scanning Tunneling Microscopy (PSTM) experiment.     

Derivation of a modified paraxial formalism for two-dimensional trajectories in electron and ion sources of non-simple geometries

A modified paraxial formalism has been developed which describes two-dimensional charged particle beam dynamics in electron and ion sources with pointed or needle-type geometries. A Hamiltonian formalism, and a more exact treatment of energy conservation is used to derive the modified paraxial equation for two-dimensional trajectories in systems with axially symmetric prolate-spheroidal beams. Calculations have been done for a gallium field emission liquid metal ion source modeled by a hyperboloid of revolution and planar extractor. Two important conclusions emerge from these calculations: i) The dominant effect of space-charge, for source geometries with small radii of curvature, occurs in the region close to the apex (<0.05 n) and ii) beam divergence has a strong dependence on geometry. This latter effect is a consequence of large two-dimensional field gradients near the apex of sources with needle-type or pointed geometry.This work was supported in part by the Division of Materials Research, National Science Foundation, Grant No. DMR-8108829     

Photoluminescence of Al x Ga1−x As/GaAs quantum well heterostructures grown by molecular beam epitaxy

Nominally undoped Al_xGa_1–xAs grown by molecular beam epitaxy from As₄ species at elevated substrate temperatures of 670°C exhibits well-resolved excitonic fine structure in the low-temperature photoluminescence spectra, if the effective As-to-(Al+Ga) flux ratio on the growth surface is kept within a rather narrow range of clearly As-stabilized conditions. In contrast to previous results on Al_xGa_1–xAs of composition 0.15not to shift in energy by changing the excitation intensity. This implies a simple freeelectron carbon-acceptor recombination mechanism for the line without any participation of a donor. In Al_xGa_1–xAs of composition close to the direct-to-indirect cross-over point, two distinct LO-phonons separated by 34 and 48 meV from the (D ⁰,C ⁰) peak position at x=0.43 were observed which were before only detectable by Raman scattering experiments. The intensity of the carbon-impurity related luminescence lines in bulk-type Al_xGa_1–xAs and GaAs layers was found to be strongly reduced, as compared to the excitonic recombination lines, if the respective active layer was covered by a very thin confinement layer of either GaAs on top of Al_xGa_1–xAs or vice versa grown in the same growth cycle.