Threshold Behavior Of (gaal)as-gaas Lasers At Low Temperatures

The temperature dependence of the threshold current, differential quantum efficiency, and internal loss have been measured in the temperature range 10-293°K. The threshold current increases relatively slowly with temperature above 100°K and is independent of the impurity concentration. Theoretical calculation shows that this behavior is to be expected for a band-to-band transition that follows k selection. The threshold behavior at low temperatures (≤ 80°K) depends strongly on the type and concentration of the impurity. The relatively fast decrease in threshold below 100°K shows saturation for an active layer with n-type impurities or with high-concentration p-type impurities. The saturation is attributed to the carrier diffusion length becoming smaller than the active-layer thickness. The internal differential quantum efficiency is near unity and is independent of temperature. The internal loss, however, decreases with temperature due to reduction in free-carrier absorption.491293

Lattice effects on the current-voltage characteristics of superconducting arrays

The lattice effects on the current-voltage characteristics of two-dimensional arrays of resistively shunted Josephson junctions are investigated. The lattice potential energies due to the discrete lattice structure are calculated for several geometries and directions of current injection. We compare the energy barrier for vortex-pair unbinding with the lattice pinning potential, which shows that lattice effects are negligible in the low-current limit as well as in the high-current limit. At intermediate currents, on the other hand, the lattice potential becomes comparable to the barrier height and the lattice effects may be observed in the current-voltage characteristics.Comment: 5 pages including 5 figures in two columns, to appear in Phys. Rev.

Signatures of Electronic Nematic Phase at Isotropic-Nematic Phase Transition

The electronic nematic phase occurs when the point-group symmetry of the lattice structure is broken, due to electron-electron interactions. We study a model for the nematic phase on a square lattice with emphasis on the phase transition between isotropic and nematic phases within mean field theory. We find the transition to be first order, with dramatic changes in the Fermi surface topology accompanying the transition. Furthermore, we study the conductivity tensor and Hall constant as probes of the nematic phase and its transition. The relevance of our findings to Hall resistivity experiments in the high-$T_c$ cuprates is discussed.Comment: 5 pages, 3 figure

Dynamics of false vacuum bubbles: beyond the thin shell approximation

We numerically study the dynamics of false vacuum bubbles which are inside an almost flat background; we assumed spherical symmetry and the size of the bubble is smaller than the size of the background horizon. According to the thin shell approximation and the null energy condition, if the bubble is outside of a Schwarzschild black hole, unless we assume Farhi-Guth-Guven tunneling, expanding and inflating solutions are impossible. In this paper, we extend our method to beyond the thin shell approximation: we include the dynamics of fields and assume that the transition layer between a true vacuum and a false vacuum has non-zero thickness. If a shell has sufficiently low energy, as expected from the thin shell approximation, it collapses (Type 1). However, if the shell has sufficiently large energy, it tends to expand. Here, via the field dynamics, field values of inside of the shell slowly roll down to the true vacuum and hence the shell does not inflate (Type 2). If we add sufficient exotic matters to regularize the curvature near the shell, inflation may be possible without assuming Farhi-Guth-Guven tunneling. In this case, a wormhole is dynamically generated around the shell (Type 3). By tuning our simulation parameters, we could find transitions between Type 1 and Type 2, as well as between Type 2 and Type 3. Between Type 2 and Type 3, we could find another class of solutions (Type 4). Finally, we discuss the generation of a bubble universe and the violation of unitarity. We conclude that the existence of a certain combination of exotic matter fields violates unitarity.Comment: 40 pages, 41 figure

Escalated regeneration in sciatic nerve crush injury by the combined therapy of human amniotic fluid mesenchymal stem cells and fermented soybean extracts, Natto

Attenuation of inflammatory cell deposits and associated cytokines prevented the apoptosis of transplanted stem cells in a sciatic nerve crush injury model. Suppression of inflammatory cytokines by fermented soybean extracts (Natto) was also beneficial to nerve regeneration. In this study, the effect of Natto on transplanted human amniotic fluid mesenchymal stem cells (AFS) was evaluated. Peripheral nerve injury was induced in SD rats by crushing a sciatic nerve using a vessel clamp. Animals were categorized into four groups: Group I: no treatment; Group II: fed with Natto (16 mg/day for 7 consecutive days); Group III: AFS embedded in fibrin glue; Group IV: Combination of group II and III therapy. Transplanted AFS and Schwann cell apoptosis, inflammatory cell deposits and associated cytokines, motor function, and nerve regeneration were evaluated 7 or 28 days after injury. The deterioration of neurological function was attenuated by AFS, Natto, or the combined therapy. The combined therapy caused the most significantly beneficial effects. Administration of Natto suppressed the inflammatory responses and correlated with decreased AFS and Schwann cell apoptosis. The decreased AFS apoptosis was in line with neurological improvement such as expression of early regeneration marker of neurofilament and late markers of S-100 and decreased vacuole formation. Administration of either AFS, or Natto, or combined therapy augmented the nerve regeneration. In conclusion, administration of Natto may rescue the AFS and Schwann cells from apoptosis by suppressing the macrophage deposits, associated inflammatory cytokines, and fibrin deposits

Meson Cloud of the Nucleon in Polarized Semi-Inclusive Deep-Inelastic Scattering

We investigate the possibility of identifying an explicit pionic component of the nucleon through measurements of polarized $\Delta^{++}$ baryon fragments produced in deep-inelastic leptoproduction off polarized protons, which may help to identify the physical mechanism responsible for the breaking of the Gottfried sum rule. The pion-exchange model predicts highly correlated polarizations of the $\Delta^{++}$ and target proton, in marked contrast with the competing diquark fragmentation process. Measurement of asymmetries in polarized $\Lambda$ production may also reveal the presence of a kaon cloud in the nucleon.Comment: 23 pages REVTeX, 7 uuencoded figures, accepted for publication in Zeit. Phys.

Cosmology from Rolling Massive Scalar Field on the anti-D3 Brane of de Sitter Vacua

We investigate a string-inspired scenario associated with a rolling massive scalar field on D-branes and discuss its cosmological implications. In particular, we discuss cosmological evolution of the massive scalar field on the ant-D3 brane of KKLT vacua. Unlike the case of tachyon field, because of the warp factor of the anti-D3 brane, it is possible to obtain the required level of amplitude of density perturbations. We study the spectra of scalar and tensor perturbations generated during the rolling scalar inflation and show that our scenario satisfies the observational constraint coming from the Cosmic Microwave Background anisotropies and other observational data. We also implement the negative cosmological constant arising from the stabilization of the modulus fields in the KKLT vacua and find that this leads to a successful reheating in which the energy density of the scalar field effectively scales as a pressureless dust. The present dark energy can be also explained in our scenario provided that the potential energy of the massive rolling scalar does not exactly cancel with the amplitude of the negative cosmological constant at the potential minimum.Comment: RevTex4, 15 pages, 5 eps figures, minor clarifications and few references added, final version to appear in PR

Shear viscous effects on the primordial power spectrum from warm inflation

We compute the primordial curvature spectrum generated during warm inflation, including shear viscous effects. The primordial spectrum is dominated by the thermal fluctuations of the radiation bath, sourced by the dissipative term of the inflaton field. The dissipative coefficient \Upsilon, computed from first principles in the close-to-equilibrium approximation, depends in general on the temperature T, and this dependence renders the system of the linear fluctuations coupled. Whenever the dissipative coefficient is larger than the Hubble expansion rate H, there is a growing mode in the fluctuations before horizon crossing. However, dissipation intrinsically means departures from equilibrium, and therefore the presence of a shear viscous pressure in the radiation fluid. This in turn acts as an extra friction term for the radiation fluctuations that tends to damp the growth of the perturbations. Independently of the T functional dependence of the dissipation and the shear viscosity, we find that when the shear viscous coefficient \zeta_s is larger than 3 \rho_r/H at horizon crossing, \rho_r being the radiation energy density, the shear damping effect wins and there is no growing mode in the spectrum.Comment: 18 pages, 6 figure

Identical transitions in the strongly deformed Sr-99 and Sr-100

The decay of the very neutron-rich nucleus Rb-100 has been studied by gamma-spectroscopy of on-line mass-separated samples. Schemes for beta-decay to Sr-100 and beta-n-decay to Sr-99 are presented. New sets of transitions in Sr-99 and Sr-100 with identical energies are observed. All identical bands so far observed in neutron-rich Sr isotopes obey a simple energy rule valid for even-even, odd-even and odd-odd bands.Comment: 31 pages, 7 figures, Phys. Rev. C, in prin