Spectral properties of Er³⁺-doped chalcogenide glasses

Lanthanum sulphides glasses formed with sufficient proportions of Ga2S3 constitute a very convenient matrix for rare earth sulphides, and Pr3+-doped Ga2S3:La2S3 (GLS) glasses have been recognized as one of the most promising candidates for fibre amplifiers operating at a wavelength of 1.3µm. In this report, the spectral properties of chalcogenide glass of the molar composition 0.7Ga2S3:0.3La2S3 doped with Er3+ are presented and discussed. Emission and absorption spectra and lifetimes of energy levels have been measured. The 2.7µm emission, as shown below, has been observed from chalcogenide glass for the first time. Radiative and non-radiative transition rates are calculated and compared with the measured lifetimes of interesting energy levels

Exo-hydrogenated Single Wall Carbon Nanotubes

An extensive first-principles study of fully exo-hydrogenated zigzag (n,0) and armchair (n,n) single wall carbon nanotubes (C$_n$H$_n$), polyhedral molecules including cubane, dodecahedrane, and C$_{60}$H$_{60}$ points to crucial differences in the electronic and atomic structures relevant to hydrogen storage and device applications. C$_n$H$_n$'s are estimated to be stable up to the radius of a (8,8) nanotube, with binding energies proportional to 1/R. Attaching a single hydrogen to any nanotube is always exothermic. Hydrogenation of zigzag nanotubes is found to be more likely than armchair nanotubes with similar radius. Our findings may have important implications for selective functionalization and finding a way of separating similar radius nanotubes from each other.Comment: 5 pages, 4 postscript figures, Revtex file, To be appear in Physical Review

Basis Functions for Linear-Scaling First-Principles Calculations

In the framework of a recently reported linear-scaling method for density-functional-pseudopotential calculations, we investigate the use of localized basis functions for such work. We propose a basis set in which each local orbital is represented in terms of an array of `blip functions'' on the points of a grid. We analyze the relation between blip-function basis sets and the plane-wave basis used in standard pseudopotential methods, derive criteria for the approximate equivalence of the two, and describe practical tests of these criteria. Techniques are presented for using blip-function basis sets in linear-scaling calculations, and numerical tests of these techniques are reported for Si crystal using both local and non-local pseudopotentials. We find rapid convergence of the total energy to the values given by standard plane-wave calculations as the radius of the linear-scaling localized orbitals is increased.Comment: revtex file, with two encapsulated postscript figures, uses epsf.sty, submitted to Phys. Rev.

A POD reduced order model for resolving angular direction in neutron/photon transport problems

publisher: Elsevier articletitle: A POD reduced order model for resolving angular direction in neutron/photon transport problems journaltitle: Journal of Computational Physics articlelink: http://dx.doi.org/10.1016/j.jcp.2015.04.043 content_type: article copyright: Copyright © 2015 Elsevier Inc. All rights reserved.publisher: Elsevier articletitle: A POD reduced order model for resolving angular direction in neutron/photon transport problems journaltitle: Journal of Computational Physics articlelink: http://dx.doi.org/10.1016/j.jcp.2015.04.043 content_type: article copyright: Copyright © 2015 Elsevier Inc. All rights reserved.publisher: Elsevier articletitle: A POD reduced order model for resolving angular direction in neutron/photon transport problems journaltitle: Journal of Computational Physics articlelink: http://dx.doi.org/10.1016/j.jcp.2015.04.043 content_type: article copyright: Copyright © 2015 Elsevier Inc. All rights reserved

First-principles Simulations of the stretching and final breaking of Al nanowires: Mechanical properties and electrical conductance

The evolution of the structure and conductance of an Al nanowire subject to a tensile stress has been studied by first-principles total-energy simulations. Our calculations show the correlation between discontinuous changes in the force (associated to changes in the bonding structure of the nanowire) and abrupt modifications of the conductance as the nanowire develops a thinner neck, in agreement with the experiments. We reproduce the characteristic increase of the conductance in the last plateau, reaching a value close to the conductance quantum $G_0 = 2 e^2 / h$ before the breaking of the nanowire. A dimer defines the contact geometry at these last stages, with three channels (one dominant) contributing to the conductance.Comment: 4 pages, 4 figure

Measurement of the B0-anti-B0-Oscillation Frequency with Inclusive Dilepton Events

The $B^0$-$\bar B^0$ oscillation frequency has been measured with a sample of 23 million \B\bar B pairs collected with the BABAR detector at the PEP-II asymmetric B Factory at SLAC. In this sample, we select events in which both B mesons decay semileptonically and use the charge of the leptons to identify the flavor of each B meson. A simultaneous fit to the decay time difference distributions for opposite- and same-sign dilepton events gives $\Delta m_d = 0.493 \pm 0.012{(stat)}\pm 0.009{(syst)}$ ps$^{-1}$.Comment: 7 pages, 1 figure, submitted to Physical Review Letter

Measurement of the CP-Violating Asymmetry Amplitude sin2β\beta

We present results on time-dependent CP-violating asymmetries in neutral B decays to several CP eigenstates. The measurements use a data sample of about 88 million Y(4S) --> B Bbar decays collected between 1999 and 2002 with the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We study events in which one neutral B meson is fully reconstructed in a final state containing a charmonium meson and the other B meson is determined to be either a B0 or B0bar from its decay products. The amplitude of the CP-violating asymmetry, which in the Standard Model is proportional to sin2beta, is derived from the decay-time distributions in such events. We measure sin2beta = 0.741 +/- 0.067 (stat) +/- 0.033 (syst) and |lambda| = 0.948 +/- 0.051 (stat) +/- 0.017 (syst). The magnitude of lambda is consistent with unity, in agreement with the Standard Model expectation of no direct CP violation in these modes

A search for the decay B+→K+ννˉB^+ \to K^+ \nu \bar{\nu}

We search for the rare flavor-changing neutral-current decay $B^+ \to K^+ \nu \bar{\nu}$ in a data sample of 82 fb$^{-1}$ collected with the {\sl BABAR} detector at the PEP-II B-factory. Signal events are selected by examining the properties of the system recoiling against either a reconstructed hadronic or semileptonic charged-B decay. Using these two independent samples we obtain a combined limit of ${\mathcal B}(B^+ \to K^+ \nu \bar{\nu})<5.2 \times 10^{-5}$ at the 90% confidence level. In addition, by selecting for pions rather than kaons, we obtain a limit of ${\mathcal B}(B^+ \to \pi^+ \nu \bar{\nu})<1.0 \times 10^{-4}$ using only the hadronic B reconstruction method.Comment: 7 pages, 8 postscript figures, submitted to Phys. Rev. Let

High-reflectivity broadband distributed Bragg reflector lattice matched to ZnTe

We report on the realization of a high quality distributed Bragg reflector with both high and low refractive index layers lattice matched to ZnTe. Our structure is grown by molecular beam epitaxy and is based on binary compounds only. The high refractive index layer is made of ZnTe, while the low index material is made of a short period triple superlattice containing MgSe, MgTe, and ZnTe. The high refractive index step of Delta_n=0.5 in the structure results in a broad stopband and the reflectivity coefficient exceeding 99% for only 15 Bragg pairs.Comment: 4 pages, 3 figure