Nanoscale Effects on Heterojunction Electron Gases in GaN/AlGaN Core/Shell Nanowires

The electronic properties of heterojunction electron gases formed in GaN/AlGaN core/shell nanowires with hexagonal and triangular cross-sections are studied theoretically. We show that at nanoscale dimensions, the non-polar hexagonal system exhibits degenerate quasi-one-dimensional electron gases at the hexagon corners, which transition to a core-centered electron gas at lower doping. In contrast, polar triangular core/shell nanowires show either a non-degenerate electron gas on the polar face or a single quasi-one-dimensional electron gas at the corner opposite the polar face, depending on the termination of the polar face. More generally, our results indicate that electron gases in closed nanoscale systems are qualitatively different from their bulk counterparts.Comment: 16 pages, 7 figures. To appear in Nano Letters. Corrected some typo

Long Minority Carrier Diffusion Lengths in Bridged Silicon Nanowires

Nanowires have large surface areas that create new challenges for their optoelectronic applications. Lithographic processes involved in device fabrication and substrate interfaces can lead to surface defects and substantially reduce charge carrier lifetimes and diffusion lengths. Here, we show that using a bridging method to suspend pristine nanowires allows for circumventing detrimental fabrication steps and interfacial effects associated with planar device architectures. We report electron diffusion lengths up to 2.7 μm in bridged silicon nanowire devices, much longer than previously reported values for silicon nanowires with a diameter of 100 nm. Strikingly, electron diffusion lengths are reduced to only 45 nm in planar devices incorporating nanowires grown under the same conditions. The highly scalable silicon nanobridge devices with the demonstrated long diffusion lengths may find exciting applications in photovoltaics, sensing, and photodetectors

(Di)-aminoguanidine Functionalization through Transamination: An Avenue to an Auspicious Class of Supramolecular Synthons

N-4H-1,2,4-Triazol-4-yl-guanidine hydrochloride (L1) and a heterocyclized 3,4-diamino-4H-1,2,4-triazole hydrochloride (L2) were synthesized in good yields thanks to a transamination reaction of aminoguanidine and diaminoguanidine hydrochlorides, whose mechanism is presented. L1 and L2 crystallize in monoclinic (P21/a) and triclinic (P1̅) space groups, respectively. Hydrogen bonding interactions dictated by H-donor, acceptor rich ligand framework together with lattice chloride ions in L1 and L2 organize the crystal packing into a three-dimensional (3D) supramolecular network. The crystal structure of L1 is the first one for a 1,2,4-triazole ligand constructed from aminoguanidine. Both L1 and L2 that exist in monocationic form were introduced as supramolecular synthons to prepare Zn(II) complexes. Scanning electron microscopy (SEM) images on a bulk sample of [Zn(L1)2Cl2]Cl2·H2O (1) reveals a spontaneous aggregation of porous balls of coccolith morphology with surface decorated sickle-shaped particles of 100 nm thickness, whereas a powder sample of [Zn(L2)2Cl2]·H2O (2) shows neat rectangular blocks of 470 nm thickness. Colorless crystals of 2 crystallize in a monoclinic space group (C2/c). Zn(II) ions bound chloride with NH6B···Cl(1) = 2.495(2) Å and N···H type bonding (N6···H3C3 = 2.743(2) Å; N2···H7N7 = 1.989(2) Å) affording a 3D supramolecular network. The degree of pyramidality, ΣN = 38°, observed at one of the terminal amines in L2 is still retained in 2, but “flipping” of orientation of attached protons due to directional H-bonding is observed too. Porosity partitioning by mercury porosimetry measurements on 1 and 2 reveals interparticular porosities between 7 and 15 μm. N2(g) and H2(g) gas adsorption capacities of 1 and 2 were measured by BET which shows no preference for N2(g) but a low irreversible H2(g) uptake of 3 cm3/g

Search for a light charged Higgs boson decaying to c-sbar in pp collisions at sqrt(s) = 8 TeV

see paper for full list of authorsInternational audienceA search for a light charged Higgs boson, originating from the decay of a top quark and subsequently decaying into a charm quark and a strange antiquark, is presented. The data used in the analysis correspond to an integrated luminosity of 19.7 inverse-femtobarns recorded in proton-proton collisions at sqrt(s) = 8 TeV by the CMS experiment at the LHC. The search is performed in the process t tbar to W+/- b H-/+ bbar, where the W boson decays to a lepton (electron or muon) and a neutrino. The decays lead to a final state comprising an isolated lepton, at least four jets and large missing transverse energy. No significant deviation is observed in the data with respect to the standard model predictions, and model-independent upper limits are set on the branching fraction BF( t to H+ b ), ranging from 1.2 to 6.5% for a charged Higgs boson with mass between 90 and 160 GeV, under the assumption that BF( H+ to c sbar ) = 100%

Measurement of charged pion, kaon, and proton production in proton-proton collisions at root s=13 TeV

Transverse momentum spectra of charged pions, kaons, and protons are measured in proton-proton collisions at √s = 13 TeV with the CMS detector at the LHC. The particles, identified via their energy loss in the silicon tracker, are measured in the transverse momentum range of pT ∼ 0.1-1.7 GeV/c and rapidities /y/ < 1. The pT spectra and integrated yields are compared to previous results at smaller √s and to predictions of Monte Carlo event generators. The average pT increases with particle mass and charged particle multiplicity of the event. Comparisons with previous CMS results at √s = 0.9, 2.76, and 7 TeV show that the average pT and the ratios of hadron yields feature very similar dependences on the particle multiplicity in the event, independently of the center-of-mass energy of the pp collision

Measurement of the ratio B(t›Wb)/B(t›Wq) in pp collisions at s=8 TeV

The ratio of the top-quark branching fractions R=B(t›Wb)/B(t›Wq), where the denominator includes the sum over all down-type quarks (q=b,s,d), is measured in the tt¯ dilepton final state with proton–proton collision data at s=8 TeV from an integrated luminosity of 19.7 fb-1, collected with the CMS detector. In order to quantify the purity of the signal sample, the cross section is measured by fitting the observed jet multiplicity, thereby constraining the signal and background contributions. By counting the number of b jets per event, an unconstrained value of R=1.014±0.003(stat.)±0.032(syst.) is measured, in a good agreement with current precision measurements in electroweak and flavour sectors. A lower limit R>0.955 at the 95% confidence level is obtained after requiring R?1, and a lower limit on the Cabibbo–Kobayashi–Maskawa matrix element |Vtb|>0.975 is set at 95% confidence level. The result is combined with a previous CMS measurement of the t-channel single-top-quark cross section to determine the top-quark total decay width, ?t=1.36±0.02(stat.)-0.11 +0.14(syst.) GeV. © 2014 The Author

Search for supersymmetry in hadronic final states with missing transverse energy using the variables αT and b-quark multiplicity in pp collisions at √s = 8 TeV

Submitted by Vitor Silverio Rodrigues ([email protected]) on 2014-05-27T11:30:34Z No. of bitstreams: 0Bitstream added on 2014-05-27T14:48:49Z : No. of bitstreams: 1 2-s2.0-84883824987.pdf: 1540579 bytes, checksum: be6bbff5192436d9fcdbc367ffd1650e (MD5) Made available in DSpace on 2014-05-27T11:30:34Z (GMT). No. of bitstreams: 0 Previous issue date: 2013-09-01 An inclusive search for supersymmetric processes that produce final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of 8 TeV. The data sample corresponds to an integrated luminosity of 11.7 fb-1 collected by the CMS experiment at the LHC. In this search, a dimensionless kinematic variable, αT, is used to discriminate between events with genuine and misreconstructed missing transverse energy. The search is based on an examination of the number of reconstructed jets per event, the scalar sum of transverse energies of these jets, and the number of these jets identified as originating from bottom quarks. No significant excess of events over the standard model expectation is found. Exclusion limits are set in the parameter space of simplified models, with a special emphasis on both compressed-spectrum scenarios and direct or gluino-induced production of third-generation squarks. For the case of gluino-mediated squark production, gluino masses up to 950-1125 GeV are excluded depending on the assumed model. For the direct pair-production of squarks, masses up to 450 GeV are excluded for a single light first- or second-generation squark, increasing to 600 GeV for bottom squarks. © 2013 CERN for the benefit of the CMS collaboration. CERN, Geneva Yerevan Physics Institute, Yerevan Institut für Hochenergiephysik der OeAW, Wien National Centre for Particle and High Energy Physics, Minsk Universiteit Antwerpen, Antwerpen Vrije Universiteit Brussel, Brussel Université Libre de Bruxelles, Bruxelles Ghent University, Ghent Université Catholique de Louvain, Louvain-la-Neuve Université de Mons, Mons Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro Universidade do Estado do Rio de Janeiro, Rio de Janeiro Universidade Estadual Paulista, São Paulo Universidade Federal do ABC, São Paulo Institute for Nuclear Research and Nuclear Energy, Sofia University of Sofia, Sofia Institute of High Energy Physics, Beijing State Key Laboratory of Nuclear Physics and Technology Peking University, Beijing Universidad de Los Andes, Bogota Technical University of Split, Split University of Split, Split Institute Rudjer Boskovic, Zagreb University of Cyprus, Nicosia Charles University, Prague Academy of Scientific Research and Technology of the Arab Republic of Egypt Egyptian Network of High Energy Physics, Cairo National Institute of Chemical Physics and Biophysics, Tallinn Department of Physics University of Helsinki, Helsinki Helsinki Institute of Physics, Helsinki Lappeenranta University of Technology, Lappeenranta DSM/IRFU CEA/Saclay, Gif-sur-Yvette Laboratoire Leprince-Ringuet, Ecole Polytechnique IN2P3-CNRS, Palaiseau Institut Pluridisciplinaire Hubert Curien, Universite de Strasbourg, Universite de Haute Alsace Mulh CNRS/IN2P3, Strasbourg CNRS-IN2P3, Institut de Physique Nucléaire de Lyon Université de Lyon, Université Claude Bernard Lyon 1, Villeurbanne Institute of High Energy Physics and Informatization Tbilisi State University, Tbilisi I. Physikalisches Institut RWTH Aachen University, Aachen III. Physikalisches Institut A RWTH Aachen University, Aachen III. 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