402 research outputs found

    Transport in suspended graphene

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    Motivated by recent experiments on suspended graphene showing carrier mobilities as high as 200,000 cm^2/Vs, we theoretically calculate transport properties assuming Coulomb impurities as the dominant scattering mechanism. We argue that the substrate-free experiments done in the diffusive regime are consistent with our theory and verify many of our earlier predictions including (i) removal of the substrate will increase mobility since most of the charged impurities are in the substrate, (ii) the minimum conductivity is not universal, but depends on impurity concentration with cleaner samples having a higher minimum conductivity. We further argue that experiments on suspended graphene put strong constraints on the two parameters involved in our theory, namely, the charged impurity concentration n_imp and d, the typical distance of a charged impurity from the graphene sheet. The recent experiments on suspended graphene indicate a residual impurity density of 1-2 \times 10^{10} cm^{-2} which are presumably stuck to the graphene interface, compared to impurity densities of ~10^{12} cm^{-2} for graphene on SiO_2 substrate. Transport experiments can therefore be used as a spectroscopic tool to identify the properties of the remaining impurities in suspended graphene.Comment: 6 pages, 4 figures. Related papers available at http://www.physics.umd.edu/cmtc

    Thermal expansion in small metal clusters and its impact on the electric polarizability

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    The thermal expansion coefficients of NaN\mathrm{Na}_{N} clusters with 8N408 \le N \le 40 and Al7\mathrm{Al}_{7}, Al13\mathrm{Al}_{13}^- and Al14\mathrm{Al}_{14}^- are obtained from {\it ab initio} Born-Oppenheimer LDA molecular dynamics. Thermal expansion of small metal clusters is considerably larger than that in the bulk and size-dependent. We demonstrate that the average static electric dipole polarizability of Na clusters depends linearly on the mean interatomic distance and only to a minor extent on the detailed ionic configuration when the overall shape of the electron density is enforced by electronic shell effects. The polarizability is thus a sensitive indicator for thermal expansion. We show that taking this effect into account brings theoretical and experimental polarizabilities into quantitative agreement.Comment: 4 pages, 2 figures, one table. Accepted for publication in Physical Review Letters. References 10 and 23 update

    Tri-axial Octupole Deformations and Shell Structure

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    Manifestations of pronounced shell effects are discovered when adding nonaxial octupole deformations to a harmonic oscillator model. The degeneracies of the quantum spectra are in a good agreement with the corresponding main periodic orbits and winding number ratios which are found by classical analysis.Comment: 10 pages, Latex, 4 postscript figures, to appear in JETP Letter

    Profile monitors for the Super-FRS

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    Determinants of distribution and prevalence of avian malaria in blue tit populations across Europe : separating host and parasite effects

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    Although avian malarial parasites are globally distributed, the factors that affect the geographical distribution and local prevalence of different parasite lineages across host populations or species are still poorly understood. Based on the intense screening of avian malarial parasites in nine European blue tit populations, we studied whether distribution ranges as well as local adaptation, host specialization and phylogenetic relationships can determine the observed prevalences within populations. We found that prevalence differed consistently between parasite lineages and host populations, indicating that the transmission success of parasites is lineage specific but is partly shaped by locality-specific effects. We also found that the lineage-specific estimate of prevalence was related to the distribution range of parasites: lineages found in more host populations were generally more prevalent within these populations. Additionally, parasites with high prevalence that were also widely distributed among blue tit populations were also found to infect more host species. These findings suggest that parasites reaching high local prevalence can also realize wide distribution at a global scale that can have further consequences for host specialization. Although phylogenetic relationships among parasites did not predict prevalence, we detected a close match between a tree based on the geographic distance of the host populations and the parasite phylogenetic tree, implying that neighbouring host populations shared a related parasite fauna

    Close-Packing of Clusters: Application to Al_100

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    The lowest energy configurations of close-packed clusters up to N=110 atoms with stacking faults are studied using the Monte Carlo method with Metropolis algorithm. Two types of contact interactions, a pair-potential and a many-atom interaction, are used. Enhanced stability is shown for N=12, 26, 38, 50, 59, 61, 68, 75, 79, 86, 100 and 102, of which only the sizes 38, 75, 79, 86, and 102 are pure FCC clusters, the others having stacking faults. A connection between the model potential and density functional calculations is studied in the case of Al_100. The density functional calculations are consistent with the experimental fact that there exist epitaxially grown FCC clusters starting from relatively small cluster sizes. Calculations also show that several other close-packed motifs existwith comparable total energies.Comment: 9 pages, 7 figure

    Soft-Dielectron Excess in Proton-Proton Collisions at root s=13 TeV

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    A measurement of dielectron production in proton-proton (pp) collisions at root s = 13 TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron (ee) invariant mass m(ee) and pair transverse momentum p(T,ee) that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of m(ee), p(T,ee), and event multiplicity dNch=d.. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured eta/pi(0) ratio in pp and proton-nucleus collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at 0.15 < m(ee) < 0.6 GeV/c(2) and for p(T,ee) < 0.4 GeV/c indicates an enhancement of soft dielectrons, reminiscent of the "anomalous" soft-photon and soft-dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to 1.61 +/- 0.13(stat) +/- 0.17(syst, data) +/- 0.34osyst; cocktailTHORN in the ALICE acceptance. Acceptance-corrected excess spectra in mee and pT; ee are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.Peer reviewe

    Ionic and electronic structure of sodium clusters up to N=59

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    We determined the ionic and electronic structure of sodium clusters with even electron numbers and 2 to 59 atoms in axially averaged and three-dimensional density functional calculations. A local, phenomenological pseudopotential that reproduces important bulk and atomic properties and facilitates structure calculations has been developed. Photoabsorption spectra have been calculated for Na2\mathrm{Na}_2, Na8\mathrm{Na}_8, and Na9+\mathrm{Na}_9^+ to Na59+\mathrm{Na}_{59}^+. The consistent inclusion of ionic structure considerably improves agreement with experiment. An icosahedral growth pattern is observed for Na19+\mathrm{Na}_{19}^+ to Na59+\mathrm{Na}_{59}^+. This finding is supported by photoabsorption data.Comment: To appear in Phys. Rev. B 62. Version with figures in better quality can be requested from the author

    First measurement of the vertical bar t vertical bar-dependence of coherent J/psi photonuclear production

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    The first measurement of the cross section for coherent J/Psi photoproduction as a function of vertical bar t vertical bar, the square of the momentum transferred between the incoming and outgoing target nucleus, is presented. The data were measured with the ALICE detector in ultra-peripheral Pb-Pbcollisions at a centre-of-mass energy per nucleon pair root s(NN) = 5.02 TeV with the J/Psi produced in the central rapidity region vertical bar y vertical bar < 0.8, which corresponds to the small Bjorken-xrange (0.3 - 1.4) x 10(-3). The measured vertical bar t vertical bar-dependence is not described by computations based only on the Pb nuclear form factor, while the photonuclear cross section is better reproduced by models including shadowing according to the leading-twist approximation, or gluon-saturation effects from the impact-parameter dependent Balitsky-Kovchegov equation. These new results are therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken- x. (C) 2021 The Author(s). Published by Elsevier B.V.Peer reviewe

    Measurements of mixed harmonic cumulants in Pb-Pb collisions at root s(NN)=5.02 TeV

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    Correlations between moments of different flow coefficients are measured in Pb-Pb collisions at root s(NN) = 5.02 TeV recorded with the ALICE detector. These new measurements are based on multiparticle mixed harmonic cumulants calculated using charged particles in the pseudorapidity region vertical bar eta vertical bar < 0.8 with the transverse momentum range 0.2 < p(T) < 5.0 GeV/c. The centrality dependence of correlations between two flow coefficients as well as the correlations between three flow coefficients, both in terms of their second moments, are shown. In addition, a collection of mixed harmonic cumulants involving higher moments of v(2) and v(3) is measured for the first time, where the characteristic signature of negative, positive and negative signs of four-, six- and eight-particle cumulants are observed, respectively. The measurements are compared to the hydrodynamic calculations using iEBE-VISHNU with AMPT and TRENTo initial conditions. It is shown that the measurements carried out using the LHC Run 2 data in 2015 have the precision to explore the details of initial-state fluctuations and probe the nonlinear hydrodynamic response of v(2) and v(3) to their corresponding initial anisotropy coefficients epsilon(2) and epsilon(3). These new studies on correlations between three flow coefficients as well as correlations between higher moments of two different flow coefficients will pave the way to tighten constraints on initial-state models and help to extract precise information on the dynamic evolution of the hot and dense matter created in heavy-ion collisions at the LHC. (C) 2021 The Author. Published by Elsevier B.V.Peer reviewe
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