17,757 research outputs found

    A study on correlation effects in two dimensional topological insulators

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    We investigate correlation effects in two dimensional topological insulators (TI). In the first part, we discuss finite size effects for interacting systems of different sizes in a ribbon geometry. For large systems, there are two pairs of well separated massless modes on both edges. For these systems, we analyze the finite size effects using a standard bosonization approach. For small systems, where the edge states are massive Dirac fermions, we use the inhomogeneous dynamical mean field theory (DMFT) combined with iterative perturbation theory as an impurity solver to study interaction effects. We show that the finite size gap in the edge states is renormalized for weak interactions, which is consistent with a Fermi-liquid picture for small size TIs. In the second part, we investigate phase transitions in finite size TIs at zero temperature focusing on the effects of possible inter-edge Umklapp scattering for the edge states within the inhomogeneous DMFT using the numerical renormalization group. We show that correlation effects are effectively stronger near the edge sites because the coordination number is smaller than in the bulk. Therefore, the localization of the edge states around the edge sites, which is a fundamental property in TIs, is weakened for strong coupling strengths. However, we find no signs for "edge Mott insulating states" and the system stays in the topological insulating state, which is adiabatically connected to the non-interacting state, for all interaction strengths smaller than the critical value. Increasing the interaction further, a nearly homogeneous Mott insulating state is stabilized.Comment: 20 page

    Renormalization Group Analysis of \rho-Meson Properties at Finite Density

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    We calculate the density dependence of the ρ\rho-meson mass and coupling constant(gρNNg_{\rho NN}) for ρ\rho-nucleon-nucleon vertex at one loop using the lagrangian where the ρ\rho-meson is included as a dynamical gauge boson of a hidden local symmetry. From the condition that thermodynamic potential should not depend on the arbitrary energy scale, renormalization scale, one can construct a renormalization group equation for the thermodynamic potential and argue that the various renormalization group coefficients are functions of the density or temperature. We calculate the β\beta-function for ρ\rho-nucleon-nucleon coupling constant (gρNNg_{\rho NN}) and γ\gamma-function for ρ\rho-meson mass (γmρ\gamma_{m_\rho}). We found that the ρ\rho-meson mass and the coupling constant for gρNNg_{\rho NN} drop as density increases in the low energy limit.Comment: 24 pages, 10 figures, revised versio

    End-to-End Joint Antenna Selection Strategy and Distributed Compress and Forward Strategy for Relay Channels

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    Multi-hop relay channels use multiple relay stages, each with multiple relay nodes, to facilitate communication between a source and destination. Previously, distributed space-time codes were proposed to maximize the achievable diversity-multiplexing tradeoff, however, they fail to achieve all the points of the optimal diversity-multiplexing tradeoff. In the presence of a low-rate feedback link from the destination to each relay stage and the source, this paper proposes an end-to-end antenna selection (EEAS) strategy as an alternative to distributed space-time codes. The EEAS strategy uses a subset of antennas of each relay stage for transmission of the source signal to the destination with amplify and forwarding at each relay stage. The subsets are chosen such that they maximize the end-to-end mutual information at the destination. The EEAS strategy achieves the corner points of the optimal diversity-multiplexing tradeoff (corresponding to maximum diversity gain and maximum multiplexing gain) and achieves better diversity gain at intermediate values of multiplexing gain, versus the best known distributed space-time coding strategies. A distributed compress and forward (CF) strategy is also proposed to achieve all points of the optimal diversity-multiplexing tradeoff for a two-hop relay channel with multiple relay nodes.Comment: Accepted for publication in the special issue on cooperative communication in the Eurasip Journal on Wireless Communication and Networkin

    Meta-orbital Transition in Heavy-fermion Systems: Analysis by Dynamical Mean Field Theory and Self-consistent Renormalization Theory of Orbital Fluctuations

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    We investigate a two-orbital Anderson lattice model with Ising orbital intersite exchange interactions by means of dynamical mean field theory combined with the static mean field approximation of the intersite orbital interactions. Focusing on Ce-based heavy-fermion compounds, we examine the orbital crossover between the two orbital states, when the total f-electron number per site n_f is n_f ~ 1. We show that a "meta-orbital" transition, at which the occupancy of the two orbitals changes steeply, occurs when the hybridization between the ground-state f-electron orbital and conduction electrons are smaller than that between the excited f-electron orbital and conduction electrons. Near the meta-orbital critical end point, the orbital fluctuations are enhanced, and couple with the charge fluctuations. A critical theory of the meta-orbital fluctuations is also developed by applying the self-consistent renormalization theory of itinerant electron magnetism to the orbital fluctuations. The critical end point, first-order transition and crossover are described within Gaussian approximations of orbital fluctuations. We discuss the relevance of our results to CeAl2, CeCu2Si2, CeCu2Ge2 and the related compounds, which all have low-lying crystalline-electric-field excited states.Comment: 11 pages, 6 figures, J. Phys. Soc. Jpn. 79, (2010) 11471

    Effect of Disorder on Fermi surface in Heavy Electron Systems

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    The Kondo lattice model with substitutional disorder is studied with attention to the size of the Fermi surface and the associated Dingle temperature. The model serves for understanding heavy-fermion Ce compounds alloyed with La according to substitution Ce{x}La{1-x}. The Fermi surface is identified from the steepest change of the momentum distribution of conduction electrons, and is derived at low enough temperature by the dynamical mean-field theory (DMFT) combined with the coherent potential approximation (CPA). The Fermi surface without magnetic field increases in size with decreasing x from x=1 (Ce end), and disappears at such x that gives the same number of localized spins as that of conduction electrons. From the opposite limit of x=0 (La end), the Fermi surface broadens quickly as x increases, but stays at the same position as that of the La end. With increasing magnetic field, a metamagnetic transition occurs, and the Fermi surface above the critical field changes continuously across the whole range of x. The Dingle temperature takes a maximum around x=0.5. Implication of the results to experimental observation is discussed.Comment: 5 pages, 5 figure

    Nearly horizon skimming orbits of Kerr black holes

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    An unusual set of orbits about extreme Kerr black holes resides at the Boyer-Lindquist radius r=Mr = M, the coordinate of the hole's event horizon. These ``horizon skimming'' orbits have the property that their angular momentum LzL_z {\it increases} with inclination angle, opposite to the familiar behavior one encounters at larger radius. In this paper, I show that this behavior is characteristic of a larger family of orbits, the ``nearly horizon skimming'' (NHS) orbits. NHS orbits exist in the very strong field of any black hole with spin a\agt 0.952412M. Their unusual behavior is due to the locking of particle motion near the event horizon to the hole's spin, and is therefore a signature of the Kerr metric's extreme strong field. An observational hallmark of NHS orbits is that a small body spiraling into a Kerr black hole due to gravitational-wave emission will be driven into orbits of progressively smaller inclination angle, toward the equator. This is in contrast to the ``normal'' behavior. For circular orbits, the change in inclination is very small, and unlikely to be of observational importance. I argue that the change in inclination may be considerably larger when one considers the evolution of inclined eccentric orbits. If this proves correct, then the gravitational waves produced by evolution through the NHS regime may constitute a very interesting and important probe of the strong-field nature of rotating black holes.Comment: 9 pages, 5 figures, accepted for publication in PR

    Gravitational Waves from Mergin Compact Binaries: How Accurately Can One Extract the Binary's Parameters from the Inspiral Waveform?

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    The most promising source of gravitational waves for the planned detectors LIGO and VIRGO are merging compact binaries, i.e., neutron star/neutron star (NS/NS), neutron star/black hole (NS/BH), and black hole/black-hole (BH/BH) binaries. We investigate how accurately the distance to the source and the masses and spins of the two bodies will be measured from the gravitational wave signals by the three detector LIGO/VIRGO network using ``advanced detectors'' (those present a few years after initial operation). The combination M(M1M2)3/5(M1+M2)1/5{\cal M} \equiv (M_1 M_2)^{3/5}(M_1 +M_2)^{-1/5} of the masses of the two bodies is measurable with an accuracy 0.1%1%\approx 0.1\%-1\%. The reduced mass is measurable to 10%15%\sim 10\%-15\% for NS/NS and NS/BH binaries, and 50%\sim 50\% for BH/BH binaries (assuming 10M10M_\odot BH's). Measurements of the masses and spins are strongly correlated; there is a combination of μ\mu and the spin angular momenta that is measured to within 1%\sim 1\%. We also estimate that distance measurement accuracies will be 15%\le 15\% for 8%\sim 8\% of the detected signals, and 30%\le 30\% for 60%\sim 60\% of the signals, for the LIGO/VIRGO 3-detector network.Comment: 103 pages, 20 figures, submitted to Phys Rev D, uses revtex macros, Caltech preprint GRP-36

    Thermoelectric properties of lead chalcogenide core-shell nanostructures

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    We present the full thermoelectric characterization of nanostructured bulk PbTe and PbTe-PbSe samples fabricated from colloidal core-shell nanoparticles followed by spark plasma sintering. An unusually large thermopower is found in both materials, and the possibility of energy filtering as opposed to grain boundary scattering as an explanation is discussed. A decreased Debye temperature and an increased molar specific heat are in accordance with recent predictions for nanostructured materials. On the basis of these results we propose suitable core-shell material combinations for future thermoelectric materials of large electric conductivities in combination with an increased thermopower by energy filtering.Comment: 12 pages, 8 figure

    Dendritic Spine Shape Analysis: A Clustering Perspective

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    Functional properties of neurons are strongly coupled with their morphology. Changes in neuronal activity alter morphological characteristics of dendritic spines. First step towards understanding the structure-function relationship is to group spines into main spine classes reported in the literature. Shape analysis of dendritic spines can help neuroscientists understand the underlying relationships. Due to unavailability of reliable automated tools, this analysis is currently performed manually which is a time-intensive and subjective task. Several studies on spine shape classification have been reported in the literature, however, there is an on-going debate on whether distinct spine shape classes exist or whether spines should be modeled through a continuum of shape variations. Another challenge is the subjectivity and bias that is introduced due to the supervised nature of classification approaches. In this paper, we aim to address these issues by presenting a clustering perspective. In this context, clustering may serve both confirmation of known patterns and discovery of new ones. We perform cluster analysis on two-photon microscopic images of spines using morphological, shape, and appearance based features and gain insights into the spine shape analysis problem. We use histogram of oriented gradients (HOG), disjunctive normal shape models (DNSM), morphological features, and intensity profile based features for cluster analysis. We use x-means to perform cluster analysis that selects the number of clusters automatically using the Bayesian information criterion (BIC). For all features, this analysis produces 4 clusters and we observe the formation of at least one cluster consisting of spines which are difficult to be assigned to a known class. This observation supports the argument of intermediate shape types.Comment: Accepted for BioImageComputing workshop at ECCV 201

    Association of pregnancy with engagement in HIV care among women with HIV in the UK: a cohort study

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    BACKGROUND: Women with HIV face challenges in engaging in HIV care post partum. We aimed to examine changes in engagement in HIV care through clinic attendance before, during, and after pregnancy, compared with matched women with HIV who had never had a recorded pregnancy. METHODS: In this cohort study, we describe changes in engagement in HIV care before, during, and after pregnancy among women with HIV from the UK Collaborative HIV Cohort (CHIC) study from 25 HIV clinics in the UK with a livebirth reported to the National Surveillance of HIV in Pregnancy and Childhood between Jan 1, 2000, and Dec 31, 2017. To investigate whether changes were specific to HIV, we compared these changes to those over equivalent periods among non-pregnant women with HIV in the UK CHIC study matched for ethnicity, year of conception, age, CD4 cell count, viral suppression, and antiretroviral therapy use. Analyses were via logistic regression using generalised estimated equations with an interaction between case-control status (pregnant women vs non-pregnant women) and pregnancy or pseudo pregnancy (for non-pregnant women) stage. FINDINGS: 1116 matched pairs of pregnant and non-pregnant women were included (median age 34 years [IQR 30-38], 80·1% Black African, 12·5% white). 69 330 person-months of follow-up were recorded, 25 412 in the before stage, 18 897 during, and 25 021 after pregnancy or pseudo pregnancy stages. Among pregnant women, the proportion of time engaged in care increased during pregnancy (8477 [90·5%] of 9371 person-months) and after pregnancy (10 501 [84·6%] of 12 407), compared with before pregnancy (9979 [78·5%] of 12 707). Among non-pregnant women in the control group, engagement in HIV care remained stable across the three equivalent stages (9688 [76·3%] of 12 705 person-months before pseudo pregnancy; 7463 [78·3%] of 9526 during pseudo pregnancy; and 9892 [78·4%] of 12 614 after pseudo pregnancy). The association of engagement in HIV care with pregnancy or pseudo pregnancy stage differed significantly by case-control status (pinteraction<0·0001); the odds of engagement in HIV care were higher during pregnancy (odds ratio [OR] 3·32, 95% CI 2·68-4·12) and after pregnancy (OR 1·49, 1·24-1·79) only among pregnant women, and not among non-pregnant women, when compared with the before pseudo pregnancy stage. INTERPRETATION: Women with HIV and a pregnancy resulting in a livebirth were more likely to engage in HIV care post partum when compared with before pregnancy. A detailed understanding of the reason for this finding could support interventions to maximise engagement in HIV care for all women with HIV. FUNDING: Medical Research Council and National Institute for Health Research
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