239 research outputs found
The ground state of clean and defected graphene: Coulomb interactions of massless Dirac fermions, pair-distribution functions and spin-polarized phases
First-principles density functional calculations for graphene and defected
graphene are used to examine when the quasi-2D electrons near the Fermi energy
in graphene could be represented by massless fermions obeying a Dirac-Weyl (DW)
equation. The DW model is found to be inapplicable to defected graphene
containing even 3% vacancies or N substitution. However, the DW model
holds in the presence of weakly adsorbed molecular layers. The possibility of
spin-polarized phases (SPP) of DW-massless fermions in pure graphene is
considered. The exchange energy is evaluated from the analytic
pair-distribution functions as well as in -space. The kinetic energy
enhancement of the sipn-polarized phase nearly cancels the exchange
enhancement, and the correlation energy plays a dominant residual role. The
correlation energies are estimated via a model four-component 2D electron fluid
whose Coulomb coupling matches that of graphene. While SPPs appear with
exchange only, the inclusion of correlations suppresses them in ideal graphene.Comment: ~7 pages, 6 figure
Accessing the topological susceptibility via the Gribov horizon
The topological susceptibility, , following the work of Witten and
Veneziano, plays a key role in identifying the relative magnitude of the
mass, the so-called problem. A nonzero is
caused by the Veneziano ghost, the occurrence of an unphysical massless pole in
the correlation function of the topological current. In a recent paper
(Phys.Rev.Lett.114 (2015) 24, 242001), an explicit relationship between this
Veneziano ghost and color confinement was proposed, by connecting the dynamics
of the Veneziano ghost, and thus the topological susceptibility, with Gribov
copies. However, the analysis is incompatible with BRST symmetry (Phys.Rev.D 93
(2016) no.8, 085010). In this paper, we investigate the topological
susceptibility, , in SU(3) and SU(2) Euclidean Yang-Mills theory using
an appropriate Pad\'e approximation tool and a non-perturbative gluon
propagator, within a BRST invariant framework and by taking into account Gribov
copies in a general linear covariant gauge.Comment: 17 pages, 4 figures. v2: corrected typos, new figures, improved style
of presentatio
BRS Cohomology of Zero Curvature Systems II. The Noncomplete Ladder Case
The Yang-Mills type theories and their BRS cohomology are analysed within the
zero curvature formalism.Comment: 14 pages, latex, no figures, latex improve
Polynomial Time Approximation Schemes for All 1-Center Problems on Metric Rational Set Similarities
In this paper, we investigate algorithms for finding centers of a given collection N of sets. In particular, we focus on metric rational set similarities, a broad class of similarity measures including Jaccard and Hamming. A rational set similarity S is called metric if D= 1 - S is a distance function. We study the 1-center problem on these metric spaces. The problem consists of finding a set C that minimizes the maximum distance of C to any set of N. We present a general framework that computes a (1 + ε) approximation for any metric rational set similarity
Increasing d-wave superconductivity by on site repulsion
We study by Variational Monte Carlo an extended Hubbard model away from half
filled band density which contains two competing nearest-neighbor interactions:
a superexchange favoring d-wave superconductivity and a repulsion
opposing against it. We find that the on-site repulsion effectively
enhances the strength of meanwhile suppressing that of , thus favoring
superconductivity. This result shows that attractions which do not involve
charge fluctuations are very well equipped against strong electron-electron
repulsion so much to get advantage from it.Comment: 4 pages, 3 figure
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