1,002 research outputs found
Quantum Monte Carlo Calculations for Carbon Nanotubes
We show how lattice Quantum Monte Carlo can be applied to the electronic
properties of carbon nanotubes in the presence of strong electron-electron
correlations. We employ the path-integral formalism and use methods developed
within the lattice QCD community for our numerical work. Our lattice
Hamiltonian is closely related to the hexagonal Hubbard model augmented by a
long-range electron-electron interaction. We apply our method to the
single-quasiparticle spectrum of the (3,3) armchair nanotube configuration, and
consider the effects of strong electron-electron correlations. Our approach is
equally applicable to other nanotubes, as well as to other carbon
nanostructures. We benchmark our Monte Carlo calculations against the two- and
four-site Hubbard models, where a direct numerical solution is feasible.Comment: 54 pages, 16 figures, published in Physical Review
Critical exponents of the semimetal-insulator transition in graphene: A Monte Carlo study
The low-energy theory of graphene exhibits spontaneous chiral symmetry
breaking due to pairing of quasiparticles and holes, corresponding to a
semimetal-insulator transition at strong Coulomb coupling. We report a Lattice
Monte Carlo study of the critical exponents of this transition as a function of
the number of Dirac flavors , finding for
, for and for , with throughout. We compare our
results with recent analytical work for graphene and closely related systems,
and discuss scenarios for the fate of the chiral transition at finite
temperature and carrier density, an issue of relevance for upcoming experiments
with suspended graphene samples.Comment: 5 pages, 5 figures. Published versio
Exchange Current Operators and Electromagnetic Dipole Transitions in Heavy Quarkonia
The electromagnetic E1 and M1 transitions in heavy quarkonia (,
, ) and the magnetic moment of the are calculated
within the framework of the covariant Blankenbecler-Sugar (BSLT) equation. The
aim of this paper is to study the effects of two-quark exchange current
operators which involve the interaction, that arise in the BSLT (or
Schr\"odinger) reduction of the Bethe-Salpeter equation. These are found to be
small for E1 dominated decays such as and
, but significant for the M1 dominated
transitions. It is shown that a satisfactory description of the empirical data
on E1 and M1 transitions in charmonium and bottomonium requires unapproximated
treatment of the Dirac currents of the quarks. Finally, it is demonstrated that
many of the transitions are sensitive to the form of the
wavefunctions, and thus require a realistic treatment of the large hyperfine
splittings in the heavy quarkonium systems.Comment: 30 pages, 2 figures, uses Feynmf. Submitted to Nucl. Phys. A Accepted
versio
Two-loop Sunset Integrals at Finite Volume
We show how to compute the two-loop sunset integrals at finite volume, for
non-degenerate masses and non-zero momentum. We present results for all
integrals that appear in the Chiral Perturbation Therory (PT) calculation
of the pseudoscalar meson masses and decay constants at NNLO, including the
case of Partially Quenched PT. We also provide numerical implementations
of the finite-volume sunset integrals, and review the results for one-loop
integrals at finite volume.Comment: 45 page
Masses and Decay Constants of Pseudoscalar Mesons to Two Loops in Two-Flavor Partially Quenched Chiral Perturbation Theory
This paper presents a first study of the masses and decay constants of the
charged, or flavor-off-diagonal, pseudoscalar mesons to two loops for two
flavors of sea-quarks, in Partially Quenched Chiral Perturbation Theory
(PQPT). Explicit analytical expressions up to in the
momentum expansion are given. The calculations have been performed within the
supersymmetric formulation of PQPT. A numerical analysis is done to
indicate the size of the corrections.Comment: 20 pages, subsection about determining the LECs from lattice results
added, v3 one misprint correcte
Graphene: from materials science to particle physics
Since its discovery in 2004, graphene, a two-dimensional hexagonal carbon
allotrope, has generated great interest and spurred research activity from
materials science to particle physics and vice versa. In particular, graphene
has been found to exhibit outstanding electronic and mechanical properties, as
well as an unusual low-energy spectrum of Dirac quasiparticles giving rise to a
fractional quantum Hall effect when freely suspended and immersed in a magnetic
field. One of the most intriguing puzzles of graphene involves the
low-temperature conductivity at zero density, a central issue in the design of
graphene-based nanoelectronic components. While suspended graphene experiments
have shown a trend reminiscent of semiconductors, with rising resistivity at
low temperatures, most theories predict a constant or even decreasing
resistivity. However, lattice field theory calculations have revealed that
suspended graphene is at or near the critical coupling for excitonic gap
formation due to strong Coulomb interactions, which suggests a simple and
straightforward explanation for the experimental data. In this contribution we
review the current status of the field with emphasis on the issue of gap
formation, and outline recent progress and future points of contact between
condensed matter physics and Lattice QCD.Comment: 14 pages, 6 figures. Plenary talk given at the XXVIII International
Symposium on Lattice Field Theory (Lattice 2010), June 14-19, 2010,
Villasimius, Sardinia, Ital
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