5,826 research outputs found
Strong Coupling Model for String Breaking on the Lattice
A model for SU(n) string breaking on the lattice is formulated using strong
coupling ideas. Although necessarily rather crude, the model gives an explicit
picture of string breaking in the presence of dynamical quarks as a mixing
phenomenon that involves the string state and a two-meson state. A careful
analysis, within the model, of the Wilson loop shows that the evolution of the
mixing angle as a function of separation may obscure the expected crossover
effect. If a sufficiently extensive mixing region exists then an appropriate
combination of transition amplitudes can help in revealing the effect.
The sensitivity of the mixing region to the values of the meson energy and
the dynamical quark mass is explored and an assessment made of the
dectectibility of string breaking in a practical lattice simulation.Comment: 14 pages, LATEX2e, 2 figures, uses styles[12pt,a4,epsf
Mixing Scenarios for Lattice String Breaking
We present some simple scenarios for string breaking on the lattice based on
a crude strong coupling model introduced previously. We review the dependence
of the model on lattice spacing and extend it to include degenerate dynamical
quarks and also meson exchange diagrams. A comparison is made between quenched
and unquenched calculations. We examine string breaking in the presence of a
static quark-diquark system, a situation that is specific to SU(3).Comment: 15 pages 5 fig
The contribution of O(alpha) radiative corrections to the renormalised anisotropy and application to general tadpole improvement schemes: addendum to "One loop calculation of the renormalised anisotropy for improved anisotropic gluon actions on a lattice" [hep-lat/0208010]
General O(alpha) radiative corrections to lattice actions may be interpreted
as counterterms that give additive contributions to the one-loop
renormalisation of the anisotropy. The effect of changing the radiative
coefficients is thus easily calculable. In particular, the results obtained in
a previous paper for Landau mean link improved actions apply in any tadpole
improvement scheme. We explain how this method can be exploited when tuning
radiatively improved actions. Efficient methods for self-consistently tuning
tadpole improvement factors are also discussed.Comment: 3 pages of revte
Spherical Formulation for Diagramatic Evaluations on a Manifold with Boundary
The mathematical formalism necessary for the diagramatic evaluation of
quantum corrections to a conformally invariant field theory for a
self-interacting scalar field on a curved manifold with boundary is considered.
The evaluation of quantum corrections to the effective action past one-loop
necessitates diagramatic techniques. Diagramatic evaluations and higher
loop-order renormalisation can be best accomplished on a Riemannian manifold of
constant curvature accommodating a boundary of constant extrinsic curvature. In
such a context the stated evaluations can be accomplished through a consistent
interpretation of the Feynman rules within the spherical formulation of the
theory for which the method of images allows. To this effect, the mathematical
consequences of such an interpretation are analyzed and the spherical
formulation of the Feynman rules on the bounded manifold is, as a result,
developed.Comment: 12 pages, references added. To appear in Classical and Quantum
Gravit
Issues for countries considering introducing the "fourth hurdle": The case of Hungary
Objectives: This study outlines the needs and current development of the "fourth hurdle" (i.e., requirement of effectiveness and cost-effectiveness data for drug coverage policy decisions) in Hungary, describes the legal background and seeks to address some of the most important questions in this field. Methods: The study draws on international experiences and discusses five issues that a given jurisdiction needs to consider before introducing the "fourth hurdle" for pharmaceuticals. Results: The "fourth hurdle" is very relevant in Hungary because many existing drugs are unevaluated and many new, expensive drugs are becoming available. On the other hand, the existing resources for health technology assessment, including economic evaluation, are quite limited. All the five issues are relevant in the Hungarian setting and were helpful in determining exactly how the "fourth hurdle" should be applied. Conclusions: The most important issue seems to be that the implementation of the "fourth hurdle" needs to be achieved in a way consistent with the limited resources for HTA in Hungary. Specifically this means that, in setting priorities for drugs to evaluate, additional criteria need to be applied. In particular, priority should be given to assessing drugs that have been evaluated in other countries, because this affords the opportunity to adapt existing studies or models to the Hungarian situation
Electrically Tunable Band Gap in Silicene
We report calculations of the electronic structure of silicene and the
stability of its weakly buckled honeycomb lattice in an external electric field
oriented perpendicular to the monolayer of Si atoms. We find that the electric
field produces a tunable band gap in the Dirac-type electronic spectrum, the
gap being suppressed by a factor of about eight by the high polarizability of
the system. At low electric fields, the interplay between this tunable band
gap, which is specific to electrons on a honeycomb lattice, and the Kane-Mele
spin-orbit coupling induces a transition from a topological to a band
insulator, whereas at much higher electric fields silicene becomes a semimetal
Quantum Monte Carlo Calculation of the Binding Energy of Bilayer Graphene
We report diffusion quantum Monte Carlo calculations of the interlayer
binding energy of bilayer graphene. We find the binding energies of the AA- and
AB-stacked structures at the equilibrium separation to be 11.5(9) and 17.7(9)
meV/atom, respectively. The out-of-plane zone-center optical phonon frequency
predicted by our binding-energy curve is consistent with available experimental
results. As well as assisting the modeling of interactions between graphene
layers, our results will facilitate the development of van der Waals
exchange-correlation functionals for density functional theory calculations.Comment: 5 pages and 3 figures, submitted to Phys. Rev. Lett.; supplemental
material is available on arXiv via the ancillary files attached to this
submissio
Electrons and phonons in single layers of hexagonal indium chalcogenides from ab initio calculations
We use density functional theory to calculate the electronic band structures,
cohesive energies, phonon dispersions, and optical absorption spectra of
two-dimensional InX crystals, where X is S, Se, or Te. We identify two
crystalline phases (alpha and beta) of monolayers of hexagonal InX, and
show that they are characterized by different sets of Raman-active phonon
modes. We find that these materials are indirect-band-gap semiconductors with a
sombrero-shaped dispersion of holes near the valence-band edge. The latter
feature results in a Lifshitz transition (a change in the Fermi-surface
topology of hole-doped InX) at hole concentrations cm, cm,
and cm for X=S, Se, and Te,
respectively, for alpha-InX and
cm, cm, and cm for beta-InX.Comment: 9 pages. arXiv admin note: text overlap with arXiv:1302.606
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