1,147 research outputs found
Errors in hybrid computers
Method is described for reduction of error components in numerical integration, sampling with zero hold order, and execution time delay
Growth of Epitaxial Oxide Thin Films on Graphene
The transfer process of graphene onto the surface of oxide substrates is well known. However, for many devices, we require high quality oxide thin films on the surface of graphene. This step is not understood. It is not clear why the oxide should adopt the epitaxy of the underlying oxide layer when it is deposited on graphene where there is no lattice match. To date there has been no explanation or suggestion of mechanisms which clarify this step. Here we show a mechanism, supported by first principles simulation and structural characterisation results, for the growth of oxide thin films on graphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a graphene and show that local defects in the graphene layer (e.g. grain boundaries) act as bridgepillar spots that enable the epitaxial growth of STO thin films on the surface of the graphene layer. This study, and in particular the suggestion of a mechanism for epitaxial growth of oxides on graphene, offers new directions to exploit the development of oxide/graphene multilayer structures and devices
String Breaking in Four Dimensional Lattice QCD
Virtual quark pair screening leads to breaking of the string between
fundamental representation quarks in QCD. For unquenched four dimensional
lattice QCD, this (so far elusive) phenomenon is studied using the recently
developed truncated determinant algorithm (TDA). The dynamical configurations
were generated on an Athlon 650 MHz PC. Quark eigenmodes up to 420 MeV are
included exactly in these TDA studies performed at low quark mass on large
coarse (but O() improved) lattices. A study of Wilson line correlators in
Coulomb gauge extracted from an ensemble of 1000 two-flavor dynamical
configurations reveals evidence for flattening of the string tension at
distances R approximately 1 fm.Comment: 16 pages, 5 figures, Latex (deleted extraneous eps figure file
Structure of Strange Dwarfs with Color Superconducting Core
We study effects of two-flavor color superconductivity on the structure of
strange dwarfs, which are stellar objects with similar masses and radii with
ordinary white dwarfs but stabilized by the strange quark matter core. We find
that unpaired quark matter is a good approximation to the core of strange
dwarfs.Comment: 8 pages 5 figures, J. Phys. G, accepte
Perturbation theory vs. simulation for tadpole improvement factors in pure gauge theories
We calculate the mean link in Landau gauge for Wilson and improved SU(3)
anisotropic gauge actions, using two loop perturbation theory and Monte Carlo
simulation employing an accelerated Langevin algorithm. Twisted boundary
conditions are employed, with a twist in all four lattice directions
considerably improving the (Fourier accelerated) convergence to an improved
lattice Landau gauge. Two loop perturbation theory is seen to predict the mean
link extremely well even into the region of commonly simulated gauge couplings
and so can be used remove the need for numerical tuning of self-consistent
tadpole improvement factors. A three loop perturbative coefficient is inferred
from the simulations and is found to be small. We show that finite size effects
are small and argue likewise for (lattice) Gribov copies and double Dirac
sheets.Comment: 13 pages of revtex
Critical temperature for kaon condensation in color-flavor locked quark matter
We study the behavior of Goldstone bosons in color-flavor-locked (CFL) quark
matter at nonzero temperature. Chiral symmetry breaking in this phase of cold
and dense matter gives rise to pseudo-Goldstone bosons, the lightest of these
being the charged and neutral kaons K^+ and K^0. At zero temperature,
Bose-Einstein condensation of the kaons occurs. Since all fermions are gapped,
this kaon condensed CFL phase can, for energies below the fermionic energy gap,
be described by an effective theory for the bosonic modes. We use this
effective theory to investigate the melting of the condensate: we determine the
temperature-dependent kaon masses self-consistently using the two-particle
irreducible effective action, and we compute the transition temperature for
Bose-Einstein condensation. Our results are important for studies of transport
properties of the kaon condensed CFL phase, such as bulk viscosity.Comment: 24 pages, 8 figures, v2: new section about effect of electric
neutrality on critical temperature added; references added; version to appear
in J.Phys.
String breaking by dynamical fermions in three-dimensional lattice QCD
The first observation is made of hadronic string breaking due to dynamical
fermions in zero temperature lattice QCD. The simulations are done for SU(2)
color in three dimensions, with two flavors of staggered fermions. The results
have clear implications for the large scale simulations that are being done to
search (so far, without success) for string breaking in four-dimensional QCD.
In particular, string breaking is readily observed using only Wilson loops to
excite a static quark-antiquark pair. Improved actions on coarse lattices are
used, providing an extremely efficient means to access the quark separations
and propagation times at which string breaking occurs.Comment: Revised version to appear in Physical Review D, has additional
discussion of the results, additional references, modified title, larger
figure
Non-Abelian discrete gauge symmetries in 4d string models
We study the realization of non-Abelian discrete gauge symmetries in 4d field
theory and string theory compactifications. The underlying structure
generalizes the Abelian case, and follows from the interplay between gaugings
of non-Abelian isometries of the scalar manifold and field identifications
making axion-like fields periodic. We present several classes of string
constructions realizing non-Abelian discrete gauge symmetries. In particular,
compactifications with torsion homology classes, where non-Abelianity arises
microscopically from the Hanany-Witten effect, or compactifications with
non-Abelian discrete isometry groups, like twisted tori. We finally focus on
the more interesting case of magnetized branes in toroidal compactifications
and quotients thereof (and their heterotic and intersecting duals), in which
the non-Abelian discrete gauge symmetries imply powerful selection rules for
Yukawa couplings of charged matter fields. In particular, in MSSM-like models
they correspond to discrete flavour symmetries constraining the quark and
lepton mass matrices, as we show in specific examples.Comment: 58 pages; minor typos corrected and references adde
Metastability in Two Dimensions and the Effective Potential
We study analytically and numerically the decay of a metastable phase in
(2+1)-dimensional classical scalar field theory coupled to a heat bath, which
is equivalent to two-dimensional Euclidean quantum field theory at zero
temperature. By a numerical simulation we obtain the nucleation barrier as a
function of the parameters of the potential, and compare it to the theoretical
prediction from the bounce (critical bubble) calculation. We find the
nucleation barrier to be accurately predicted by theory using the bounce
configuration obtained from the tree-level (``classical'') effective action.
Within the range of parameters probed, we found that using the bounce derived
from the one-loop effective action requires an unnaturally large prefactor to
match the lattice results. Deviations from the tree-level prediction are seen
in the regime where loop corrections would be expected to become important.Comment: 13pp, LaTex with Postscript figs, CLNS 93/1202, DART-HEP-93/0
Static SU(3) potentials for sources in various representations
The potentials and string tensions between static sources in a variety of
representations (fundamental, 8, 6, 15-antisymmetric, 10, 27 and 15-symmetric)
have been computed by measuring Wilson loops in pure gauge SU(3). The
simulations have been done primarily on anisotropic lattices, using a tadpole
improved action improved to O(a_{s}^4). A range of lattice spacings (0.43 fm,
0.25 fm and 0.11 fm) and volumes (, , and ) has been used in an attempt to control
discretization and finite volume effects. At intermediate distances, the
results show approximate Casimir scaling. Finite lattice spacing effects
dominate systematic error, and are particularly large for the representations
with the largest string tensions.Comment: Version to appear in PR
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