1,573 research outputs found
Compact QED under scrutiny: it's first order
We report new results from our finite size scaling analysis of 4d compact
pure U(1) gauge theory with Wilson action. Investigating several cumulants of
the plaquette energy within the Borgs-Kotecky finite size scaling scheme we
find strong evidence for a first-order phase transition and present a high
precision value for the critical coupling in the thermodynamic limit.Comment: Lattice2002(Spin
A High Precision Study of the QQ(bar) Potential from Wilson Loops in the Regime of String Breaking
For lattice QCD with two sea quark flavours we compute the static quark
antiquark potential V(R) in the regime where string breaking is expected. In
order to increase statistics, we make full use of the lattice information by
including all lattice vectors R to any possible lattice separation in the
infrared regime. The corresponding paths between the lattice points are
constructed by means of a generalized Bresenham algorithm as known from
computer graphics. As a results we achieve a determination of the unquenched
potential in the range .8 to 1.5 fm with hitherto unknown precision.
Furthermore, we demonstrate some error reducing methods for the evaluation of
the transition matrix element between two- and four-quark states.Comment: 6 pages, 7 figure
Decorrelating Topology with HMC
The investigation of the decorrelation efficiency of the HMC algorithm with
respect to vacuum topology is a prerequisite for trustworthy full QCD
simulations, in particular for the computation of topology sensitive
quantities. We demonstrate that for mpi/mrho ratios <= 0.69 sufficient
tunneling between the topological sectors can be achieved, for two flavours of
dynamical Wilson fermions close to the scaling region beta=5.6. Our results are
based on time series of length 5000 trajectories.Comment: change of comments: LATTICE98(confine
Scanning the Topological Sectors of the QCD Vacuum with Hybrid Monte Carlo
We address a long standing issue and determine the decorrelation efficiency
of the Hybrid Monte Carlo algorithm (HMC), for full QCD with Wilson fermions,
with respect to vacuum topology. On the basis of five state-of-the art QCD
vacuum field ensembles (with 3000 to 5000 trajectories each and
m_pi/m_rho-ratios in the regime >0.56, for two sea quark flavours) we are able
to establish, for the first time, that HMC provides sufficient tunneling
between the different topological sectors of QCD. This will have an important
bearing on the prospect to determine, by lattice techniques, the topological
susceptibility of the vacuum, and topology sensitive quantities like the spin
content of the proton, or the eta' mass.Comment: 5 pages, 4 eps-figure
Flavour singlet pseudoscalar masses in N_f = 2 QCD
We perform a lattice mass analysis in the flavour singlet pseudoscalar
channel on the SESAM and TXL full QCD vacuum configurations, with 2 active
flavours of dynamical Wilson fermions at beta = 5.6. At our inverse lattice
spacing, a^-1 = 2.3 GeV, we retrieve by a chiral extrapolation to the physical
light quark masses the value m_eta' = 3.7(+8)(-4) m_pi. A crude extrapolation
from (N_f = 3) phenomenology would suggest m_eta' \approx 5.1 m_pi for N_f = 2
QCD. we verify that the mass gap between the singlet state eta' and the pi
flavour triplt state is due to gauge configurations with non-trivial topology.Comment: 8 pages, 10 figure
Multicanonical Hybrid Monte Carlo: Boosting Simulations of Compact QED
We demonstrate that substantial progress can be achieved in the study of the
phase structure of 4-dimensional compact QED by a joint use of hybrid Monte
Carlo and multicanonical algorithms, through an efficient parallel
implementation. This is borne out by the observation of considerable speedup of
tunnelling between the metastable states, close to the phase transition, on the
Wilson line. We estimate that the creation of adequate samples (with order 100
flip-flops) becomes a matter of half a year's runtime at 2 Gflops sustained
performance for lattices of size up to 24^4.Comment: 15 pages, 8 figure
Characterisation of the effectiveness of carbon incorporation in SiGe for the elimination of parasitic energy barriers in SiGe HBT's
An electrical method is applied to SiGe and SiGeC HBTs to extract the bandgap narrowing in the base layer and to characterise the presence of parasitic energy barriers in the conduction band, arising from transient enhanced boron diffusion from the SiGe layer. It is shown that a background carbon concentration with the base of approximately 1E20cm-3 eliminates parasitic energy barriers at the C/B junction and hence shows that transient enhanced diffusion of boron from the base has been completely suppressed
Precursors, black holes, and a locality bound
We revisit the problem of precursors in the AdS/CFT correspondence.
Identification of the precursors is expected to improve our understanding of
the tension between holography and bulk locality and of the resolution of the
black hole information paradox. Previous arguments that the precursors are
large, undecorated Wilson loops are found to be flawed. We argue that the role
of precursors should become evident when one saturates a certain locality
bound. The spacetime uncertainty principle is a direct consequence of this
bound.Comment: 26 pages, 8 figs; reference added, minor clarification in sec. 2;
incorrect draft mistakenly used in version
Massive Parallel Quantum Computer Simulator
We describe portable software to simulate universal quantum computers on
massive parallel computers. We illustrate the use of the simulation software by
running various quantum algorithms on different computer architectures, such as
a IBM BlueGene/L, a IBM Regatta p690+, a Hitachi SR11000/J1, a Cray X1E, a SGI
Altix 3700 and clusters of PCs running Windows XP. We study the performance of
the software by simulating quantum computers containing up to 36 qubits, using
up to 4096 processors and up to 1 TB of memory. Our results demonstrate that
the simulator exhibits nearly ideal scaling as a function of the number of
processors and suggest that the simulation software described in this paper may
also serve as benchmark for testing high-end parallel computers.Comment: To appear in Comp. Phys. Com
Preconditioning of Improved and ``Perfect'' Fermion Actions
We construct a locally-lexicographic SSOR preconditioner to accelerate the
parallel iterative solution of linear systems of equations for two improved
discretizations of lattice fermions: the Sheikholeslami-Wohlert scheme where a
non-constant block-diagonal term is added to the Wilson fermion matrix and
renormalization group improved actions which incorporate couplings beyond
nearest neighbors of the lattice fermion fields. In case (i) we find the block
llssor-scheme to be more effective by a factor about 2 than odd-even
preconditioned solvers in terms of convergence rates, at beta=6.0. For type
(ii) actions, we show that our preconditioner accelerates the iterative
solution of a linear system of hypercube fermions by a factor of 3 to 4.Comment: 27 pages, Latex, 17 Figures include
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