1,018 research outputs found
Improving the low-lying spectrum of the overlap kernel
The action of the overlap-Dirac operator on a vector is typically implemented
in directly through a multi-shift conjugate gradient solver. The compute-time
this takes to evaluate depends upon the condition number of the matrix
that is used as the overlap kernel. We examine the low-lying spectra of various
candidate kernels in an effort to optimise , thereby speeding up the
overlap evaluation.Comment: 5 pages, 8 figure
FLIC-Overlap Fermions and Topology
APE smearing the links in the irrelevant operators of clover fermions
(Fat-Link Irrelevant Clover (FLIC) fermions) provides significant improvement
in the condition number of the Hermitian-Dirac operator and gives rise to a
factor of two savings in computing the overlap operator. This report
investigates the effects of using a highly-improved definition of the lattice
field-strength tensor F_mu_nu in the fermion action, made possible through the
use of APE-smeared fat links in the construction of the irrelevant operators.
Spurious double-zero crossings in the spectral flow of the Hermitian-Wilson
Dirac operator associated with lattice artifacts at the scale of the lattice
spacing are removed with FLIC fermions composed with an O(a^4)-improved lattice
field strength tensor. Hence, FLIC-Overlap fermions provide an additional
benefit to the overlap formalism: a correct realization of topology in the
fermion sector on the lattice.Comment: Lattice2002(chiral
Towards the Continuum Limit of the Overlap Quark Propagator in Landau Gauge
The properties of the momentum space quark propagator in Landau gauge are
examined for the overlap quark action in quenched lattice QCD. Numerical
calculations were done on two lattices with different lattice spacing and
similar physical volumes to explore the quark propagator in the continuum
limit. We have calculated the nonperturbative wavefunction renormalization
function and the nonperturbative mass function for a variety of
bare quark masses and perform a simple linear extrapolation to the chiral
limit. We find the behaviour of and in the chiral limit are in
good agreement between the two lattices.Comment: 3 pages, 2 figures, talk, Lattice2002(Chiral Fermion
Boson induced s-wave pairing in dilute boson-fermion mixtures
We show that in dilute boson-fermion mixtures with fermions in two internal
states, even when the bare fermion-fermion interaction is repulsive, the
exchange of density fluctuations of the Bose condensate may lead to an
effective fermion-fermion attraction, and thus to a Cooper instability in the
s-wave channel. We give an analytical method to derive the associated in
the limit where the phonon branch of the Bogoliubov excitation spectrum of the
bosons is important. We find a of the same order as for a pure Fermi gas
with bare attraction.Comment: 12 pages, no figure
Light Quark Simulations With FLIC Fermions
Hadron masses are calculated in quenched lattice QCD in order to probe the
scaling behavior of a novel fat-link clover fermion action in which only the
irrelevant operators of the fermion action are constructed using APE-smeared
links. Light quark masses corresponding to an m_pi / m_rho ratio of 0.35 are
considered to assess the exceptional configuration problem of clover-fermion
actions. This Fat-Link Irrelevant Clover (FLIC) fermion action provides scaling
which is superior to mean-field improvement and offers advantages over
nonperturbative improvement, including reduced exceptional configurations.Comment: 3 pages, 2 figures, Lattice2002(QCD Spectrum and Quark Masses
Quantum thermodynamic fluctuations of a chaotic Fermi-gas model
We investigate the thermodynamics of a Fermi gas whose single-particle energy
levels are given by the complex zeros of the Riemann zeta function. This is a
model for a gas, and in particular for an atomic nucleus, with an underlying
fully chaotic classical dynamics. The probability distributions of the quantum
fluctuations of the grand potential and entropy of the gas are computed as a
function of temperature and compared, with good agreement, with general
predictions obtained from random matrix theory and periodic orbit theory (based
on prime numbers). In each case the universal and non--universal regimes are
identified.Comment: 23 pages, 4 figures, 1 tabl
Novel fat-link fermion actions
The hadron mass spectrum is calculated in lattice QCD using a novel fat-link
clover fermion action in which only the irrelevant operators of the fermion
action are constructed using smeared links. The simulations are performed on a
16^3 X 32 lattice with a lattice spacing of a=0.125 fm. We compare actions with
n=4 and 12 smearing sweeps with a smearing fraction of 0.7. The n=4 Fat Link
Irrelevant Clover (FLIC) action provides scaling which is superior to
mean-field improvement, and offers advantages over nonperturbative O(a)
improvement.Comment: 5 pages, 2 figures, talk given by J.Zanotti at LHP 2001 workshop,
Cairns, Australi
Thermodynamics of small Fermi systems: quantum statistical fluctuations
We investigate the probability distribution of the quantum fluctuations of
thermodynamic functions of finite, ballistic, phase-coherent Fermi gases.
Depending on the chaotic or integrable nature of the underlying classical
dynamics, on the thermodynamic function considered, and on temperature, we find
that the probability distributions are dominated either (i) by the local
fluctuations of the single-particle spectrum on the scale of the mean level
spacing, or (ii) by the long-range modulations of that spectrum produced by the
short periodic orbits. In case (i) the probability distributions are computed
using the appropriate local universality class, uncorrelated levels for
integrable systems and random matrix theory for chaotic ones. In case (ii) all
the moments of the distributions can be explicitly computed in terms of
periodic orbit theory, and are system-dependent, non-universal, functions. The
dependence on temperature and number of particles of the fluctuations is
explicitly computed in all cases, and the different relevant energy scales are
displayed.Comment: 24 pages, 7 figures, 5 table
Effects of inhomogeneities on apparent cosmological observables: "fake" evolving dark energy
Using the exact Lemaitre-Bondi-Tolman solution with a non-vanishing
cosmological constant , we investigate how the presence of a local
spherically-symmetric inhomogeneity can affect apparent cosmological
observables, such as the deceleration parameter or the effective equation of
state of dark energy (DE), derived from the luminosity distance under the
assumption that the real space-time is exactly homogeneous and isotropic. The
presence of a local underdensity is found to produce apparent phantom behavior
of DE, while a locally overdense region leads to apparent quintessence
behavior. We consider relatively small large scale inhomogeneities which today
are not linear and could be seeded by primordial curvature perturbations
compatible with CMB bounds. Our study shows how observations in an
inhomogeneous CDM universe with initial conditions compatible with the
inflationary beginning, if interpreted under the wrong assumption of
homogeneity, can lead to the wrong conclusion about the presence of "fake"
evolving dark energy instead of .Comment: 22 pages, 19 figures,Final version to appear in European Physical
Journal
Sedimentary ancient DNA from Lake Skartjorna, Svalbard: assessing the resilience of arctic flora to Holocene climate change
Reconstructing past vegetation and species diversity from arctic lake sediments can be challenging because of low pollen and plant macrofossil concentrations. Information may be enhanced by metabarcoding of sedimentary ancient DNA (sedaDNA). We developed a Holocene record from Lake Skartjørna, Svalbard, using sedaDNA, plant macrofossils and sediment properties, and compared it with published records. All but two genera of vascular plants identified as macrofossils in this or a previous study were identified with sedaDNA. Six additional vascular taxa were found, plus two algal and 12 bryophyte taxa, by sedaDNA analysis, which also detected more species per sample than macrofossil analysis. A shift from Salix polaris-dominated vegetation, with Koenigia islandica, Ranunculaceae and the relatively thermophilic species Arabis alpina and Betula, to Dryas octopetala-dominated vegetation ~6600–5500 cal. BP suggests a transition from moist conditions 1–2°C warmer than today to colder/drier conditions. This coincides with a decrease in runoff, inferred from core lithology, and an independent record of declining lacustrine productivity. This mid-Holocene change in terrestrial vegetation is broadly coincident with changes in records from marine sediments off the west coast of Svalbard. Over the Holocene sedaDNA records little floristic change, and it clearly shows species persisted near the lake during time intervals when they are not detected as macrofossils. The flora has shown resilience in the presence of a changing climate, and, if future warming is limited to 2°C or less, we might expect only minor floristic changes in this region. However, the Holocene record provides no analogues for greater warming
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