17,738 research outputs found
Lattice study of trapped fermions at unitarity
We present a lattice study of up to N=20 unitary fermions confined to a
harmonic trap. Our preliminary results show better than 1% agreement with high
precision solutions to the many-body Schrodinger equation for up to N=6. We are
able to make predictions for larger N which were inaccessible by the
Hamiltonian approach due to computational limitations. Harmonic traps are used
experimentally to study cold atoms tuned to a Feshbach resonance. We show that
they also provide certain benefits to numerical studies of many-body
correlators on the lattice. In particular, we anticipate that the methods
described here could be used for studying nuclear physics.Comment: 7 pages, 5 figures, presented at the XXVIII International Symposium
on Lattice Field Theory (Lattice 2010), Villasimius, Italy, June 14-19 201
Lattice calculation for unitary fermions in a finite box
A fundamental constant in systems of unitary fermions is the so-called
Bertsch parameter, the ratio of the ground state energy for spin paired unitary
fermions to that for free fermions at the same density. I discuss how we
computed this parameter as well as the pairing gap using a recently developed
lattice construction for unitary fermions, by measuring correlation functions
for up to 38 fermions in a finite box. Our calculation illustrates interesting
issues facing the study of many-body states on the lattice, which may
eventually be confronted in QCD calculations as well.Comment: 7 pages, 6 figures, The XXVIII International Symposium on Lattice
Field Theory, Lattice2010, June 14-19, 2010, Villasimius, Ital
Sign problems, noise, and chiral symmetry breaking in a QCD-like theory
The Nambu-Jona-Lasinio model reduced to 2+1 dimensions has two different path
integral formulations: at finite chemical potential one formulation has a
severe sign problem similar to that found in QCD, while the other does not. At
large N, where N is the number of flavors, one can compute the probability
distributions of fermion correlators analytically in both formulations. In the
former case one finds a broad distribution with small mean; in the latter one
finds a heavy tailed positive distribution amenable to the cumulant expansion
techniques developed in earlier work. We speculate on the implications of this
model for QCD.Comment: 16 pages, 5 figures; Published version with minor changes from the
origina
Lattice Monte Carlo calculations for unitary fermions in a finite box
We perform lattice Monte Carlo simulations for up to 66 unitary fermions in a
finite box using a highly improved lattice action for nonrelativistic spin 1/2
fermions. We obtain a value of for the Bertsch
parameter, defined as the energy of the unitary Fermi gas measured in units of
the free gas energy in the thermodynamic limit. In addition, for up to four
unitary fermions, we compute the spectrum of the lattice theory by exact
diagonalization of the transfer matrix projected onto irreducible
representations of the octahedral group for small to moderate size lattices,
providing an independent check of our few-body simulation results. We compare
our exact numerical and simulation results for the spectrum to benchmark
studies of other research groups, as well as perform an extended analysis of
our lattice action improvement scheme, including an analysis of the errors
associated with higher partial waves and finite temporal discretization.Comment: Significant revisions from previous version. Included data at a
larger volume and performed an infinite volume extrapolation of the Bertsch
parameter. Published versio
Rubidium and lead abundances in giant stars of the globular clusters M4 and M5
We present measurements of the neutron-capture elements Rb and Pb for bright
giants in the globular clusters M4 and M5. The clusters are of similar
metallicity ([Fe/H] = -1.2) but M4 is decidedly s-process enriched relative to
M5: [Ba/Fe] = +0.6 for M4 but 0.0 for M5. The Rb and Pb abundances were derived
by comparing synthetic spectra with high-resolution, high signal-to-noise ratio
spectra obtained with MIKE on the Magellan telescope. Abundances of Y, Zr, La,
and Eu were also obtained. In M4, the mean abundances from 12 giants are
[Rb/Fe] = 0.39 +/- 0.02 (sigma = 0.07), [Rb/Zr] = 0.17 +/- 0.03 (sigma = 0.08),
and [Pb/Fe] = 0.30 +/- 0.02 (sigma = 0.07). In M5, the mean abundances from two
giants are [Rb/Fe] = 0.00 +/- 0.05 (sigma = 0.06), [Rb/Zr] = 0.08 +/- 0.08
(sigma = 0.11), and [Pb/Fe] = -0.35 +/- 0.02 (sigma = 0.04). Within the
measurement uncertainties, the abundance ratios [Rb/Fe], [Pb/Fe] and [Rb/X] for
X = Y, Zr, La are constant from star-to-star in each cluster and none of these
ratios are correlated with O or Na abundances. While M4 has a higher Rb
abundance than M5, the ratios [Rb/X] are similar in both clusters indicating
that the nature of the s-products are very similar for each cluster but the gas
from which M4's stars formed had a higher concentration of these products.Comment: Accepted for publication in Ap
Lattice Monte Carlo calculations for unitary fermions in a harmonic trap
We present a new lattice Monte Carlo approach developed for studying large
numbers of strongly interacting nonrelativistic fermions, and apply it to a
dilute gas of unitary fermions confined to a harmonic trap. Our lattice action
is highly improved, with sources of discretization and finite volume errors
systematically removed; we are able to demonstrate the expected volume scaling
of energy levels of two and three untrapped fermions, and to reproduce the high
precision calculations published previously for the ground state energies for N
= 3 unitary fermions in a box (to within our 0.3% uncertainty), and for N = 3,
. . ., 6 unitary fermions in a harmonic trap (to within our ~ 1% uncertainty).
We use this action to determine the ground state energies of up to 70
unpolarized fermions trapped in a harmonic potential on a lattice as large as
64^3 x 72; our approach avoids the use of importance sampling or calculation of
a fermion determinant and employs a novel statistical method for estimating
observables, allowing us to generate ensembles as large as 10^8 while requiring
only relatively modest computational resources.Comment: 48 pages, 19 figures, published version, some text revised, typos
corrected, title changed in journal; previously "Unitary fermions on the
lattice I: in a harmonic trap
Extended study for unitary fermions on a lattice using the cumulant expansion technique
A recently developed lattice method for large numbers of strongly interacting
nonrelativistic fermions exhibits a heavy tail in the distributions of
correlators for large Euclidean time {\tau} and large number of fermions N,
which only allows the measurement of ground state energies for a limited number
of fermions using standard techniques. In such cases, it is suggested that
measuring the log of the correlator is more efficient, and a cumulant expansion
of this quantity can be exactly related to the correlation function. The
cumulant expansion technique allows us to determine the ground state energies
of up to 66 unpolarized unitary fermions on lattices as large as
7214^3, and up to 70 unpolarized unitary fermions trapped in a harmonic
potential on lattices as large as 7264^3. We have also improved our
lattice action with a Galilean invariant form for the four-fermion interaction,
which results in predictive volume scaling of the lowest energy of three
fermions in a periodic box and in good agreement of our results for N \leq 6
trapped unitary fermions with those from other benchmark calculations.Comment: 7 pages, 6 figures, Presented at 29th International Symposium on
Lattice Field Theory (Lattice2011), Squaw Valley, Lake Tahoe, CA, USA, 10-16
July 201
Spiky Strings on I-brane
We study rigidly rotating strings in the near horizon geometry of the 1+1
dimensional intersection of two orthogonal stacks of NS5-branes, the so called
I-brane background. We solve the equations of motion of the fundamental string
action in the presence of two form NS-NS fluxes that the I-brane background
supports and write down general form of conserved quantities. We further find
out two limiting cases corresponding to giant magnon and single spike like
strings in various parameter space of solutions.Comment: 17 pages, major restructuring of text, added a referenc
Deposition of platinum clusters on surface-modified tobacco mosaic virus
Nanoscaled Pt conductors were prepared from genetically engineered Tobacco mosaic virus (TMV) templates through Pt cluster deposition on the outer surface of the TMV. Pt clusters were synthesized and deposited on the engineered TMV with surface-exposed cysteine via the in situ mineralization of hexachloroplatinate anions. This deposition was driven by the specific binding between thiols and the solid metal clusters. In addition, Pt-thiolate adducts are suggested to form on the engineered TMV in aqueous solutions that work as nucleation sites for the formation of the Pt clusters. The specific binding between Pt clusters and the engineered TMV template was investigated using UV/vis spectrophotometry and quartz crystal microbalance (QCM) analysis. The electric conductance of Pt-deposited TMV was greater than that of the uncoated TMV virion particles. This result suggests the application of metal cluster-deposited engineered TMV in future electrical devices such as rapid response sensors
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