6,737 research outputs found
Pairing Correlations in the Two-Dimensional Hubbard Model
We present the results of a quantum Monte Carlo study of the extended and
the pairing correlation functions for the two-dimensional Hubbard
model, computed with the constrained-path method. For small lattice sizes and
weak interactions, we find that the pairing correlations are
stronger than the extended pairing correlations and are positive when the
pair separation exceeds several lattice constants. As the system size or the
interaction strength increases, the magnitude of the long-range part of both
correlation functions vanishes.Comment: 4 pages, RevTex, 4 figures included; submitted to Phys. Rev. Let
Theory of monolayers with boundaries: Exact results and Perturbative analysis
Domains and bubbles in tilted phases of Langmuir monolayers contain a class
of textures knows as boojums. The boundaries of such domains and bubbles may
display either cusp-like features or indentations. We derive analytic
expressions for the textures within domains and surrounding bubbles, and for
the shapes of the boundaries of these regions. The derivation is perturbative
in the deviation of the bounding curve from a circle. This method is not
expected to be accurate when the boundary suffers large distortions, but it
does provide important clues with regard to the influence of various energetic
terms on the order-parameter texture and the shape of the domain or bubble
bounding curve. We also look into the effects of thermal fluctuations, which
include a sample-size-dependent effective line tension.Comment: replaced with published version, 21 pages, 16 figures include
Hot carrier diffusion in graphene
We report an optical study of charge transport in graphene. Diffusion of hot
carriers in epitaxial graphene and reduced graphene oxide samples are studied
using an ultrafast pump-probe technique with a high spatial resolution.
Spatiotemporal dynamics of hot carriers after a point-like excitation are
monitored. Carrier diffusion coefficients of 11,000 and 5,500 squared
centimeters per second are measured in epitaxial graphene and reduced graphene
oxide samples, respectively, with a carrier temperature on the order of 3,600
K. The demonstrated optical techniques can be used for non-contact and
non-invasive in-situ detection of transport properties of graphene.Comment: 5 pages, 3 figure
High-energy gamma-ray observations of the accreting black hole V404 Cygni during its June 2015 outburst
We report on Fermi/Large Area Telescope observations of the accreting black
hole low-mass X-ray binary V404 Cygni during its outburst in June-July 2015.
Detailed analyses reveal a possible excess of -ray emission on 26 June
2015, with a very soft spectrum above MeV, at a position consistent with
the direction of V404 Cyg (within the confidence region and a chance
probability of ). This emission cannot be associated with any
previously-known Fermi source. Its temporal coincidence with the brightest
radio and hard X-ray flare in the lightcurve of V404 Cyg, at the end of the
main active phase of its outburst, strengthens the association with V404 Cyg.
If the -ray emission is associated with V404 Cyg, the simultaneous
detection of keV annihilation emission by INTEGRAL requires that the
high-energy rays originate away from the corona, possibly in a
Blandford-Znajek jet. The data give support to models involving a
magnetically-arrested disk where a bright -ray jet can re-form after
the occurrence of a major transient ejection seen in the radio.Comment: 5 pages, 3 figures, accepted for publication in MNRA
A Constrained Path Quantum Monte Carlo Method for Fermion Ground States
We propose a new quantum Monte Carlo algorithm to compute fermion
ground-state properties. The ground state is projected from an initial
wavefunction by a branching random walk in an over-complete basis space of
Slater determinants. By constraining the determinants according to a trial
wavefunction , we remove the exponential decay of
signal-to-noise ratio characteristic of the sign problem. The method is
variational and is exact if is exact. We report results on the
two-dimensional Hubbard model up to size , for various electron
fillings and interaction strengths.Comment: uuencoded compressed postscript file. 5 pages with 1 figure. accepted
by PRL
A Survey of Molecular Hydrogen in the Crab Nebula
We have carried out a near-infrared, narrow-band imaging survey of the Crab
Nebula, in the H2 2.12 micron and Br-gamma 2.17 micron lines, using the Spartan
Infrared camera on the SOAR Telescope. Over a 2.8' x 5.1' area that encompasses
about 2/3 of the full visible extent of the Crab, we detect 55 knots that emit
strongly in the H2 line. We catalog the observed properties of these knots. We
show that they are in or next to the filaments that are seen in
optical-passband emission lines. Comparison to HST [S II] and [O III] images
shows that the H2 knots are strongly associated with compact regions of
low-ionization gas. We also find evidence of many additional, fainter H2
features, both discrete knots and long streamers following gas that emits
strongly in [S II]. A pixel-by-pixel analysis shows that about 6 percent of the
Crab's projected surface area has significant H2 emission that correlates with
[S II] emission. We measured radial velocities of the [S II] lambda6716
emission lines from 47 of the cataloged knots and find that most are on the far
(receding) side of the nebula. We also detect Br-gamma emission. It is right at
the limit of our survey, and our Br-gamma filter cuts off part of the expected
velocity range. But clearly the Br-gamma emission has a quite different
morphology than the H2 knots, following the long linear filaments that are seen
in H-alpha and in [O III] optical emission lines.Comment: Accepted for publication in the ApJ
Documentation of orthopaedic trauma theatre cancellations in a large teaching hospital in Scotland: Closing the loop
Characterisation of alpha-dystrobrevin in muscle
Dystrophin-related and associated proteins are important for the formation and maintenance of the mammalian neuromuscular junction. Initial studies in the electric organ of Torpedo californica showed that the dystrophin-related protein dystrobrevin (87K) co-purifies with the acetylcholine receptors and other postsynaptic proteins. Dystrobrevin is also a major phosphotyrosine-containing protein in the postsynaptic membrane. Since inhibitors of tyrosine protein phosphorylation block acetylcholine receptor clustering in cultured muscle cells, we examined the role of alpha-dystrobrevin during synapse formation and in response to agrin. Using specific antibodies, we show that C2 myoblasts and early myotubes only produce alpha-dystrobrevin-1, the mammalian orthologue of Torpedo dystrobrevin, whereas mature skeletal muscle expresses three distinct alpha-dystrobrevin isoforms. In myotubes, alpha-dystrobrevin-1 is found on the cell surface and also in acetylcholine receptor-rich domains. Following agrin stimulation, alpha-dystrobrevin-1 becomes re-localised beneath the cell surface into macroclusters that contain acetylcholine receptors and another dystrophin-related protein, utrophin. This redistribution is not associated with tyrosine phosphorylation of alpha-dystrobrevin-1 by agrin. Furthermore, we show that alpha-dystrobrevin-1 is associated with both utrophin in C2 cells and dystrophin in mature skeletal muscle. Thus alpha-dystrobrevin-1 is a component of two protein complexes in muscle, one with utrophin at the neuromuscular junction and the other with dystrophin at the sarcolemma. These results indicate that alpha-dystrobrevin-1 is not involved in the phosphorylation-dependent, early stages of receptor clustering, but rather in the stabilisation and maturation of clusters, possibly via an interaction with utrophin
Finite-Temperature Monte Carlo Calculations For Systems With Fermions
We present a quantum Monte Carlo method which allows calculations on
many-fermion systems at finite temperatures without any sign decay. This
enables simulations of the grand-canonical ensemble at large system sizes and
low temperatures. Both diagonal and off-diagonal expectations can be computed
straightforwardly. The sign decay is eliminated by a constraint on the fermion
determinant. The algorithm is approximate. Tests on the Hubbard model show that
accurate results on the energy and correlation functions can be obtained.Comment: 5 pages, RevTex; to appear in Phys. Rev. Let
- …