498 research outputs found
Superconductivity and antiferromagnetism in the two-dimensional Hubbard model: a variational study
A variational ground state of the repulsive Hubbard model on a square lattice
is investigated numerically for an intermediate coupling strength (U = 8t) and
for moderate sizes (from 6 x 6 to 10 x 10). Our ansatz is clearly superior to
other widely used variational wave functions. The results for order parameters
and correlation functions provide new insight for the antiferromagnetic state
at half filling as well as strong evidence for a superconducting phase away
from half filling.Comment: 4 pages, 4 figure
Variational ground states of the two-dimensional Hubbard model
Recent refinements of analytical and numerical methods have improved our
understanding of the ground-state phase diagram of the two-dimensional (2D)
Hubbard model. Here we focus on variational approaches, but comparisons with
both Quantum Cluster and Gaussian Monte Carlo methods are also made. Our own
ansatz leads to an antiferromagnetic ground state at half filling with a
slightly reduced staggered order parameter (as compared to simple mean-field
theory). Away from half filling, we find d-wave superconductivity, but confined
to densities where the Fermi surface passes through the antiferromagnetic zone
boundary (if hopping between both nearest-neighbour and next-nearest-neighbour
sites is considered). Our results agree surprisingly well with recent numerical
studies using the Quantum Cluster method. An interesting trend is found by
comparing gap parameters (antiferromagnetic or superconducting) obtained with
different variational wave functions. They vary by an order of magnitude and
thus cannot be taken as a characteristic energy scale. In contrast, the order
parameter is much less sensitive to the degree of sophistication of the
variational schemes, at least at and near half filling.Comment: 18 pages, 4 figures, to be published in New J. Phy
Diluted Josephson-junction arrays in a magnetic field: phase coherence and vortex glass thresholds
The effects of random dilution of junctions on a two-dimensional
Josephson-junction array in a magnetic field are considered. For rational
values of the average flux quantum per plaquette , the superconducting
transition temperature vanishes, for increasing dilution, at a critical value
, while the vortex ordering remains stable up to , much
below the value corresponding to the geometric percolation threshold. For
, the array behaves as a zero-temperature vortex-glass.
Numerical results for from defect energy calculations are presented
which are consistent with this scenario.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Dynamic transition in vortex flow in strongly disordered Josephson junction arrays and superconducting thin films
We study the dynamics of vortices in strongly disordered Josephson
junction arrays and superconducting films driven by a current. We find a
dynamic phase transition in vortex flow at a current . Below
there is plastic flow characterized by an average-velocity correlation length
scale in the direction of motion, which diverges when approaching
. Above we find a moving vortex phase with homogeneous flow and
short range smectic order. A finite size analysis shows that this phase becomes
asymptotically a liquid for large length scales.Comment: 4 pages, 4 figure
Current-voltage characteristics of diluted Josephson-junction arrays: scaling behavior at current and percolation threshold
Dynamical simulations and scaling arguments are used to study the
current-voltage (IV) characteristics of a two-dimensional model of resistively
shunted Josephson-junction arrays in presence of percolative disorder, at zero
external field. Two different limits of the Josephson-coupling concentration
are considered, where is the percolation threshold. For
and zero temperature, the IV curves show power-law behavior above a disorder
dependent critical current. The power-law behavior and critical exponents are
consistent with a simple scaling analysis. At and finite temperature ,
the results show the scaling behavior of a T=0 superconducting transition. The
resistance is linear but vanishes for decreasing with an apparent
exponential behavior. Crossover to non-linearity appears at currents
proportional to , with a thermal-correlation length exponent
consistent with the corresponding value for the diluted XY model at
.Comment: Revtex, 9 postscript pages, to appear in Phys. Rev.
Quantum Metrology Triangle Experiments: A Status Review
Quantum Metrology Triangle experiments combine three quantum electrical
effects (the Josephson effect, the quantum Hall effect and the single-electron
transport effect) used in metrology. These experiments allow important
fundamental consistency tests on the validity of commonly assumed relations
between fundamental constants of nature and the quantum electrical effects.
This paper reviews the history, results and the present status and perspectives
of Quantum Metrology Triangle experiments. It also reflects on the possible
implications of results for the knowledge on fundamental constants and the
quantum electrical effects.Comment: 36 pages, 8 figure
Evaluation of SLC11A1 as an inflammatory bowel disease candidate gene
BACKGROUND: Significant evidence suggests that a promoter polymorphism withinthe gene SLC11A1 is involved in susceptibility to both autoimmune and infectious disorders. The aim of this study was to evaluate whether SLC11A1 has a role in the susceptibility to inflammatory bowel disease (IBD) by characterizing a promoter polymorphism within the gene and two short tandem repeat (STR) markers in genetic proximity to SLC11A1. METHODS: The studied population consisted of 484 Caucasians with IBD, 144 population controls, and 348 non-IBD-affected first-degree relatives of IBD patients. IBD subjects were re-categorized at the sub-disease phenotypic level to characterize possible SLC11A1 genotype-phenotype correlations. Polymorphic markers were amplified from germline DNA and typed using gel electrophoresis. Genotype-phenotype correlations were defined using case-control, haplotype, and family-based association studies. RESULTS: This study did not provide compelling evidence for SLC11A1 disease association; most significantly, there was no apparent evidence of SLC11A1 promoter allele association in the studied Crohn's disease population. CONCLUSION: Our results therefore refute previous studies that have shown SLC11A1 promoter polymorphisms are involved in susceptibility to this form of IBD
Decatransin, a novel natural product inhibiting protein translocation at the Sec61/SecY translocon
A new cyclic decadepsipeptide was isolated from Chaetosphaeria tulasneorum with potent bioactivity on mammalian and yeast cells. Chemogenomic profiling in S. cerevisiae indicated that the Sec61 translocon, the machinery for protein translocation and membrane insertion at the endoplasmic reticulum, is the target. The profiles were similar to those of cyclic heptadepsipeptides of a distinct chemotype (HUN-7293/cotransin) that had previously been shown to inhibit cotranslational translocation at the mammalian Sec61 translocon. Unbiased, genome-wide mutagenesis followed by full-genome sequencing in both fungal and mammalian cells identified dominant mutations in Sec61p/Sec61α1 to confer resistance. Most, but not all, of these mutations affected inhibition by both chemotypes, despite an absence of structural similarity. Biochemical analysis confirmed inhibition of protein translocation into the endoplasmic reticulum of both co- and posttranslationally translocated substrates by both chemotypes, demonstrating a mechanism independent of a translating ribosome. Most interestingly, both chemotypes were found to also inhibit SecYEG, the bacterial Sec61 homolog. We suggest "decatransin" as the name for this novel decadepsipeptide translocation inhibitor
Observation of Radiative Leptonic Decay of the Tau Lepton
Using 4.68 fb^{-1} of e^+e^- annihilation data collected with the CLEO II
detector at the Cornell Electron Storage Ring (CESR) we have studied tau
radiative decays tau -> mu nu nu gamma and tau -> e nu nu gamma. For a 10 MeV
minimum photon energy in the tau rest frame, the branching fraction of
radiative tau decay to a muon or electron is measured to be
(3.61+-0.16+-0.35)*10^{-3} or (1.75+-0.06+-0.17)*10^{-2}, respectively. The
branching fractions are in agreement with the Standard Model theoretical
predictions.Comment: 11 pages postscript, also available through
http://w4.lns.cornell.edu/public/CLN
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