270 research outputs found
Liquid 4He near the superfluid transition in the presence of a heat current and gravity
The effects of a heat current and gravity in liquid 4He near the superfluid
transition are investigated for temperatures above and below T_lambda. We
present a renormalization-group calculation based on model F for the Green's
function in a self-consistent approximation which in quantum many-particle
theory is known as the Hartree approximation. The approach can handle a zero
average order parameter above and below T_lambda and includes effects of
vortices. We calculate the thermal conductivity and the specific heat for all
temperatures T and heat currents Q in the critical regime. Furthermore, we
calculate the temperature profile. Below T_lambda we find a second correlation
length which describes the dephasing of the order parameter field due to
vortices. We find dissipation and mutual friction of the superfluid-normal
fluid counterflow and calculate the Gorter-Mellink coefficient A. We compare
our theoretical results with recent experiments.Comment: 26 pages, 9 figure
Criticality and Superfluidity in liquid He-4 under Nonequilibrium Conditions
We review a striking array of recent experiments, and their theoretical
interpretations, on the superfluid transition in He in the presence of a
heat flux, . We define and evaluate a new set of critical point exponents.
The statics and dynamics of the superfluid-normal interface are discussed, with
special attention to the role of gravity. If is in the same direction as
gravity, a self-organized state can arise, in which the entire sample has a
uniform reduced temperature, on either the normal or superfluid side of the
transition. Finally, we review recent theory and experiment regarding the heat
capacity at constant . The excitement that surrounds this field arises from
the fact that advanced thermometry and the future availability of a
microgravity experimental platform aboard the International Space Station will
soon open to experimental exploration decades of reduced temperature that were
previously inaccessible.Comment: 16 pages, 9 figures, plus harvard.sty style file for references
Accepted for publication in Colloquia section of Reviews of Modern Physic
The environmental and genetic determinants of chick telomere length in Tree Swallows (Tachycineta bicolor)
Conditions during early life can have dramatic effects on adult characteristics and fitness. However, we still know little about the mechanisms that mediate these relationships. Telomere shortening is one possibility. Telomeres are long sequences of DNA that protect the ends of chromosomes. They shorten naturally throughout an individual's life, and individuals with short telomeres tend to have poorer health and reduced survival. Given this connection between telomere length (TL) and fitness, natural selection should favor individuals that are able to retain longer telomeres for a greater portion of their lives. However, the ability of natural selection to act on TL depends on the extent to which genetic and environmental factors influence TL. In this study, we experimentally enlarged broods of Tree Swallows (Tachycineta bicolor) to test the effects of demanding early-life conditions on TL, while simultaneously cross-fostering chicks to estimate heritable genetic influences on TL. In addition, we estimated the effects of parental age and chick sex on chick TL. We found that TL is highly heritable in Tree Swallow chicks, and that the maternal genetic basis for TL is stronger than is the paternal genetic basis. In contrast, the experimental manipulation of brood size had only a weak effect on chick TL, suggesting that the role of environmental factors in influencing TL early in life is limited. There was no effect of chick sex or parental age on chick TL. While these results are consistent with those reported in some studies, they are in conflict with others. These disparate conclusions might be attributable to the inherent complexity of telomere dynamics playing out differently in different populations or to study-specific variation in the age at which subjects were measured.John Weber endowment; Athena fund at the Cornell Lab of Ornithology; Department of Ecology and Evolutionary Biology; Andrew W. Mellon Student research Grants at Cornell University; Sigma Xi; Society for Integrative and comparative Biology; American Ornithologists' Union; NSF LTREB grants [DEB-0717021, DEB-1242573]Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
A First-Landau-Level Laughlin/Jain Wave Function for the Fractional Quantum Hall Effect
We show that the introduction of a more general closed-shell operator allows
one to extend Laughlin's wave function to account for the richer hierarchies
(1/3, 2/5, 3/7 ...; 1/5, 2/9, 3/13, ..., etc.) found experimentally. The
construction identifies the special hierarchy states with condensates of
correlated electron clusters. This clustering implies a single-particle (ls)j
algebra within the first Landau level (LL) identical to that of multiply filled
LLs in the integer quantum Hall effect. The end result is a simple generalized
wave function that reproduces the results of both Laughlin and Jain, without
reference to higher LLs or projection.Comment: Revtex. In this replacement we show how to generate the Jain wave
function explicitly, by acting with the generalized ls closed-shell operator
discussed in the original version. We also walk the reader through a
classical 1d caricature of this problem so that he/she can better understand
why 2s+1, where s is the spin, should be associated with the number of
electrons associated with the underlying clusters or composites. 11 page
Superfluid phase transition and strong-coupling effects in an ultracold Fermi gas with mass imbalance
We investigate the superfluid phase transition and effects of mass imbalance
in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation)
crossover regime of an cold Fermi gas. We point out that the Gaussian
fluctuation theory developed by Nozi\`eres and Schmitt-Rink and the -matrix
theory, that are now widely used to study strong-coupling physics of cold Fermi
gases, give unphysical results in the presence of mass imbalance. To overcome
this problem, we extend the -matrix theory to include higher-order pairing
fluctuations. Using this, we examine how the mass imbalance affects the
superfluid phase transition. Since the mass imbalance is an important key in
various Fermi superfluids, such as K-Li Fermi gas mixture, exciton
condensate, and color superconductivity in a dense quark matter, our results
would be useful for the study of these recently developing superfluid systems.Comment: 7 pages, 4 figures, Proceedings of QFS-201
Optically Pumped NMR Measurements of the Electron Spin Polarization in GaAs Quantum Wells near Landau Level Filling Factor nu=1/3
The Knight shift of Ga-71 nuclei is measured in two different electron-doped
multiple quantum well samples using optically pumped NMR. These data are the
first direct measurements of the electron spin polarization,
P(nu,T)=/max, near nu=1/3. The P(T) data at nu=1/3 probe the
neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In
addition, the saturated P(nu) drops on either side of nu=1/3, even in a Btot=12
Tesla field. The observed depolarization is quite small, consistent with an
average of about 0.1 spin-flips per quasihole (or quasiparticle), a value which
does not appear to be explicable by the current theoretical understanding of
the FQHE near nu=1/3.Comment: 4 pages (REVTEX), 5 eps figures embedded in text; minor changes,
published versio
Stability of condensate in superconductors
According to the BCS theory the superconducting condensate develops in a
single quantum mode and no Cooper pairs out of the condensate are assumed. Here
we discuss a mechanism by which the successful mode inhibits condensation in
neighboring modes and suppresses a creation of noncondensed Cooper pairs. It is
shown that condensed and noncondensed Cooper pairs are separated by an energy
gap which is smaller than the superconducting gap but large enough to prevent
nucleation in all other modes and to eliminate effects of noncondensed Cooper
pairs on properties of superconductors. Our result thus justifies basic
assumptions of the BCS theory and confirms that the BCS condensate is stable
with respect to two-particle excitations
Hidden symmetry and knot solitons in a charged two-condensate Bose system
We show that a charged two-condensate Ginzburg-Landau model or equivalently a
Gross-Pitaevskii functional for two charged Bose condensates, can be mapped
onto a version of the nonlinear O(3) -model. This implies in particular
that such a system possesses a hidden O(3) symmetry and allows for the
formation of stable knotted solitons. The results, in particular, should be
relevant to the superconducting MgB_2.Comment: This version will appear in Phys. Rev. B, added a comment on the case
when condensates in two bands do not independently conserve, also added a
figure and references to experimental papers on MgB_2 (for which our study is
relevant). Miscellaneous links on knot solitons are also available at the
homepage of one of the authors http://www.teorfys.uu.se/PEOPLE/egor/ .
Animations of knot solitons are available at
http://users.utu.fi/h/hietarin/knots/c45_p2.mp
Ginzburg-Landau theory of superconductors with short coherence length
We consider Fermions in two dimensions with an attractive interaction in the
singlet d-wave channel of arbitrary strength. By means of a
Hubbard-Stratonovich transformation a statistical Ginzburg-Landau theory is
derived, which describes the smooth crossover from a weak-coupling BCS
superconductor to a condensate of composite Bosons. Adjusting the interaction
strength to the observed slope of H_c2 at T_c in the optimally doped high-T_c
compounds YBCO and BSCCO, we determine the associated values of the
Ginzburg-Landau correlation length xi and the London penetration depth lambda.
The resulting dimensionless ratio k_F xi(0) approx 5-8 and the Ginzburg-Landau
parameter kappa=lambda xi approx 90-100 agree well with the experimentally
observed values. These parameters indicate that the optimally doped materials
are still on the weak coupling side of the crossover to a Bose regime.Comment: 12 pages, RevTeX, 6 postscript figures, resubmitted with minor
changes in section III, to appear in Physical Review
Effective action for Superconductors and BCS-Bose crossover
A standard perturbative expansion around the mean-field solution is used to
derive the low-energy effective action for superconductors at T=0. Taking into
account the density fluctuations at the outset we get the effective action
where the density is the conjugated momentum to the phase of
the order parameter. In the hydrodynamic regime, the dynamics of the
superconductor is described by a time dependent non-linear Schr\"odinger
equation (TDNLS) for the field . The
evolution of the density fluctuations in the crossover from weak-coupling (BCS)
to strong-coupling (Bose condensation of localized pairs) superconductivity is
discussed for the attractive Hubbard model. In the bosonic limit, the TDNLS
equation reduces to the the Gross-Pitaevskii equation for the order parameter,
as in the standard description of superfluidity. The conditions under which a
phase-only action can be derived in the presence of a long-range interaction to
describe the physics of the superconductivity of ``bad metals'' are discussed.Comment: 13 pages, accepted for publication on Phys. Rev.
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