18,730 research outputs found
Pairing without Superfluidity: The Ground State of an Imbalanced Fermi Mixture
Radio-frequency spectroscopy is used to study pairing in the normal and
superfluid phases of a strongly interacting Fermi gas with imbalanced spin
populations. At high spin imbalances the system does not become superfluid even
at zero temperature. In this normal phase full pairing of the minority atoms is
observed. This demonstrates that mismatched Fermi surfaces do not prevent
pairing but can quench the superfluid state, thus realizing a system of fermion
pairs that do not condense even at the lowest temperature
Fermionic Superfluidity with Imbalanced Spin Populations and the Quantum Phase Transition to the Normal State
Whether it occurs in superconductors, helium-3 or inside a neutron star,
fermionic superfluidity requires pairing of fermions, particles with
half-integer spin. For an equal mixture of two states of fermions ("spin up"
and "spin down"), pairing can be complete and the entire system will become
superfluid. When the two populations of fermions are unequal, not every
particle can find a partner. Will the system nevertheless stay superfluid? Here
we study this intriguing question in an unequal mixture of strongly interacting
ultracold fermionic atoms. The superfluid region vs population imbalance is
mapped out by employing two complementary indicators: The presence or absence
of vortices in a rotating mixture, as well as the fraction of condensed fermion
pairs in the gas. Due to the strong interactions near a Feshbach resonance, the
superfluid state is remarkably stable in response to population imbalance. The
final breakdown of superfluidity marks a new quantum phase transition, the
Pauli limit of superfluidity.Comment: 15 pages, 5 figure
Spin relaxation in mesoscopic superconducting Al wires
We studied the diffusion and the relaxation of the polarized quasiparticle
spins in superconductors. To that end, quasiparticles of polarized spins were
injected through an interface of a mesoscopic superconducting Al wire in
proximity contact with an overlaid ferromagnetic Co wire in the single-domain
state. The superconductivity was observed to be suppressed near the
spin-injecting interface, as evidenced by the occurrence of a finite voltage
for a bias current below the onset of the superconducting transition. The spin
diffusion length, estimated from finite voltages over a certain length of Al
wire near the interface, was almost temperature independent in the temperature
range sufficiently below the superconducting transition but grew as the
transition temperature was approached. This temperature dependence suggests
that the relaxation of the spin polarization in the superconducting state is
governed by the condensation of quasiparticles to the paired state. The spin
relaxation in the superconducting state turned out to be more effective than in
the normal state.Comment: 9 pages, 8 figure
Tomographic RF Spectroscopy of a Trapped Fermi Gas at Unitarity
We present spatially resolved radio-frequency spectroscopy of a trapped Fermi
gas with resonant interactions and observe a spectral gap at low temperatures.
The spatial distribution of the spectral response of the trapped gas is
obtained using in situ phase-contrast imaging and 3D image reconstruction. At
the lowest temperature, the homogeneous rf spectrum shows an asymmetric
excitation line shape with a peak at 0.48(4) with respect to the
free atomic line, where is the local Fermi energy
Optical Weak Link between Two Spatially Separate Bose-Einstein Condensates
Two spatially separate Bose-Einstein condensates were prepared in an optical
double-well potential. A bidirectional coupling between the two condensates was
established by two pairs of Bragg beams which continuously outcoupled atoms in
opposite directions. The atomic currents induced by the optical coupling depend
on the relative phase of the two condensates and on an additional controllable
coupling phase. This was observed through symmetric and antisymmetric
correlations between the two outcoupled atom fluxes. A Josephson optical
coupling of two condensates in a ring geometry is proposed. The continuous
outcoupling method was used to monitor slow relative motions of two elongated
condensates and characterize the trapping potential.Comment: 4 pages, 5 figure
Nonlinear Response of Cylindrical Shells to Underwater Explosion: Testings and Numerical Prediction Using USA/DYNA3D / June 1, 1991 - March 1, 1992
The views expressed are those of the authors and do not reflect the offical policy or position of DoD or US Government.Nonlinear 3-D Dynamic Analysis Code (VEC/DYNA3D) has been interfaced with Underwater Shock Analysis Code (USA) and capabilities were developed to perform numerical analysis of submerged and semi-submerged marine structures subjected to underwater explosion. A series of numerical analysis were performed to determine the elastic and elasto-plastic responses of cylindrica shell type structures. The results were favorably compared with those of underwater explosion testings. The coupled code USA/DYNA3D makes possible to predict shock-induced damage response of naval structure. In addition, numerical sensitivity analyses were undertaken to determine the importance of various physical and numerical modeling factors. This study showed clearly three types of response modes of cylinder subjected to a side-on explosion: accordion mode, breathing mode and whipping mode.This report was prepared for and funded by both Defense Nuclear Agency,
Alexandria, VA 20311 and Naval Postgraduate School, Monterey, CA 93943.Approved for public release; distribution is unlimited
Cranked Hartree-Fock-Bogoliubov Calculation for Rotating Bose-Einstein Condensates
A rotating bosonic many-body system in a harmonic trap is studied with the
3D-Cranked Hartree-Fock-Bogoliubov method at zero temperature, which has been
applied to nuclear many-body systems at high spin. This method is a variational
method extended from the Hartree-Fock theory, which can treat the pairing
correlations in a self-consistent manner. An advantage of this method is that a
finite-range interaction between constituent particles can be used in the
calculation, unlike the original Gross-Pitaevskii approach. To demonstrate the
validity of our method, we present a calculation for a toy model, that is, a
rotating system of ten bosonic particles interacting through the repulsive
quadrupole-quadrupole interaction in a harmonic trap. It is found that the
yrast states, the lowest-energy states for the given total angular momentum,
does not correspond to the Bose-Einstein condensate, except a few special
cases. One of such cases is a vortex state, which appears when the total
angular momentum is twice the particle number (i.e., ).Comment: accepted to Phys. Rev.
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