5,122 research outputs found
Pseudogap phenomenon in an ultracold Fermi gas with a p-wave pairing interaction
We investigate single-particle properties of a one-component Fermi gas with a
tunable p-wave interaction. Including pairing fluctuations associated with this
anisotropic interaction within a -matrix theory, we calculate the
single-particle density of states, as well as the spectral weight, above the
superfluid transition temperature . Starting from the weak-coupling
regime, we show that the so-called pseudogap first develops in these quantities
with increasing the interaction strength. However, when the interaction becomes
strong to some extent, the pseudogap becomes obscure to eventually disappear in
the strong-coupling regime. This non-monotonic interaction dependence is quite
different from the case of an s-wave interaction, where the pseudogap simply
develops with increasing the interaction strength. The difference between the
two cases is shown to originate from the momentum dependence of the p-wave
interaction, which vanishes in the low momentum limit. We also identify the
pseudogap regime in the phase diagram with respect to the temperature and the
p-wave interaction strength. Since the pseudogap is a precursor phenomenon of
the superfluid phase transition, our results would be useful for the research
toward the realization of p-wave superfluid Fermi gases.Comment: 21 pages, 9 figure
Detection of an X-Ray Hot Region in the Virgo Cluster of Galaxies with ASCA
Based on mapping observations with ASCA, an unusual hot region with a spatial
extent of 1 square degree was discovered between M87 and M49 at a center
coordinate of R. A. = 12h 27m 36s and Dec. = (J2000). The X-ray
emission from the region has a 2-10 keV flux of ergs
s cm and a temperature of keV, which is
significantly higher than that in the surrounding medium of keV. The
internal thermal energy in the hot region is estimated to be ergs with a gas density of cm. A power-law
spectrum with a photon index is also allowed by the data. The hot
region suggests there is an energy input due to a shock which is probably
caused by the motion of the gas associated with M49, infalling toward the M87
cluster with a velocity km s.Comment: 12 pages, 3 figures, accepted to ApJ
Adiabatic Phase Diagram of an Ultracold Atomic Fermi Gas with a Feshbach Resonance
We determine the adiabatic phase diagram of a resonantly-coupled system of
Fermi atoms and Bose molecules confined in the harmonic trap by using the local
density approximation. The adiabatic phase diagram shows the fermionic
condensate fraction composed of condensed molecules and Cooper pair atoms. The
key idea of our work is conservation of entropy through the adiabatic process,
extending the study of Williams et al. [Williams et al., New J. Phys. 6, 123
(2004)] for an ideal gas mixture to include the resonant interaction in a
mean-field theory. We also calculate the molecular conversion efficiency as a
function of initial temperature. Our work helps to understand recent
experiments on the BCS-BEC crossover, in terms of the initial temperature
measured before a sweep of the magnetic field.Comment: 13 pages, 8 figures. In press, "Journal of the Physical Society of
Japan", Vol.76, No.
Superfluid density of states and pseudogap phenomenon in the BCS-BEC crossover regime of a superfluid Fermi gas
We investigate single-particle excitations and strong-coupling effects in the
BCS-BEC crossover regime of a superfluid Fermi gas. Including phase and
amplitude fluctuations of the superfluid order parameter within a -matrix
theory, we calculate the superfluid density of states (DOS), as well as
single-particle spectral weight, over the entire BCS-BEC crossover region below
the superfluid transition temperature . We clarify how the pseudogap
in the normal state evolves into the superfluid gap, as one passes through
. While the pseudogap in DOS continuously evolves into the
superfluid gap in the weak-coupling BCS regime, the superfluid gap in the
crossover region is shown to appear in DOS after the pseudogap disappears below
. In the phase diagram with respect to the temperature and
interaction strength, we determine the region where strong pairing fluctuations
dominate over single-particle properties of the system. Our results would be
useful for the study of strong-coupling phenomena in the BCS-BEC crossover
regime of a superfluid Fermi gas.Comment: 22 pages, 8 figure
CCS Imaging of the Starless Core L1544: An Envelope with Infall and Rotation
We have carried out observations of the starless core L1544 in the CCS
(J_N=3_2-2_1) line at 9 millimeters wavelength using the BIMA array. The maps
show an elongated condensation, 0.15 x 0.045 pc in size, with stronger emission
at the edges. The appearance is consistent with a flattened, ringlike structure
viewed at high inclination to the line of sight. The CCS molecule is likely
heavily depleted in the inner part of the core. The position velocity diagram
along the major axis shows a remarkable pattern, a "tilted ellipse", that can
be reproduced by a simple model ring with motions of both infall and rotation.
The models suggest comparable velocities for infall and rotation, ~0.1 km/s, in
the outermost envelope, at radius 15000 AU.Comment: 14 pages, 4 figures, AAS-LaTex v4.0, will be published in ApJ
Kohn's theorem in a superfluid Fermi gas with a Feshbach resonance
We investigate the dipole mode in a superfluid gas of Fermi atoms trapped in
a harmonic potential. According to Kohn's theorem, the frequency of this
collective mode is not affected by an interaction between the atoms and is
always equal to the trap frequency. This remarkable property, however, does not
necessarily hold in an approximate theory. We explicitly prove that the
Hartree-Fock-Bogoliubov generalized random phase approximation (HFB-GRPA),
including a coupling between fluctuations in the density and Cooper channels,
is consistent with both Kohn's theorem as well as Goldstone's theorem. This
proof can be immediately extended to the strong-coupling superfluid theory
developed by Nozi\'eres and Schmitt-Rink (NSR), where the effect of superfluid
fluctuations is included within the Gaussian level. As a result, the NSR-GRPA
formalism can be used to study collective modes in the BCS-BEC crossover region
in a manner which is consistent with Kohn's theorem. We also include the effect
of a Feshbach resonance and a condensate of the associated molecular bound
states. A detailed discussion is given of the unusual nature of the Kohn mode
eigenfunctions in a Fermi superfluid, in the presence and absence of a Feshbach
resonance. When the molecular bosons feel a different trap frequency from the
Fermi atoms, the dipole frequency is shown to {\it depend} on the strength of
effective interaction associated with the Feshbach resonance.Comment: 29 pages, 1 figure
Single-particle excitations in the BCS-BEC crossover region II: Broad Feshbach resonance
We apply the formulation developed in a recent paper [Y. Ohashi and A.
Griffin, Phys. Rev. A {\bf 72}, 013601, (2005)] for single-particle excitations
in the BCS-BEC crossover to the case of a broad Feshbach resonance. At T=0, we
solve the Bogoliubov-de Gennes coupled equations taking into account a Bose
condensate of bound states (molecules). In the case of a broad resonance, the
density profile , as well as the profile of the superfluid order
parameter , are spatially spread out to the Thomas-Fermi
radius, even in the crossover region. This order parameter
suppresses the effects of low-energy Andreev bound states on the rf-tunneling
current. As a result, the peak energy in the rf-spectrum is found to occur at
an energy equal to the superfluid order parameter at the
center of the trap, in contrast to the case of a narrow resonance, and in
agreement with recent measurements. The LDA is found to give a good
approximation for the rf-tunneling spectrum.Comment: 14 pages, 8 figure
Equation of state of a superfluid Fermi gas in the BCS-BEC crossover
We present a theory for a superfluid Fermi gas near the BCS-BEC crossover,
including pairing fluctuation contributions to the free energy similar to that
considered by Nozieres and Schmitt-Rink for the normal phase. In the strong
coupling limit, our theory is able to recover the Bogoliubov theory of a weakly
interacting Bose gas with a molecular scattering length very close to the known
exact result. We compare our results with recent Quantum Monte Carlo
simulations both for the ground state and at finite temperature. Excellent
agreement is found for all interaction strengths where simulation results are
available.Comment: 7 pages, 4 figures, published version in Europhysics Letters, a long
preprint with details will appear soo
Pseudogap temperature and effects of a harmonic trap in the BCS-BEC crossover regime of an ultracold Fermi gas
We theoretically investigate excitation properties in the pseudogap regime of
a trapped Fermi gas. Using a combined -matrix theory with the local density
approximation, we calculate strong-coupling corrections to single-particle
local density of states (LDOS), as well as the single-particle local spectral
weight (LSW). Starting from the superfluid phase transition temperature , we clarify how the pseudogap structures in these quantities disappear with
increasing the temperature. As in the case of a uniform Fermi gas, LDOS and LSW
give different pseudogap temperatures and at which the pseudogap
structures in these quantities completely disappear. Determining and
over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein
condensate) crossover region, we identify the pseudogap regime in the phase
diagram with respect to the temperature and the interaction strength. We also
show that the so-called back-bending peak recently observed in the
photoemission spectra by JILA group may be explained as an effect of pseudogap
phenomenon in the trap center. Since strong pairing fluctuations, spatial
inhomogeneity, and finite temperatures, are important keys in considering real
cold Fermi gases, our results would be useful for clarifying normal state
properties of this strongly interacting Fermi system.Comment: 25 pages, 12 figure
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