4,341 research outputs found
Mott transition in Kagom\'e lattice Hubbard model
We investigate the Mott transition in the Kagom\'e lattice Hubbard model
using a cluster extension of dynamical mean field theory. The calculation of
the double occupancy, the density of states, the static and dynamical spin
correlation functions demonstrates that the system undergoes the first-order
Mott transition at the Hubbard interaction (:bandwidth). In
the metallic phase close to the Mott transition, we find the strong
renormalization of three distinct bands, giving rise to the formation of heavy
quasiparticles with strong frustration. It is elucidated that the quasiparticle
states exhibit anomalous behavior in the temperature-dependent spin correlation
functions.Comment: 4 pages, 6 figure
Temperature Variation in the Cluster of Galaxies Abell 115 Studied with ASCA
Abell 115 exhibits two distinct peaks in the surface brightness distribution.
ASCA observation shows a significant temperature variation in this cluster,
confirmed by a hardness ratio analysis and spectral fits. A linking region
between main and sub clusters shows a high temperature compared with other
regions. Two possibilities are examined as the cause of the temperature
variation: cooling flows in the main cluster and a shock heating due to the
collision of the subcluster into the main system. Spectral fits with cooling
flow models to the main-cluster data show a mass-deposition rate less than 419
solar-mass/yr. Temperatures in the main cluster, the linking region, and the
subcluster are estimated by correcting for the effects of X-ray telescope
response as 4.9 (+0.7/-0.6), 11 (+12/-4), and 5.2 (+1.4/-1.0) keV,
respectively. The high temperature in the linking region implies that Abell 115
is indeed a merger system, with possible contribution from cooling flows on the
temperature structure.Comment: 23 pages, including 7 Postscript figures, accepted for publication in
Ap
Anomalous proximity effect in gold coated (110) films: Penetration of the Andreev bound states
Scanning tunneling spectroscopy of (110) bi-layers
reveal a proximity effect markedly different from the conventional one. While
proximity-induced mini-gaps rarely appear in the Au layer, the Andreev bound
states clearly penetrate into the metal. Zero bias conductance peaks are
measured on Au layers thinner than 7 nm with magnitude similar to those
detected on the bare superconductor films. The peaks then decay abruptly with
Au thickness and disappear above 10 nm. This length is shorter than the normal
coherence length and corresponds to the (ballistic) mean free path.Comment: 5 prl format pages, 4 figures, to be published in PR
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
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
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
Static black hole uniqueness and Penrose inequality
Under certain conditions, we give a new way to prove the uniqueness of static
black hole in higher dimensional asymptotically flat spacetimes. In the proof,
the Penrose inequality plays a key role in higher dimensions as well as four
dimensions.Comment: 6 page
Andreev reflections on Y1-xCaxBa2Cu3O7-delta evidence for an unusual proximity effect
We have measured Andreev reflections between an Au tip and
Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7 - \delta} thin films in the in-plane orientation.
The conductance spectra are best fitted with a pair potential having the
"d_{x^{2}-y^{2}+is" symmetry. We find that the amplitude of the "is" component
is enhanced as the contact transparency is increased. This is an indication for
an unusual proximity effect that modifies the pair potential in the
superconductor near the surface with the normal metal.Comment: 4 pages, 4 figure
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.
Formation of magnetic impurities and pair-breaking effect in a superfluid Fermi gas
We theoretically investigate a possible idea to introduce magnetic impurities
to a superfluid Fermi gas. In the presence of population imbalance
(, where is the number of Fermi atoms with
pseudospin ), we show that nonmagnetic potential
scatterers embedded in the system are magnetized in the sense that some of
excess -spin atoms are localized around them. They destroy the
superfluid order parameter around them, as in the case of magnetic impurity
effect discussed in the superconductivity literature. This pair-breaking effect
naturally leads to localized excited states below the superfluid excitation
gap. To confirm our idea in a simply manner, we treat an attractive Fermi
Hubbard model within the mean-field theory at T=0. We self-consistently
determine superfluid properties around a nonmagnetic impurity, such as the
superfluid order parameter, local population imbalance, as well as
single-particle density of states, in the presence of population imbalance.
Since the competition between superconductivity and magnetism is one of the
most fundamental problems in condensed matter physics, our results would be
useful for the study of this important issue in cold Fermi gases.Comment: 27 pages, 14 figure
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