1,966 research outputs found
Feshbach Resonances and Medium Effects in ultracold atomic Gases
We develop an effective low energy theory for multi-channel scattering of
cold atomic alkali atoms with particular focus on Feshbach resonances. The
scattering matrix is expressed in terms of observables only and the theory
allows for the inclusion of many-body effects both in the open and in the
closed channels.
We then consider the frequency and damping of collective modes for Fermi
gases and demonstrate how medium effects significantly increase the scattering
rate determining the nature of the modes. Our results obtained with no fitting
parameters are shown to compare well with experimental data.Comment: Presented at the 5th workshop on Critical Stability, Erice, Italy
13-17 October 2008. 8 pages, 3 figures. Figure caption correcte
Validity of the Gor'kov expansion near the upper critical field in type II superconductors
We have examined the validity of the Gor'kov expansion in the strength of the
order parameter of type II superconductors near the upper critical field.
Although the degeneracy of the electron levels in a magnetic field gives non-
perturbative terms in the solution to the Bogoliubov-de Gennes equations we
find, contrary to recent claims, that these non-perturbative terms cancel in
the expression for the thermodynamic potential, and that the traditional
Gor'kov theory is correct sufficiently close to Hc2 at finite temperature. We
have derived conditions for the validity of the Gor'kov theory which
essentially state, that the change in the quasiparticle energies as compared to
the normal state energies cannot be too large compared to the temperature.Comment: 5 pages, 3 figures. One reference adde
Low energy monopole Modes of a Trapped atomic Fermi Gas
We consider the low energy collective monopole modes of a trapped weakly
interacting atomic Fermi gas in the collisionless regime. The spectrum is
calculated for varying coupling strength and chemical potential. Using an
effective Hamiltonian, we derive analytical results that agree well with
numerical calculations in various regimes. The onset of superfluidity is shown
to lead to effects such as the vanishing of the energy required to create a
Cooper molecule at a critical coupling strength and to the emergence of pair
vibration excitations. Our analysis suggests ways to experimentally detect the
presence of the superfluid phase in trapped atomic Fermi gases.Comment: 5 pages & 1 figure. Accepted for Phys. Rev. Let
Viscous relaxation and collective oscillations in a trapped Fermi gas near the unitarity limit
The viscous relaxation time of a trapped two-component gas of fermions in its
normal phase is calculated as a function of temperature and scattering length,
with the collision probability being determined by an energy-dependent s-wave
cross section. The result is used for calculating the temperature dependence of
the frequency and damping of collective modes studied in recent experiments,
starting from the kinetic equation for the fermion distribution function with
mean-field effects included in the streaming terms.Comment: 10 pages, 9 figures; proof version, corrected typo in Eq. (23);
accepted for publication in PR
Laser probing of Cooper-paired trapped atoms
We consider a gas of trapped Cooper-paired fermionic atoms which are
manipulated by laser light. The laser induces a transition from an internal
state with large negative scattering length (superfluid) to one with weaker
interactions (normal gas). We show that the process can be used to detect the
presence of the superconducting order parameter. Also, we propose a direct way
of measuring the size of the gap in the trap. The efficiency and feasibility of
this probing method is investigated in detail in different physical situations.Comment: 9 pages, 8 figure
Pairing of fermions in atomic traps and nuclei
Pairing gaps for fermionic atoms in harmonic oscillator traps are calculated
for a wide range of interaction strengths and particle number, and compared to
pairing in nuclei. Especially systems, where the pairing gap exceeds the level
spacing but is smaller than the shell splitting , are studied
which applies to most trapped Fermi atomic systems as well as to finite nuclei.
When solving the gap equation for a large trap with such multi-level pairing,
one finds that the matrix elements between nearby harmonic oscillator levels
and the quasi-particle energies lead to a double logarithm of the gap, and a
pronounced shell structure at magic numbers. It is argued that neutron and
proton pairing in nuclei belongs to the class of multi-level pairing, that
their shell structure follows naturally and that the gaps scale as - all in qualitative agreement with odd-even staggering of nuclear
binding energies. Pairing in large systems are related to that in the bulk
limit. For large nuclei the neutron and proton superfluid gaps approach the
asymptotic value in infinite nuclear matter: MeV.Comment: 11 pages, 5 figure
Twin peaks in rf spectra of Fermi gases at unitarity
We calculate the radio-frequency spectrum of balanced and imbalanced
ultracold Fermi gases in the normal phase at unitarity.
For the homogeneous case the spectrum of both the majority and minority
components always has a single peak even in the pseudogap regime.
We furthermore show how the double-peak structures observed in recent
experiments arise due to the inhomogeneity of the trapped gas.
The main experimental features observed above the critical temperature in the
recent experiment of Schunck et al. [Science 316, 867, (2007)] are recovered
with no fitting parameters.Comment: v3: version accepted for publication as a Rapid Communication in PRA.
With respect to v2, minor changes in the text and in the inset of Fig.
Spin Excitations in a Fermi Gas of Atoms
We have experimentally investigated a spin excitation in a quantum degenerate
Fermi gas of atoms. In the hydrodynamic regime the damping time of the
collective excitation is used to probe the quantum behavior of the gas. At
temperatures below the Fermi temperature we measure up to a factor of 2
reduction in the excitation damping time. In addition we observe a strong
excitation energy dependence for this quantum statistical effect.Comment: 4 pages, 3 figure
Analytical treatment of interacting Fermi gas in arbitrary dimensional harmonic trap
We study normal state properties of an interacting Fermi gas in an isotropic
harmonic trap of arbitrary dimensions. We exactly calculate the first-order
perturbation terms in the ground state energy and chemical potential, and
obtain simple analytic expressions of the total energy and chemical potential.
At zero temperature, we find that Thomas-Fermi approximation agrees well with
exact results for any dimension even though system is dilute and small, i.e.
when the Thomas-Fermi approximation is generally expected to fail. In the high
temperature (classical) region, we find interaction energy decreases in
proportion to T^(-d/2), where T is temperature and d is dimension of the
system. Effect of interaction in the ground state in two and three-dimensional
systems is also discussed.Comment: 15 pages, 4 figure
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