33 research outputs found
Comment on "Collective excitations of a degenerate gas at the BEC-BCS crossover"
Very recent experiments have studied for the first time collective
excitations of an ultracold Li gas covering in particular the BEC-BCS
crossover domain. We point out that the results for the axial mode, through
hydrodynamics, give direct access to the (3D) equation of state of the strongly
interacting gas, mostly near the unitarity limit. On the other hand the
surprising results found for the radial mode are actually not necessarily in
contradiction with the expectations from superfluid hydrodynamics.Comment: 2 pages, 1 figur
Analytical theory of the dressed bound state in highly polarized Fermi gases
We present an analytical treatment of a single \down atom within a Fermi sea
of \up atoms, when the interaction is strong enough to produce a bound state,
dressed by the Fermi sea. Our method makes use of a diagrammatic analysis, with
the involved diagrams taking only into account at most two particle-hole pairs
excitations. The agreement with existing Monte-Carlo results is excellent. In
the BEC limit our equation reduces exactly to the Skorniakov and
Ter-Martirosian equation. We present results when \up and \down atoms have
different masses, which is of interest for experiments in progress.Comment: 5 pages, 3 figure
A simple theory for high ratio in d-wave superconductors
We investigate a simple explanation for the high maximum gap to ratio
found experimentally in high compounds. We ascribe this observation to
the lowering of by boson scattering of electrons between parts of the
Fermi surface with opposite sign for the order parameter. We study the simplest
possible model within this picture. Our quantitative results show that we can
account for experiment for a rather small value of the coupling constant, all
the other ingredients of our model being already known to exist in these
compounds. A striking implication of this theory is the fairly high value of
the critical temperature in the absence of boson scattering.Comment: 5 pages, revtex, 2 figure
Collective modes of a trapped Lieb-Liniger gas: a hydrodynamic approach
We consider a trapped repulsive one-dimensional (1D) Bose gas at very low
temperature. In order to study the collective modes of this strongly
interacting system, we use a hydrodynamic approach, where the gas is locally
described by the Lieb-Liniger model of bosons interacting via a repulsive delta
potential. Solving the corresponding linearized hydrodynamic equations, we
obtain the collective modes and concentrate more specifically on the lowest
compressional mode. This is done by finding models, approaching very closely
the exact equation of stae of the gas, for which the linearized hydrodynamic
equations are exactly solvable. Results are in excellent agreement with those
of the sum rule approach of Menotti and Stringari.Comment: Proceedings of the Laser Physics Workshop held in Hamburg (August
2003), Seminar on the Physics of Cold Trapped Atom
Equation of state and collective frequencies of a trapped Fermi gas along the BEC-unitarity crossover
We show that the study of the collective oscillations in a harmonic trap
provides a very sensitive test of the equation of state of a Fermi gas near a
Feshbach resonance. Using a scaling approach, whose high accuracy is proven by
comparison with exact hydrodynamic solutions, the frequencies of the lowest
compressional modes are calculated at T=0 in terms of a dimensionless parameter
characterizing the equation of state. The predictions for the collective
frequencies, obtained from the equations of state of mean field BCS theory and
of recent Monte-Carlo calculations, are discussed in detail.Comment: 4 pages, 3 figure
Axial collective excitations of a degenerate Fermi gas in the BEC to unitarity crossover
We show that, with reasonable hypotheses leading to a simple modeling,a link
can be obtained from experiments on the axial low frequency collective modes
between the molecular scattering length and the energy parameter of the gas at the unitarity limit. We also point out that, in
order to reach the range where the features of the Bose limit can be clearly
seen, experiments have to go to more dilute situations than have been achieved
presently.Comment: 5 pages, revtex, 2 figure
Atom-dimer scattering length for fermions with different masses: analytical study of limiting cases
We consider the problem of obtaining the scattering length for a fermion
colliding with a dimer, formed from a fermion identical to the incident one and
another different fermion. This is done in the universal regime where the range
of interactions is short enough so that the scattering length for non
identical fermions is the only relevant quantity. This is the generalization to
fermions with different masses of the problem solved long ago by Skorniakov and
Ter-Martirosian for particles with equal masses. We solve this problem
analytically in the two limiting cases where the mass of the solitary fermion
is very large or very small compared to the mass of the two other identical
fermions. This is done both for the value of the scattering length and for the
function entering the Skorniakov-Ter-Martirosian integral equation, for which
simple explicit expressions are obtained.Comment: Very simple form for the solution added; conclusion adde