2,101 research outputs found
Tunable Wigner States with Dipolar Atoms and Molecules
We study the few-body physics of trapped atoms or molecules with electric or
magnetic dipole moments aligned by an external field. Using exact numerical
diagonalization appropriate for the strongly correlated regime, as well as a
classical analysis, we show how Wigner localization emerges with increasing
coupling strength. The Wigner states exhibit non-trivial geometries due to the
anisotropy of the interaction. This leads to transitions between different
Wigner states as the tilt angle of the dipoles with the confining plane is
changed. Intriguingly, while the individual Wigner states are well described by
a classical analysis, the transitions between different Wigner states are
strongly affected by quantum statistics. This can be understood by considering
the interplay between quantum-mechanical and spatial symmetry properties.
Finally, we demonstrate that our results are relevant to experimentally
realistic systems.Comment: 4 pages, 6 figure
Spin diffusion in trapped clouds of strongly interacting cold atoms
We show that puzzling recent experimental results on spin diffusion in a
strongly interacting atomic gas may be understood in terms of the predicted
spin diffusion coefficient for a generic strongly interacting system. Three
important features play a central role: a) Fick's law for diffusion must be
modified to allow for the trapping potential, b) the diffusion coefficient is
inhomogeneous, due to the density variations in the cloud and c) the diffusion
approximation fails in the outer parts of the cloud, where the mean free path
is long.Comment: 4 pages, 6 figures, minor modifications to the text and figures in 2.
versio
Polarons and Molecules in a Two-Dimensional Fermi Gas
We study an impurity atom in a two-dimensional Fermi gas using variational
wave functions for (i) an impurity dressed by particle-hole excitations
(polaron) and (ii) a dimer consisting of the impurity and a majority atom. In
contrast to three dimensions, where similar calculations predict a sharp
transition to a dimer state with increasing interspecies attraction, we show
that the polaron ansatz always gives a lower energy. However, the exact
solution for a heavy impurity reveals that both a two-body bound state and
distortions of the Fermi sea are crucial. This reflects the importance of
particle-hole pairs in lower dimensions and makes simple variational
calculations unreliable. We show that the energy of an impurity gives important
information about its dressing cloud, for which both ans\"atze give inaccurate
results.Comment: 5 pages, 2 figures, minor change
Clock shifts in a Fermi gas interacting with a minority component: a soluble model
We consider the absorption spectrum of a Fermi gas mixed with a minority
species when majority fermions are transferred to another internal state by an
external probe. In the limit when the minority species is much more massive
than the majority one, we show that the minority species may be treated as
static impurities and the problem can be solved in closed form. The analytical
results bring out the importance of vertex corrections, which change
qualitatively the nature of the absorption spectrum. It is demonstrated that
large line shifts are not associated with resonant interactions in general. We
also show that the commonly used ladder approximation fails when the majority
component is degenerate for large mass ratios between the minority and majority
species and that bubble diagrams, which correspond to the creation of many
particle--hole pairs, must be taken into account. We carry out detailed
numerical calculations, which confirm the analytical insights and we point out
the connection to shadowing phenomena in nuclear physics.Comment: 8 pages, 4 figures, NORDITA-2010-
An effective theory of Feshbach resonances and many-body properties of Fermi gases
For calculating low-energy properties of a dilute gas of atoms interacting
via a Feshbach resonance, we develop an effective theory in which the
parameters that enter are an atom-molecule coupling strength and the magnetic
moment of the molecular resonance. We demonstrate that for resonances in the
fermionic systems Li and K that are under experimental
investigation, the coupling is so strong that many-body effects are appreciable
even when the resonance lies at an energy large compared with the Fermi energy.
We calculate a number of many-body effects, including the effective mass and
the lifetime of atomic quasiparticles in the gas.Comment: 4 pages, 1 figure, NORDITA-2003-21 C
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
Viscosity and Thermal Relaxation for a resonantly interacting Fermi gas
The viscous and thermal relaxation rates of an interacting fermion gas are
calculated as functions of temperature and scattering length, using a many-body
scattering matrix which incorporates medium effects due to Fermi blocking of
intermediate states. These effects are demonstrated to be large close to the
transition temperature to the superfluid state. For a homogeneous gas in
the unitarity limit, the relaxation rates are increased by nearly an order of
magnitude compared to their value obtained in the absence of medium effects due
to the Cooper instability at . For trapped gases the corresponding ratio
is found to be about three due to the averaging over the inhomogeneous density
distribution. The effect of superfluidity below is considered to leading
order in the ratio between the energy gap and the transition temperature.Comment: 7 pages, 3 figure
Self-bound many-body states of quasi-one-dimensional dipolar Fermi gases: Exploiting Bose-Fermi mappings for generalized contact interactions
Using a combination of results from exact mappings and from mean-field theory
we explore the phase diagram of quasi-one-dimensional systems of identical
fermions with attractive dipolar interactions. We demonstrate that at low
density these systems provide a realization of a single-component
one-dimensional Fermi gas with a generalized contact interaction. Using an
exact duality between one-dimensional Fermi and Bose gases, we show that when
the dipole moment is strong enough, bound many-body states exist, and we
calculate the critical coupling strength for the emergence of these states. At
higher densities, the Hartree-Fock approximation is accurate, and by combining
the two approaches we determine the structure of the phase diagram. The
many-body bound states should be accessible in future experiments with
ultracold polar molecules
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