2,047 research outputs found
Few-body precursor of the Higgs mode in a superfluid Fermi gas
We demonstrate that an undamped few-body precursor of the Higgs mode can be
investigated in a harmonically trapped Fermi gas. Using exact diagonalisation,
the lowest monopole mode frequency is shown to depend non-monotonically on the
interaction strength, having a minimum in a crossover region. The minimum
deepens with increasing particle number, reflecting that the mode is the
few-body analogue of a many-body Higgs mode in the superfluid phase, which has
a vanishing frequency at the quantum phase transition point to the normal
phase. We show that this mode mainly consists of coherent excitations of
time-reversed pairs, and that it can be selectively excited by modulating the
interaction strength, using for instance a Feshbach resonance in cold atomic
gases.Comment: 9 pages, 7 figure
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
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
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
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
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-
Bragg Spectroscopy of Cold Atomic Fermi Gases
We propose a Bragg spectroscopy experiment to measure the onset of superfluid
pairing in ultracold trapped Fermi gases. In particular, we study two component
Fermi gases in the weak coupling BCS and BEC limits as well as in the strong
coupling unitarity limit. The low temperature Bragg spectrum exhibits a gap
directly related to the pair-breaking energy. Furthermore, the Bragg spectrum
has a large maximum just below the critical temperature when the gas is
superfluid in the BCS limit. In the unitarity regime, we show how the pseudogap
in the normal phase leads to a significant suppression of the low frequency
Bragg spectrum.Comment: 8 pages, 9 figures. Typos corrected. Reference update
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