1,884 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
Shear viscosity and damping for a Fermi gas in the unitarity limit
The shear viscosity of a two-component Fermi gas in the normal phase is
calculated as a function of temperature in the unitarity limit, taking into
account strong-coupling effects that give rise to a pseudogap in the spectral
density for single-particle excitations. The results indicate that recent
measurements of the damping of collective modes in trapped atomic clouds can be
understood in terms of hydrodynamics, with a decay rate given by the viscosity
integrated over an effective volume of the cloud.Comment: 7 pages, 3 figures. Discussion significantly extended. Appendix
added. To appear in PR
Using superlattice potentials to probe long-range magnetic correlations in optical lattices
In Pedersen et al. (2011) we proposed a method to utilize a temporally
dependent superlattice potential to mediate spin-selective transport, and
thereby probe long and short range magnetic correlations in optical lattices.
Specifically this can be used for detecting antiferromagnetic ordering in
repulsive fermionic optical lattice systems, but more generally it can serve as
a means of directly probing correlations among the atoms by measuring the mean
value of an observable, the number of double occupied sites. Here, we provide a
detailed investigation of the physical processes which limit the effectiveness
of this "conveyer belt method". Furthermore we propose a simple ways to improve
the procedure, resulting in an essentially perfect (error-free) probing of the
magnetic correlations. These results shows that suitably constructed
superlattices constitute a promising way of manipulating atoms of different
spin species as well as probing their interactions.Comment: 12 pages, 9 figure
Probing spatial spin correlations of ultracold gases by quantum noise spectroscopy
Spin noise spectroscopy with a single laser beam is demonstrated
theoretically to provide a direct probe of the spatial correlations of cold
fermionic gases. We show how the generic many-body phenomena of anti-bunching,
pairing, antiferromagnetic, and algebraic spin liquid correlations can be
revealed by measuring the spin noise as a function of laser width, temperature,
and frequency.Comment: Revised version. 4 pages, 3 figures. Accepted for PR
Finite-temperature behavior of the Bose polaron
We consider a mobile impurity immersed in a Bose gas at finite temperature.
Using perturbation theory valid for weak coupling between the impurity and the
bosons, we derive analytical results for the energy and damping of the impurity
for low and high temperatures, as well as for temperatures close to the
critical temperature for Bose-Einstein condensation. These results show
that the properties of the impurity vary strongly with temperature. In
particular, the energy exhibits a non-monotonic behavior close to , and
the damping rises sharply close to . We argue that this behaviour is
generic for impurities immersed in an environment undergoing a phase transition
that breaks a continuous symmetry. Finally, we discuss how these effects can be
detected experimentally.Comment: 10 pages and 6 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
Transition from Collisionless to Hydrodynamic Behaviour in an Ultracold Atomic Gas
Relative motion in a two-component, trapped atomic gas provides a sensitive
probe of interactions. By studying the lowest frequency excitations of a two
spin-state gas confined in a magnetic trap, we have explored the transition
from the collisionless to the hydrodynamic regime. As a function of collision
rate, we observe frequency shifts as large as 6% as well as a dramatic,
non-monotonic dependence of the damping rate. The measurements agree
qualitatively with expectations for behavior in the collisionless and
hydrodynamic limits and are quantitatively compared to a classical kinetic
model.Comment: 5 pages, 4 figure
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