179 research outputs found
Efimov trimers under strong confinement
The dimensionality of a system can fundamentally impact the behaviour of
interacting quantum particles. Classic examples range from the fractional
quantum Hall effect to high temperature superconductivity. As a general rule,
one expects confinement to favour the binding of particles. However,
attractively interacting bosons apparently defy this expectation: while three
identical bosons in three dimensions can support an infinite tower of Efimov
trimers, only two universal trimers exist in the two dimensional case. We
reveal how these two limits are connected by investigating the problem of three
identical bosons confined by a harmonic potential along one direction. We show
that the confinement breaks the discrete Efimov scaling symmetry and destroys
the weakest bound trimers. However, the deepest bound Efimov trimer persists
under strong confinement and hybridizes with the quasi-two-dimensional trimers,
yielding a superposition of trimer configurations that effectively involves
tunnelling through a short-range repulsive barrier. Our results suggest a way
to use strong confinement to engineer more stable Efimov-like trimers, which
have so far proved elusive.Comment: 8 pages, 4 figures. Typos corrected, published versio
Polarons, Dressed Molecules, and Itinerant Ferromagnetism in ultracold Fermi gases
In this review, we discuss the properties of a few impurity atoms immersed in
a gas of ultracold fermions, the so-called Fermi polaron problem. On one side,
this many-body system is appealing because it can be described almost exactly
with simple diagrammatic and/or variational theoretical approaches. On the
other, it provides quantitatively reliable insight into the phase diagram of
strongly interacting population imbalanced quantum mixtures. In particular, we
show that the polaron problem can be applied to study itinerant ferromagnetism,
a long standing problem in quantum mechanics.Comment: Review paper; published version, 48 pages and 23 figure
Strong-coupling ansatz for the one-dimensional Fermi gas in a harmonic potential
A major challenge in modern physics is to accurately describe strongly
interacting quantum many-body systems. One-dimensional systems provide
fundamental insights since they are often amenable to exact methods. However,
no exact solution is known for the experimentally relevant case of external
confinement. Here, we propose a powerful ansatz for the one-dimensional Fermi
gas in a harmonic potential near the limit of infinite short-range repulsion.
For the case of a single impurity in a Fermi sea, we show that our ansatz is
indistinguishable from numerically exact results in both the few- and many-body
limits. We furthermore derive an effective Heisenberg spin-chain model
corresponding to our ansatz, valid for any spin-mixture, within which we obtain
the impurity eigenstates analytically. In particular, the classical Pascal's
triangle emerges in the expression for the ground-state wavefunction. As well
as providing an important benchmark for strongly correlated physics, our
results are relevant for emerging quantum technologies, where a precise
knowledge of one-dimensional quantum states is paramount.Comment: 13 pages, 6 figures. Published versio
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
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.
Synthetic gauge fields in synthetic dimensions
We describe a simple technique for generating a cold-atom lattice pierced by
a uniform magnetic field. Our method is to extend a one-dimensional optical
lattice into the "dimension" provided by the internal atomic degrees of
freedom, yielding a synthetic 2D lattice. Suitable laser-coupling between these
internal states leads to a uniform magnetic flux within the 2D lattice. We show
that this setup reproduces the main features of magnetic lattice systems, such
as the fractal Hofstadter butterfly spectrum and the chiral edge states of the
associated Chern insulating phases.Comment: 5+4 pages, 5+3 figures, two-column revtex; v2: discussion of role of
interactions added, Fig. 1 reshaped, minor changes, references adde
Collective excitations of a trapped Bose-Einstein condensate in the presence of a 1D optical lattice
We study low-lying collective modes of a horizontally elongated 87Rb
condensate produced in a 3D magnetic harmonic trap with the addition of a 1D
periodic potential which is provided by a laser standing-wave along the
horizontal axis. While the transverse breathing mode results unperturbed,
quadrupole and dipole oscillations along the optical lattice are strongly
modified. Precise measurements of the collective mode frequencies at different
height of the optical barriers provide a stringent test of the theoretical
model recently introduced [M.Kraemer et al. Phys. Rev. Lett. 88 180404 (2002)].Comment: 4 pages, 4 figure
Decay of polarons and molecules in a strongly polarized Fermi gas
The ground state of an impurity immersed in a Fermi sea changes from a
polaron to a molecule as the interaction strength is increased.
We show here that the coupling between these two states is strongly
suppressed due to a combination of phase space effects and Fermi statistics,
and that it vanishes much faster than the energy difference between the two
states, thereby confirming the first order nature of the polaron-molecule
transition. In the regime where each state is metastable, we find quasiparticle
lifetimes which are much longer than what is expected for a usual Fermi liquid.
Our analysis indicates that the decay rates are sufficiently slow to be
experimentally observable.Comment: Version accepted in PRL. Added discussion of three-body losses to
deeply bound molecular state
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