87 research outputs found
The BCS-BEC Crossover
This chapter presents the crossover from the Bardeen-Cooper-Schrieffer (BCS)
state of weakly-correlated pairs of fermions to the Bose-Einstein condensation
(BEC) of diatomic molecules in the atomic Fermi gas. Our aim is to provide a
pedagogical review of the BCS-BEC crossover, with an emphasis on the basic
concepts, particularly those that are not generally known or are difficult to
find in the literature. We shall not attempt to give an exhaustive survey of
current research in the limited space here; where possible, we will direct the
reader to more extensive reviews.Comment: 19 pages, 6 figures. This article will be published as Chapter 9 in
"Quantum gas experiments - exploring many-body states", edited by P. Torma
and K. Sengstock, Imperial College Press, London, to be published 201
Three-body correlations in a two-dimensional SU(3) Fermi gas
We consider a three-component Fermi gas that has SU(3) symmetry and is
confined to two dimensions (2D). For realistic cold atomic gas experiments, we
show that the phase diagram of the quasi-2D system can be characterized using
two 2D scattering parameters: the scattering length and the effective range.
Unlike the case in 3D, we argue that three-body bound states (trimers) in the
quasi-2D system can be stable against three-body losses. Using a low-density
expansion coupled with a variational approach, we investigate the fate of such
trimers in the many-body system as the attractive interactions are decreased
(or, conversely, as the density of particles is increased). We find that
remnants of trimers can persist in the form of strong three-body correlations
in the weak-coupling (high-density) limit.Comment: 13 pages, 4 figure
Evaporative depolarization and spin transport in a unitary trapped Fermi gas
We consider a partially spin-polarized atomic Fermi gas in a
high-aspect-ratio trap, with a flux of predominantly spin-up atoms exiting the
center of the trap. We argue that such a scenario can be produced by
evaporative cooling, and we find that it can result in a substantially
non-equilibrium polarization pattern for typical experimental parameters. We
offer this as a possible explanation for the quantitative discrepancies in
recent experiments on spin-imbalanced unitary Fermi gases.Comment: 6 pages, 3 figures; published versio
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
Microscopic description of exciton-polaritons in microcavities
We investigate the microscopic description of exciton-polaritons that
involves electrons, holes and photons within a two-dimensional microcavity. We
show that in order to recover the simplified exciton-photon model that is
typically used to describe polaritons, one must correctly define the
exciton-photon detuning and exciton-photon (Rabi) coupling in terms of the bare
microscopic parameters. For the case of unscreened Coulomb interactions, we
find that the exciton-photon detuning is strongly shifted from its bare value
in a manner akin to renormalization in quantum electrodynamics. Within the
renormalized theory, we exactly solve the problem of a single exciton-polariton
for the first time and obtain the full spectral response of the microcavity. In
particular, we find that the electron-hole wave function of the polariton can
be significantly modified by the very strong Rabi couplings achieved in current
experiments. Our microscopic approach furthermore allows us to properly
determine the effective interaction between identical polaritons, which goes
beyond previous theoretical work. Our findings are thus important for
understanding and characterizing exciton-polariton systems across the whole
range of polariton densities.Comment: 14 pages, 5 figure
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