104 research outputs found
From quantum quasiparticles to a classical gas
Peer ReviewedPostprint (published version
Superfluid Vortex Dynamics on Planar Sectors and Cones
We study the dynamics of vortices formed in a superfluid film adsorbed on the
curved two-dimensional surface of a cone. To this aim, we observe that a cone
can be unrolled to a sector on a plane with periodic boundary conditions on the
straight sides. The sector can then be mapped conformally to the whole plane,
leading to the relevant stream function. In this way, we show that a superfluid
vortex on the cone precesses uniformly at fixed distance from the apex. The
stream function also yields directly the interaction energy of two vortices on
the cone. We then study the vortex dynamics on unbounded and bounded cones. In
suitable limits, we recover the known results for dynamics on cylinders and
planar annuli.Comment: 10 pages, 8 figure
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
Quantized superfluid vortex dynamics on cylindrical surfaces and planar annuli
Superfluid vortex dynamics on an infinite cylinder differs significantly from
that on a plane. The requirement that a condensate wave function be single
valued upon once encircling the cylinder means that such a single vortex cannot
remain stationary. Instead, it acquires one of a series of quantized
translational velocities around the circumference, the simplest being , with the mass of the superfluid particles and the radius
of the cylinder. A generalization to a finite cylinder automatically includes
these quantum-mechanical effects through the pairing of the single vortex and
its image in either the top or bottom end of the surface. The dynamics of a
single vortex on this surface provides a hydrodynamic analog of Laughlin
pumping. The interaction energy for two vortices on an infinite cylinder is
proportional to the classical stream function , and it
crosses over from logarithmic to linear when the intervortex separation becomes larger than the cylinder radius. An Appendix summarizes the
connection to an earlier study of Ho and Huang for one or more vortices on an
infinite cylinder. A second Appendix reviews the topologically equivalent
planar annulus, where such quantized vortex motion has no offset, but Laughlin
pumping may be more accessible to experimental observation.Comment: 16 pages, 7 figures; published version, with thoroughly revised
Appendice
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
Bose polarons at finite temperature and strong coupling
A mobile impurity coupled to a weakly interacting Bose gas, a Bose polaron, displays several interesting effects. While a single attractive quasiparticle is known to exist at zero temperature, we show here that the spectrum splits into two quasiparticles at finite temperatures for sufficiently strong impurity-boson interaction. The ground state quasiparticle has minimum energy at Tc, the critical temperature for Bose-Einstein condensation, and it becomes overdamped when T»Tc. The quasiparticle with higher energy instead exists only below Tc, since it is a strong mixture of the impurity with thermally excited collective Bogoliubov modes. This phenomenology is not restricted to ultracold gases, but should occur whenever a mobile impurity is coupled to a medium featuring a gapless bosonic mode with a large population for finite temperature.Peer ReviewedPostprint (author's final draft
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