596 research outputs found
Phase Separation of a Fast Rotating Boson-Fermion Mixture in the Lowest-Landau-Level Regime
By minimizing the coupled mean-field energy functionals, we investigate the
ground-state properties of a rotating atomic boson-fermion mixture in a
two-dimensional parabolic trap. At high angular frequencies in the
mean-field-lowest-Landau-level regime, quantized vortices enter the bosonic
condensate, and a finite number of degenerate fermions form the
maximum-density-droplet state. As the boson-fermion coupling constant
increases, the maximum density droplet develops into a lower-density state
associated with the phase separation, revealing characteristics of a
Landau-level structure
Vortices in quantum droplets: Analogies between boson and fermion systems
The main theme of this review is the many-body physics of vortices in quantum
droplets of bosons or fermions, in the limit of small particle numbers. Systems
of interest include cold atoms in traps as well as electrons confined in
quantum dots. When set to rotate, these in principle very different quantum
systems show remarkable analogies. The topics reviewed include the structure of
the finite rotating many-body state, universality of vortex formation and
localization of vortices in both bosonic and fermionic systems, and the
emergence of particle-vortex composites in the quantum Hall regime. An overview
of the computational many-body techniques sets focus on the configuration
interaction and density-functional methods. Studies of quantum droplets with
one or several particle components, where vortices as well as coreless vortices
may occur, are reviewed, and theoretical as well as experimental challenges are
discussed.Comment: Review article, 53 pages, 53 figure
Quantum Hall fractions for spinless Bosons
We study the Quantum Hall phases that appear in the fast rotation limit for
Bose-Einstein condensates of spinless bosonic atoms. We use exact
diagonalization in a spherical geometry to obtain low-lying states of a small
number of bosons as a function of the angular momentum. This allows to
understand or guess the physics at a given filling fraction nu, ratio of the
number of bosons to the number of vortices. This is also the filling factor of
the lowest Landau level. In addition to the well-known Bose Laughlin state at
nu =1/2 we give evidence for the Jain principal sequence of incompressible
states at nu =p/(p+- 1) for a few values of p. There is a collective mode in
these states whose phenomenology is in agreement with standard arguments coming
e.g. from the composite fermion picture. At filling factor one, the potential
Fermi sea of composite fermions is replaced by a paired state, the Moore-Read
state. This is most clearly seen from the half-flux nature of elementary
excitations. We find that the hierarchy picture does not extend up to the point
of transition towards a vortex lattice. While we cannot conclude, we
investigate the clustered Read-Rezayi states and show evidence for
incompressible states at the expected ratio of flux vs number of Bose
particles.Comment: RevTeX 4, 11 pages, 13 figure
Cooling and thermometry of atomic Fermi gases
We review the status of cooling techniques aimed at achieving the deepest
quantum degeneracy for atomic Fermi gases. We first discuss some physical
motivations, providing a quantitative assessment of the need for deep quantum
degeneracy in relevant physics cases, such as the search for unconventional
superfluid states. Attention is then focused on the most widespread technique
to reach deep quantum degeneracy for Fermi systems, sympathetic cooling of
Bose-Fermi mixtures, organizing the discussion according to the specific
species involved. Various proposals to circumvent some of the limitations on
achieving the deepest Fermi degeneracy, and their experimental realizations,
are then reviewed. Finally, we discuss the extension of these techniques to
optical lattices and the implementation of precision thermometry crucial to the
understanding of the phase diagram of classical and quantum phase transitions
in Fermi gases.Comment: 33 pages, 15 figures, contribution to the 100th anniversary of the
birth of Vitaly L. Ginzbur
Surface modes of ultracold atomic clouds with very large number of vortices
We study the surface modes of some of the vortex liquids recently found by
means of exact diagonalizations in systems of rapidly rotating bosons. In
contrast to the surface modes of Bose condensates, we find that the surface
waves have a frequency linear in the excitation angular momentum, . Furthermore, in analogy with the edge waves of electronic quantum Hall
states, these excitations are {\it chiral}, that is, they can be excited only
for values of that increase the total angular momentum of the vortex
liquid. However, differently from the quantum Hall phenomena for electrons, we
also find other excitations that are approximately degenerate in the laboratory
frame with the surface modes, and which decrease the total angular momentum by
quanta. The surface modes of the Laughlin, as well as other scalar and
vector boson states are analyzed, and their {\it observable} properties
characterized. We argue that measurement of the response of a vortex liquid to
a weak time-dependent potential that imparts angular momentum to the system
should provide valuable information to characterize the vortex liquid. In
particular, the intensity of the signal of the surface waves in the dynamic
structure factor has been studied and found to depend on the type of vortex
liquid. We point out that the existence of surface modes has observable
consequences on the density profile of the Laughlin state. These features are
due to the strongly correlated behavior of atoms in the vortex liquids. We
point out that these correlations should be responsible for a remarkable
stability of some vortex liquids with respect to three-body losses.Comment: 28 pages + 6 EPS figures. Final version as accepted for publication
in Phys. Rev.
Vortex-lattice structures in rotating Bose-Fermi superfluid mixtures
The system of Bose-Fermi superfluid mixture offers a playground to explore
rich macroscopic quantum phenomena. In a recent experiment of Yao {\it et al.}
[Phys. Rev. Lett. {\bf 117}, 145301 (2016)], K-Li superfluid
mixture is implemented. Coupled quantized vortices are generated via rotating
the superfluid mixture, and a few unconventional behaviors on the formations of
vortex numbers are observed, which can be traced to boson-fermion interactions.
Here we provide a theoretical insight into the unconventional behaviors
observed in the experiment. To this end, the orbital-free density functional
theory is hired, and its utility is validated by making comparison of numerical
results and full microscopic theory for vortex lattices in strongly interacting
Fermi superfluids alone. We also predict interesting phenomena which can be
readily explored experimentally, including the novel structures of vortex
lattices in Bose-Fermi superfluid mixtures in phase-separated regimes, and
attractive interactions between vortex lines belonging to distinct superfluids.Comment: 27 pages, 9 figures, Submission to SciPost Physic
Quantum phase transitions in the interacting boson model
This review is focused on various properties of quantum phase transitions
(QPTs) in the Interacting Boson Model (IBM) of nuclear structure. The model in
its infinite-size limit exhibits shape-phase transitions between spherical,
deformed prolate, and deformed oblate forms of the ground state. Finite-size
precursors of such behavior are verified by robust variations of nuclear
properties (nuclear masses, excitation energies, transition probabilities for
low lying levels) across the chart of nuclides. Simultaneously, the model
serves as a theoretical laboratory for studying diverse general features of
QPTs in interacting many-body systems, which differ in many respects from
lattice models of solid-state physics. We outline the most important fields of
the present interest: (a) The coexistence of first- and second-order phase
transitions supports studies related to the microscopic origin of the QPT
phenomena. (b) The competing quantum phases are characterized by specific
dynamical symmetries and novel symmetry related approaches are developed to
describe also the transitional dynamical domains. (c) In some parameter
regions, the QPT-like behavior can be ascribed also to individual excited
states, which is linked to the thermodynamic and classical descriptions of the
system. (d) The model and its phase structure can be extended in many
directions: by separating proton and neutron excitations, considering
odd-fermion degrees of freedom or different particle-hole configurations, by
including other types of bosons, higher order interactions, and by imposing
external rotation. All these aspects of IBM phase transitions are relevant in
the interpretation of experimental data and important for a fundamental
understanding of the QPT phenomenon.Comment: a review article, 71 pages, 18 figure
Rapidly Rotating Atomic Gases
This article reviews developments in the theory of rapidly rotating
degenerate atomic gases. The main focus is on the equilibrium properties of a
single component atomic Bose gas, which (at least at rest) forms a
Bose-Einstein condensate. Rotation leads to the formation of quantized vortices
which order into a vortex array, in close analogy with the behaviour of
superfluid helium. Under conditions of rapid rotation, when the vortex density
becomes large, atomic Bose gases offer the possibility to explore the physics
of quantized vortices in novel parameter regimes. First, there is an
interesting regime in which the vortices become sufficiently dense that their
cores -- as set by the healing length -- start to overlap. In this regime, the
theoretical description simplifies, allowing a reduction to single particle
states in the lowest Landau level. Second, one can envisage entering a regime
of very high vortex density, when the number of vortices becomes comparable to
the number of particles in the gas. In this regime, theory predicts the
appearance of a series of strongly correlated phases, which can be viewed as
{\it bosonic} versions of fractional quantum Hall states. This article
describes the equilibrium properties of rapidly rotating atomic Bose gases in
both the mean-field and the strongly correlated regimes, and related
theoretical developments for Bose gases in lattices, for multi-component Bose
gases, and for atomic Fermi gases. The current experimental situation and
outlook for the future are discussed in the light of these theoretical
developments.Comment: Published version + minor correction
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