11,271 research outputs found
Dynamics of the BCS-BEC crossover in a degenerate Fermi gas
We study the short-time dynamics of a degenerate Fermi gas positioned near a
Feshbach resonance following an abrupt jump in the atomic interaction resulting
from a change of external magnetic field. We investigate the dynamics of the
condensate order parameter and pair wavefunction for a range of field
strengths. When the abrupt jump is sufficient to span the BCS to BEC crossover,
we show that the rigidity of the momentum distribution precludes any
atom-molecule oscillations in the entrance channel dominated resonances
observed in the 40K and 6Li. Focusing on material parameters tailored to the
40K Feshbach resonance system at 202.1 gauss, we comment on the integrity of
the fast sweet projection technique as a vehicle to explore the condensed phase
in the crossover regionComment: 5 pages, 4 figure
Collective Dynamics of Bose--Einstein Condensates in Optical Cavities
Recent experiments on Bose--Einstein condensates in optical cavities have
reported a quantum phase transition to a coherent state of the matter-light
system -- superradiance. The time dependent nature of these experiments demands
consideration of collective dynamics. Here we establish a rich phase diagram,
accessible by quench experiments, with distinct regimes of dynamics separated
by non-equilibrium phase transitions. We include the key effects of cavity
leakage and the back-reaction of the cavity field on the condensate. Proximity
to some of these phase boundaries results in critical slowing down of the decay
of many-body oscillations. Notably, this slow decay can be assisted by large
cavity losses. Predictions include the frequency of collective oscillations, a
variety of multi-phase co-existence regions, and persistent optomechanical
oscillations described by a damped driven pendulum. These findings open new
directions to study collective dynamics and non-equilibrium phase transitions
in matter-light systems.Comment: 5 pages, 5 figure
Feshbach Resonance in Optical Lattices and the Quantum Ising Model
Motivated by experiments on heteronuclear Feshbach resonances in Bose
mixtures, we investigate s-wave pairing of two species of bosons in an optical
lattice. The zero temperature phase diagram supports a rich array of superfluid
and Mott phases and a network of quantum critical points. This topology reveals
an underlying structure that is succinctly captured by a two-component Landau
theory. Within the second Mott lobe we establish a quantum phase transition
described by the paradigmatic longitudinal and transverse field Ising model.
This is confirmed by exact diagonalization of the 1D bosonic Hamiltonian. We
also find this transition in the homonuclear case.Comment: 5 pages, 4 figure
Quantum Phase Transitions in Bosonic Heteronuclear Pairing Hamiltonians
We explore the phase diagram of two-component bosons with Feshbach resonant
pairing interactions in an optical lattice. It has been shown in previous work
to exhibit a rich variety of phases and phase transitions, including a
paradigmatic Ising quantum phase transition within the second Mott lobe. We
discuss the evolution of the phase diagram with system parameters and relate
this to the predictions of Landau theory. We extend our exact diagonalization
studies of the one-dimensional bosonic Hamiltonian and confirm additional Ising
critical exponents for the longitudinal and transverse magnetic
susceptibilities within the second Mott lobe. The numerical results for the
ground state energy and transverse magnetization are in good agreement with
exact solutions of the Ising model in the thermodynamic limit. We also provide
details of the low-energy spectrum, as well as density fluctuations and
superfluid fractions in the grand canonical ensemble.Comment: 11 pages, 14 figures. To appear in Phys. Rev.
Tail States in Disordered Superconductors with Magnetic Impurities: the Unitarity Limit
When subject to a weak magnetic impurity distribution, the order parameter
and quasi-particle energy gap of a weakly disordered bulk s-wave superconductor
are suppressed. In the Born scattering limit, recent investigations have shown
that `optimal fluctuations' of the random impurity potential can lead to the
nucleation of `domains' of localised states within the gap region predicted by
the conventional Abrikosov-Gor'kov mean-field theory, rendering the
superconducting system gapless at any finite impurity concentration. By
implementing a field theoretic scheme tailored to the weakly disordered system,
the aim of the present paper is to extend this analysis to the consideration of
magnetic impurities in the unitarity scattering limit. This investigation
reveals that the qualitative behaviour is maintained while the density of
states exhibits a rich structure.Comment: 18 pages AMSLaTeX (with LaTeX2e), 6 eps figure
Polaritons and Pairing Phenomena in Bose--Hubbard Mixtures
Motivated by recent experiments on cold atomic gases in ultra high finesse
optical cavities, we consider the problem of a two-band Bose--Hubbard model
coupled to quantum light. Photoexcitation promotes carriers between the bands
and we study the non-trivial interplay between Mott insulating behavior and
superfluidity. The model displays a global U(1) X U(1) symmetry which supports
the coexistence of Mott insulating and superfluid phases, and yields a rich
phase diagram with multicritical points. This symmetry property is shared by
several other problems of current experimental interest, including
two-component Bose gases in optical lattices, and the bosonic BEC-BCS crossover
problem for atom-molecule mixtures induced by a Feshbach resonance. We
corroborate our findings by numerical simulations.Comment: 4 pages, 3 figure
Exact calculation of the ground-state dynamical spin correlation function of a S=1/2 antiferromagnetic Heisenberg chain with free spinons
We calculate the exact dynamical magnetic structure factor S(Q,E) in the
ground state of a one-dimensional S=1/2 antiferromagnet with gapless free S=1/2
spinon excitations, the Haldane-Shastry model with inverse-square exchange,
which is in the same low-energy universality class as Bethe's nearest-neighbor
exchange model. Only two-spinon excited states contribute, and S(Q,E) is found
to be a very simple integral over these states.Comment: 11 pages, LaTeX, RevTeX 3.0, cond-mat/930903
Parametric correlations versus fidelity decay: the symmetry breaking case
We derive fidelity decay and parametric energy correlations for random matrix
ensembles where time--reversal invariance of the original Hamiltonian is broken
by the perturbation. Like in the case of a symmetry conserving perturbation a
simple relation between both quantities can be established.Comment: 8 pages, 8 figure
Ising Deconfinement Transition Between Feshbach-Resonant Superfluids
We investigate the phase diagram of bosons interacting via Feshbach-resonant
pairing interactions in a one-dimensional lattice. Using large scale density
matrix renormalization group (DMRG) and field theory techniques we explore the
atomic and molecular correlations in this low-dimensional setting. We provide
compelling evidence for an Ising deconfinement transition occurring between
distinct superfluids and extract the Ising order parameter and correlation
length of this unusual superfluid transition. This is supported by results for
the entanglement entropy which reveal both the location of the transition and
critical Ising degrees of freedom on the phase boundary.Comment: 4 pages, 4 figure
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