274 research outputs found
Bosons with long range interactions on two-leg ladders in artificial magnetic fields
Motivated by experiments exploring the physics of neutral atoms in artificial
magnetic fields, we study the ground state of bosons interacting with long
range dipolar interactions on a two-leg ladder. Using two complimentary
variational approaches, valid for weak interactions, we find rich physics
driven by the long range forces. Generically, long range interactions tend to
destroy the Meissner phase in favor of modulated density wave phases. Nearest
neighbor interactions produce a novel interleg charge density wave phase, where
the total density remains uniform, but the density on each leg of the ladder is
modulating in space, out-of-phase with one another. At weak magnetic fields,
next nearest neighbor interactions lead to a fully modulated biased ladder
phase, where all the particles are on one leg of the ladder, and the density is
modulating in space. This state simultaneously breaks reflection
symmetry and symmetry associated with translation in real space. For
values of the flux near , we find that a switching effect occurs
for arbitrarily weak interactions, where the density modulates in space, but
the chiral current changes sign on every plaquette. Arbitrarily weak attractive
interactions along the rungs destroy the Meissner phase completely, in favor of
a modulated density wave phase. Varying magnetic field produces a cascade of
first order transitions between modulated density wave states with different
wave-vectors, which manifests itself as discrete jumps in the chiral current.
Polarizing the dipoles along the ladder direction yields a region of phase
space where a stable biased ladder phase occurs even at arbitrarily weak
magnetic fields. We discuss the experimental consequences of our work, in
particular, how the interleg charge density wave can manifest itself in recent
experiments on bosons in synthetic dimensions.Comment: 12 pages, 4 figure
Spin dynamics in a spin-orbit coupled Fermi gas
We study the dynamics of a non-degenerate, harmonically trapped Fermi gas
following a sudden ramp of the spin-orbit coupling strength. In the
non-interacting limit, we solve the Boltzmann equation in the presence of spin
orbit coupling analytically, and derive expressions for the dynamics of an
arbitrary initial spin state. In particular we show that for a fully spin
polarized initial state, the total magnetization exhibits collapse and revival
dynamics in time with a period set by the trapping potential. In real space,
this corresponds to oscillations between a fully polarized state and a spin
helix. We numerically study the effect of interactions on the dynamics using a
collisionless Boltzmann equation.Comment: 8 pages, 3 figures Expanded to include discussion of dynamics in the
presence of a Zeeman field, rewritten section with interaction
Absence of damping of low energy excitations in a quasi-2D dipolar Bose gas
We develop a theory of damping of low energy, collective excitations in a
quasi-2D, homogenous, dipolar Bose gas at zero temperature, via processes
whereby an excitation decays into two excitations with lower energy. We find
that owing to the nature of the low energy spectrum of a quasi-2D dipolar gas,
such processes cannot occur unless the momentum of the incoming quasi-particle
exceeds a critical value k_{crit}. We find that as the dipolar interaction
strength is increased, this critical value shifts to larger momenta. Our
predictions can be directly verified in current experiments on dipolar Bose
condensates using Bragg spectroscopy, and provide valuable insight into the
quantum many-body physics of dipolar gases.Comment: 4 pages, 2 figure
Spin waves in a spin-1 Bose gas
We present a theory of spin waves in a non-condensed gas of spin-1 bosons:
providing both analytic calculations of the linear theory, and full numerical
simulations of the nonlinear response. We highlight the role of spin-dependent
contact interactions in the dynamics of a thermal gas. Although these
interactions are small compared to the thermal energy, they set the scale for
low energy long wavelength spin waves. In particular, we find that the polar
state of Rb-87 is unstable to collisional mixing of magnetic sublevels even in
the normal state. We augment our analytic calculations by providing full
numerical simulations of a trapped gas, explicitly demonstrating this
instability. Further we show that for strong enough anti-ferromagnetic
interactions, the polar gas is unstable. Finally we explore coherent population
dynamics in a collisionless transversely polarized gas.Comment: 10 pages, 7 figure
Landau damping in a collisionless dipolar Bose gas
We present a theory for the Landau damping of low energy quasi-particles in a
collisionless, quasi-2D dipolar Bose gas and produce expressions for the
damping rate in uniform and non-uniform systems. Using simple energy-momentum
conservation arguments, we show that in the homogeneous system, the nature of
the low energy dispersion in a dipolar Bose gas severely inhibits Landau
damping of long wave-length excitations. For a gas with contact and dipolar
interactions, the damping rate for phonons tends to decrease with increasing
dipolar interactions; for strong dipole-dipole interactions, phonons are
virtually undamped over a broad range of temperature. The damping rate for
maxon-roton excitations is found to be significantly larger than the damping
rate for phonons.Comment: 11 pages, 4 figure
Dynamics of correlations in a quasi-2D dipolar Bose gas following a quantum quench
We study the evolution of correlations in a quasi-2D dipolar gas driven
out-of-equilibrium by a sudden ramp of the interaction strength. For
sufficiently strong ramps, the momentum distribution, excited fraction and
density-density correlation function all display pronounced features that are
directly related to the appearance of a roton minimum in the underlying
spectrum. Our study suggests that the evolution of correlations following a
quench can be used as a probe of roton-like excitations in a dipolar gas. We
also find that the build up of density-density correlations following a quench
occurs much more slowly in the dipolar gas compared to a non-dipolar gas, owing
to the long-range interactions.Comment: 8 pages, 4 figures. Expanded to include self-consistent Bogoliubov
ansatz. Significance of results to experiments discusse
Coarsening dynamics of binary Bose condensates
We study the dynamics of domain formation and coarsening in a binary
Bose-Einstein condensate that is quenched across a miscible-immiscible phase
transition. The late-time evolution of the system is universal and governed by
scaling laws for the correlation functions. We numerically determine the
scaling forms and extract the critical exponents that describe the growth rate
of domain size and autocorrelations. Our data is consistent with inviscid
hydrodynamic domain growth, which is governed by a universal dynamical critical
exponent of . In addition, we analyze the effect of domain wall
configurations which introduce a nonanalytic term in the short-distance
structure of the pair correlation function, leading to a high-momentum
"Porod"-tail in the static structure factor, which can be measured
experimentally
Interaction-Tuned Dynamical Transitions in a Rashba Spin-Orbit Coupled Fermi Gas
We consider the time evolution of the magnetization in a Rashba
spin-orbit-coupled Fermi gas, starting from a fully-polarized initial state. We
model the dynamics using a Boltzmann equation, which we solve in the
Hartree-Fock approximation. The resulting non-linear system of equations gives
rise to three distinct dynamical regimes with qualitatively different
asymptotic behaviors of the magnetization at long times. The distinct regimes
and the transitions between them are controlled by the interaction strength:
for weakly interacting fermions, the magnetization decays to zero. For
intermediate interactions, it displays undamped oscillations about zero and for
strong interactions, a partially magnetized state is dynamically stabilized.
The dynamics we find is a spin analog of interaction induced self-trapping in
double-well Bose Einstein condensates. The predicted phenomena can be realized
in trapped Fermi gases with synthetic spin-orbit interactions.Comment: 5 pages, 3 figure
Stoner ferromagnetism in a thermal pseudospin-1/2 Bose gas
We compute the finite-temperature phase diagram of a pseudospin- Bose
gas with contact interactions, using two complementary methods: the random
phase approximation (RPA) and self-consistent Hartree-Fock theory. We show that
the inter-spin interactions, which break the (pseudo) spin-rotational symmetry
of the Hamiltonian, generally lead to the appearance of a magnetically ordered
phase at temperatures above the superfluid transition. In three dimensions, we
predict a normal easy-axis/easy-plane ferromagnet for sufficiently strong
repulsive/attractive inter-species interactions respectively. The normal
easy-axis ferromagnet is the bosonic analog of Stoner ferromagnetism known in
electronic systems. For the case of inter-spin attraction, we also discuss the
possibility of a \textit{bosonic} analog of the Cooper paired phase. This state
is shown to significantly lose in energy to the transverse ferromagnet in three
dimensions, but is more energetically competitive in lower dimensions.
Extending our calculations to a spin-orbit-coupled Bose gas with equal Rashba
and Dresselhaus-type couplings (as recently realized in experiment), we
investigate the possibility of stripe ordering in the normal phase. Within our
approximations however, we do not find an instability towards stripe formation,
suggesting that the stripe order melts below the condensation temperature,
which is consistent with the experimental observations of Ji \textit{et al.}
[Ji \textit{et al.}, Nature Physics \textbf{10}, 314 (2014)].Comment: 5 pages, 3 figures. published versio
Strong correlation effects in a two-dimensional Bose gas with quartic dispersion
Motivated by the fundamental question of the fate of interacting bosons in
flat bands, we consider a two-dimensional Bose gas at zero temperature with an
underlying quartic single-particle dispersion in one spatial direction. This
type of band structure can be realized using the NIST scheme of spin-orbit
coupling [Y.-J. Lin, K. Jim\'{e}nez-Garcia, and I. B. Spielman, Nature
, 83 (2011)], in the regime where the lower band dispersion has
the form , or using
the shaken lattice scheme of Parker [C. V. Parker, L.-C. Ha
and C. Chin, Nature Physics , 769 (2013)]. We numerically compare
the ground state energies of the mean-field Bose-Einstein condensate (BEC) and
various trial wave-functions, where bosons avoid each other at short distances.
We discover that, at low densities, several types of strongly correlated states
have an energy per particle (), which scales with density () as
, in contrast to for the weakly
interacting Bose gas. These competing states include a Wigner crystal,
quasi-condensates described in terms properly symmetrized fermionic states, and
variational wave-functions of Jastrow type. We find that one of the latter has
the lowest energy among the states we consider. This Jastrow-type state has a
strongly reduced, but finite condensate fraction, and true off-diagonal long
range order, which suggests that the ground state of interacting bosons with
quartic dispersion is a strongly-correlated condensate reminiscent of
superfluid Helium-4. Our results show that even for weakly-interacting bosons
in higher dimensions, one can explore the crossover from a weakly-coupled BEC
to a strongly-correlated condensate by simply tuning the single particle
dispersion or density.Comment: 10 pages, 1 figur
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