660 research outputs found
Supersolid phase with cold polar molecules on a triangular lattice
We study a system of heteronuclear molecules on a triangular lattice and
analyze the potential of this system for the experimental realization of a
supersolid phase. The ground state phase diagram contains superfluid, solid and
supersolid phases. At finite temperatures and strong interactions there is an
additional emulsion region, in contrast to similar models with short-range
interactions. We derive the maximal critical temperature and the
corresponding entropy for supersolidity and find feasible
experimental conditions for its realization.Comment: 4 pages, 4 figure
Out-of-equilibrium states and quasi-many-body localization in polar lattice gases
The absence of energy dissipation leads to an intriguing out-of-equilibrium
dynamics for ultracold polar gases in optical lattices, characterized by the
formation of dynamically-bound on-site and inter-site clusters of two or more
particles, and by an effective blockade repulsion. These effects combined with
the controlled preparation of initial states available in cold gases
experiments can be employed to create interesting out-of-equilibrium states.
These include quasi-equilibrated effectively repulsive 1D gases for attractive
dipolar interactions and dynamically-bound crystals. Furthermore,
non-equilibrium polar lattice gases can offer a promising scenario for the
study of many-body localization in the absence of quenched disorder. This
fascinating out-of-equilibrium dynamics for ultra-cold polar gases in optical
lattices may be accessible in on-going experiments.Comment: 5+1 pages, 4+1 figure
Beyond the Landau Criterion for Superfluidity
According to the Landau criterion for superfluidity, a Bose-Einstein
condensate flowing with a group velocity smaller than the sound velocity is
energetically stable to the presence of perturbing potentials. We found that
this is strictly correct only for vanishingly small perturbations. The
superfluid critical velocity strongly depends on the strength and shape of the
defect. We quantitatively study, both numerically and with an approximate
analytical model, the dynamical response of a one-dimensional condensate
flowing against an istantaneously raised spatially periodic defect. We found
that the critical velocity decreases by incresing the strength of the
defect , up to to a critical value of the defect intensity where the
critical velocity vanishes
Two-body bound and edge states in the extended SSH Bose-Hubbard model
We study the bosonic two-body problem in a Su-Schrieffer-Heeger dimerized
chain with on-site and nearest-neighbor interactions. We find two classes of
bound states. The first, similar to the one induced by on-site interactions,
has its center of mass on the strong link, whereas the second, existing only
thanks to nearest-neighbors interactions, is centered on the weak link. We
identify energy crossings between these states and analyse them using exact
diagonalization and perturbation theory. In the presence of open boundary
conditions, novel strongly-localized edge-bound states appear in the spectrum
as a consequence of the interplay between lattice geometry, on-site and
nearest-neighbor interactions. Contrary to the case of purely on-site
interactions, such EBS persist even in the strongly interacting regime.Comment: 12 pages, 8 figures; Submitted to EPJ Special Topics, Quantum Gases
and Quantum Coherenc
Metastable states of a gas of dipolar bosons in a 2D optical lattice
We investigate the physics of dipolar bosons in a two dimensional optical
lattice. It is known that due to the long-range character of dipole-dipole
interaction, the ground state phase diagram of a gas of dipolar bosons in an
optical lattice presents novel quantum phases, like checkerboard and supersolid
phases. In this paper, we consider the properties of the system beyond its
ground state, finding that it is characterised by a multitude of almost
degenerate metastable states, often competing with the ground state. This makes
dipolar bosons in a lattice similar to a disordered system and opens
possibilities of using them for quantum memories.Comment: small improvements in the text, Fig.4 replaced, added and updated
references. 4 pages, 4 figures, to appear in Phys. Rev. Let
Late-time expansion in the semiclassical theory of the Hawking radiation
We give a detailed treatment of the back-reaction effects on the Hawking
spectrum in the late-time expansion within the semiclassical approach to the
Hawking radiation. We find that the boundary value problem defining the action
of the modes which are regular at the horizon admits in general the presence of
caustics. We show that for radii less that a certain critical value no
caustic occurs for all values of the wave number and time and we give a
rigorous lower bound on such a critical value. We solve the exact system of non
linear equations defining the motion, by an iterative procedure rigorously
convergent at late times. The first two terms of such an expansion give the
correction to the Hawking spectrum.Comment: 17 pages, 1 figure, LaTex, typos corrected, one intermediate formula
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Particle-hole character of the Higgs and Goldstone modes in strongly-interacting lattice bosons
We study the low-energy excitations of the Bose-Hubbard model in the
strongly-interacting superfluid phase using a Gutzwiller approach and extract
the single-particle and single-hole excitation amplitudes for each mode. We
report emergent mode-dependent particle-hole symmetry on specific arc-shaped
lines in the phase diagram connecting the well-known Lorentz-invariant limits
of the Bose-Hubbard model. By tracking the in-phase particle-hole symmetric
oscillations of the order parameter, we provide an answer to the long-standing
question about the fate of the pure amplitude Higgs mode away from the
integer-density critical point. Furthermore, we point out that out-of-phase
oscillations are responsible for a full suppression of the condensate density
oscillations of the gapless Goldstone mode. Possible detection protocols are
also discussed.Comment: 6 pages, 3 figure
Nonlinear effects for Bose Einstein condensates in optical lattices
We present our experimental investigations on the subject of dynamical
nonlinearity-induced instabilities and of nonlinear Landau-Zener tunneling
between two energy bands in a Rubidium Bose-Einstein condensate in an
accelerated periodic potential. These two effects may be considered two
different regimes (for small and large acceleration) of the same physical
system and studied with the same experimental protocol. Nonlinearity introduces
an asymmetry in Landau-Zener tunneling; as a result, tunneling from the ground
state to the excited state is enhanced whereas in the opposite direction it is
suppressed. When the acceleration is lowered, the condensate exhibits an
unstable behaviour due to nonlinearity. We also carried out a full numerical
simulation of both regimes integrating the full Gross-Pitaevskii equation; for
the Landau-Zener effect we also used a simple two-level model. In both cases we
found good agreement with the experimental results.Comment: 9 pages, 7 figures. Submitted to Laser Physic
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