34 research outputs found
Tuning the structural and dynamical properties of a dipolar Bose-Einstein condensate: Ripples and instability islands
It is now well established that the stability of aligned dipolar Bose gases
can be tuned by varying the aspect ratio of the external harmonic confinement.
This paper extends this idea and demonstrates that a Gaussian barrier along the
strong confinement direction can be employed to tune both the structural
properties and the dynamical stability of an oblate dipolar Bose gas aligned
along the strong confinement direction. In particular, our theoretical
mean-field analysis predicts the existence of instability islands immersed in
otherwise stable regions of the phase diagram. Dynamical studies indicate that
these instability islands, which can be probed experimentally with present-day
technology, are associated with the going soft of a Bogoliubov--de Gennes
excitation frequency with radial breathing mode character. Furthermore, we find
dynamically stable ground state densities with ripple-like oscillations along
the radial direction. These structured ground states exist in the vicinity of a
dynamical radial roton-like instability.Comment: 9 pages, 11 figure
Few-body bound states in dipolar gases and their detection
We consider dipolar interactions between heteronuclear molecules in a
low-dimensional setup consisting of two one-dimensional tubes. We demonstrate
that attraction between molecules in different tubes can overcome intratube
repulsion and complexes with several molecules in the same tube are stable. In
situ detection schemes of the few-body complexes are proposed. We discuss
extensions to the case of many tubes and layers, and outline the implications
of our results on many-body physics.Comment: Published versio
Few-Body Bound Complexes in One-dimensional Dipolar Gases and Non-Destructive Optical Detection
We consider dipolar interactions between heteronuclear molecules in
low-dimensional geometries. The setup consists of two one-dimensional tubes. We
study the stability of possible few-body complexes in the regime of repulsive
intratube interaction, where the binding arises from intertube attraction. The
stable dimers, trimers, and tetramers are found and we discuss their properties
for both bosonic and fermionic molecules. To observe these complexes we propose
an optical non-destructive detection scheme that enables in-situ observation of
the creation and dissociation of the few-body complexes. A detailed description
of the expected signal of such measurements is given using the numerically
calculated wave functions of the bound states. We also discuss implications on
the many-body physics of dipolar systems in tubular geometries, as well as
experimental issues related to the external harmonic confinement along the tube
and the prospect of applying an in-tube optical lattice to increase the
effective dipole strength.Comment: 16 pages, 15 figures, published versio
Ultracold Dipolar Gases in Optical Lattices
This tutorial is a theoretical work, in which we study the physics of
ultra-cold dipolar bosonic gases in optical lattices. Such gases consist of
bosonic atoms or molecules that interact via dipolar forces, and that are
cooled below the quantum degeneracy temperature, typically in the nK range.
When such a degenerate quantum gas is loaded into an optical lattice produced
by standing waves of laser light, new kinds of physical phenomena occur. These
systems realize then extended Hubbard-type models, and can be brought to a
strongly correlated regime. The physical properties of such gases, dominated by
the long-range, anisotropic dipole-dipole interactions, are discussed using the
mean-field approximations, and exact Quantum Monte Carlo techniques (the Worm
algorithm).Comment: 56 pages, 26 figure
Density wave instabilities of tilted fermionic dipoles in a multilayer geometry
We consider the density wave instability of fermionic dipoles aligned by an
external field, and moving in equidistant layers at zero temperature. Using a
conserving Hartree-Fock approximation, we show that correlations between
dipoles in different layers significantly decrease the critical coupling
strength for the formation of density waves when the distance between the
layers is comparable to the inter-particle distance within each layer. This
effect, which is strongest when the dipoles are oriented perpendicular to the
planes, causes the density waves in neighboring layers to be in-phase for all
orientations of the dipoles. We furthermore demonstrate that the effects of the
interlayer interaction can be understood from a classical model. Finally, we
show that the interlayer correlations are important for experimentally relevant
dipolar molecules, including the chemically stable NaK and
KCs, where the density wave regime is within experimental reach.Comment: 18 pages, 11 figures; new version with expanded discussion on
experimental relevance including one new figur
Phase diagrams of the 2D t-t'-U Hubbard model from an extended mean field method
It is well-known from unrestricted Hartree-Fock computations that the 2D
Hubbard model does not have homogeneous mean field states in significant
regions of parameter space away from half filling. This is incompatible with
standard mean field theory. We present a simple extension of the mean field
method that avoids this problem. As in standard mean field theory, we restrict
Hartree-Fock theory to simple translation invariant states describing
antiferromagnetism (AF), ferromagnetism (F) and paramagnetism (P), but we use
an improved method to implement the doping constraint allowing us to detect
when a phase separated state is energetically preferred, e.g. AF and F
coexisting at the same time. We find that such mixed phases occur in
significant parts of the phase diagrams, making them much richer than the ones
from standard mean field theory. Our results for the 2D t-t'-U Hubbard model
demonstrate the importance of band structure effects.Comment: 6 pages, 5 figure