62 research outputs found
Emulating Molecular Orbitals and Electronic Dynamics with Ultracold Atoms
In recent years, ultracold atoms in optical lattices have proven their great
value as quantum simulators for studying strongly correlated phases and complex
phenomena in solid-state systems. Here we reveal their potential as quantum
simulators for molecular physics and propose a technique to image the
three-dimensional molecular orbitals with high resolution. The outstanding
tunability of ultracold atoms in terms of potential and interaction offer fully
adjustable model systems for gaining deep insight into the electronic structure
of molecules. We study the orbitals of an artificial benzene molecule and
discuss the effect of tunable interactions in its conjugated pi electron system
with special regard to localization and spin order. The dynamical time scales
of ultracold atom simulators are on the order of milliseconds, which allows for
the time-resolved monitoring of a broad range of dynamical processes. As an
example, we compute the hole dynamics in the conjugated pi system of the
artificial benzene molecule.Comment: 8 pages, 4 figure
Localization and delocalization of ultracold bosonic atoms in finite optical lattices
We study bosonic atoms in small optical lattices by exact diagonalization and
observe a striking similarity to the superfluid to Mott insulator transition in
macroscopic systems. The momentum distribution, the formation of an energy gap,
and the pair correlation function show only a weak size dependence. For
noncommensurate filling we reveal in deep lattices a mixture of localized and
delocalized particles, which is sensitive to lattice imperfections. Breaking
the lattice symmetry causes a Bose-glass-like behavior. We discuss the nature
of excited states and orbital effects by using an exact diagonalization
technique that includes higher bands.Comment: 8 pages, 10 figures. Published versio
Spontaneous pattern formation in an anti-ferromagnetic quantum gas
Spontaneous pattern formation is a phenomenon ubiquitous in nature, examples
ranging from Rayleigh-Benard convection to the emergence of complex organisms
from a single cell. In physical systems, pattern formation is generally
associated with the spontaneous breaking of translation symmetry and is closely
related to other symmetry-breaking phenomena, of which (anti-)ferromagnetism is
a prominent example. Indeed, magnetic pattern formation has been studied
extensively in both solid-state materials and classical liquids. Here, we
report on the spontaneous formation of wave-like magnetic patterns in a spinor
Bose-Einstein condensate, extending those studies into the domain of quantum
gases. We observe characteristic modes across a broad range of the magnetic
field acting as a control parameter. Our measurements link pattern formation in
these quantum systems to specific unstable modes obtainable from linear
stability analysis. These investigations open new prospects for controlled
studies of symmetry breaking and the appearance of structures in the quantum
domain
Evolution from a Bose-Einstein condensate to a Tonks-Girardeau gas: An exact diagonalization study
We study ground state properties of spinless, quasi one-dimensional bosons
which are confined in a harmonic trap and interact via repulsive
delta-potentials. We use the exact diagonalization method to analyze the pair
correlation function, as well as the density, the momentum distribution,
different contributions to the energy and the population of single-particle
orbitals in the whole interaction regime. In particular, we are able to trace
the fascinating transition from bosonic to fermi-like behavior in
characteristic features of the momentum distribution which is accessible to
experiments. Our calculations yield quantitative measures for the interaction
strength limiting the mean-field regime on one side and the Tonks-Girardeau
regime on the other side of an intermediate regime.Comment: 5 pages, 5 figure
Charge density wave and charge pump of interacting fermions in circularly shaken hexagonal optical lattices
We analyze strong correlation effects and topological properties of
interacting fermions with a Falicov-Kimball type interaction in circularly
shaken hexagonal optical lattices, which can be effectively described by the
Haldane-Falicov-Kimball model, using the real-space Floquet dynamical
mean-field theory (DMFT). The Haldane model, a paradigmatic model of the Chern
insulator, is experimentally relevant, because it has been realized using
circularly shaken hexagonal optical lattices. We show that in the presence of
staggering a charge density wave emerges, which is affected by interactions and
resonant tunneling. We demonstrate that interactions smear out the edge states
by introducing a finite life time of quasiparticles. Even though a general
method for calculating the topological invariant of a nonequilibrium steady
state is lacking, we extract the topological invariant using a Laughlin charge
pump set-up. We find and attribute to the dissipations into the bath connected
to every lattice site, which is intrinsic to real-space Floquet DMFT methods,
that the pumped charge is not an integer even for the non-interacting case at
very low reservoir temperatures. Furthermore, using the rate equation based on
the Floquet-Born-Markov approximation, we calculate the charge pump from the
rate equations for the non-interacting case to identify the role of the
spectral properties of the bath. Starting from this approach we propose an
experimental protocol for measuring quantized charge pumping.Comment: 13 pages, 12 figures, published versio
Frustrated quantum antiferromagnetism with ultracold bosons in a triangular lattice
We propose to realize the anisotropic triangular-lattice Bose-Hubbard model
with positive tunneling matrix elements by using ultracold atoms in an optical
lattice dressed by a fast lattice oscillation. This model exhibits frustrated
antiferromagnetism at experimentally feasible temperatures; it interpolates
between a classical rotor model for weak interaction, and a quantum spin-1/2
-model in the limit of hard-core bosons. This allows to explore
experimentally gapped spin liquid phases predicted recently [Schmied et al.,
New J. Phys. {\bf 10}, 045017 (2008)].Comment: 6 pages, as published in EP
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