199 research outputs found
Do mixtures of bosonic and fermionic atoms adiabatically heat up in optical lattices?
Mixtures of bosonic and fermionic atoms in optical lattices provide a
promising arena to study strongly correlated systems. In experiments realizing
such mixtures in the quantum degenerate regime the temperature is a key
parameter. In this work, we investigate the intrinsic heating and cooling
effects due to an entropy-preserving raising of the optical lattice potential.
We analyze this process, identify the generic behavior valid for a wide range
of parameters, and discuss it quantitatively for the recent experiments with
87Rb and 40K atoms. In the absence of a lattice, we treat the bosons in the
Hartree-Fock-Bogoliubov-Popov-approximation, including the fermions in a
self-consistent mean field interaction. In the presence of the full
three-dimensional lattice, we use a strong coupling expansion. As a result of
the presence of the fermions, the temperature of the mixture after the lattice
ramp-up is always higher than for the pure bosonic case. This sheds light onto
a key point in the analysis of recent experiments.Comment: 5 pages, 3 figure
Localization of bosonic atoms by fermionic impurities in a 3d optical lattice
We observe a localized phase of ultracold bosonic quantum gases in a
3-dimensional optical lattice induced by a small contribution of fermionic
atoms acting as impurities in a Fermi-Bose quantum gas mixture. In particular
we study the dependence of this transition on the fermionic 40K impurity
concentration by a comparison to the corresponding superfluid to Mott insulator
transition in a pure bosonic 87Rb gas and find a significant shift in the
transition parameter. The observed shift is larger than expected based on a
mean-field argument, which is a strong indication that disorder-related effects
play a significant role.Comment: 4 pages, 4 figure
Density and Stability in Ultracold Dilute Boson-Fermion Mixtures
We analyze in detail recent experiments on ultracold dilute 87Rb-40K mixtures
in Hamburg and in Florence within a mean-field theory. To this end we determine
how the stationary bosonic and fermionic density profiles in this mixture
depend in the Thomas-Fermi limit on the respective particle numbers.
Furthermore, we investigate how the observed stability of the Bose-Fermi
mixture with respect to collapse is crucially related to the value of the
interspecies s-wave scattering length.Comment: Author Information under
http://www.theo-phys.uni-essen.de/tp/ags/pelster_dir
Ultracold polar molecules near quantum degeneracy
We report the creation and characterization of a near quantum-degenerate gas
of polar K-Rb molecules in their absolute rovibrational ground
state. Starting from weakly bound heteronuclear KRb Feshbach molecules, we
implement precise control of the molecular electronic, vibrational, and
rotational degrees of freedom with phase-coherent laser fields. In particular,
we coherently transfer these weakly bound molecules across a 125 THz frequency
gap in a single step into the absolute rovibrational ground state of the
electronic ground potential. Phase coherence between lasers involved in the
transfer process is ensured by referencing the lasers to two single components
of a phase-stabilized optical frequency comb. Using these methods, we prepare a
dense gas of polar molecules at a temperature below 400 nK. This
fermionic molecular ensemble is close to quantum degeneracy and can be
characterized by a degeneracy parameter of . We have measured the
molecular polarizability in an optical dipole trap where the trap lifetime
gives clues to interesting ultracold chemical processes. Given the large
measured dipole moment of the KRb molecules of 0.5 Debye, the study of quantum
degenerate molecular gases interacting via strong dipolar interactions is now
within experimental reach
The electronic system and of LiCa
High resolution Fourier transform spectroscopy and Laser induced fluorescence
has been performed on LiCa in the infrared spectral range. We analyze
rovibrational transitions of the -- system of
LiCa and find the state to be perturbed by spin-orbit coupling
to the state. We study the coupled system obtaining molecular
parameters for the and the state together with
effective spin-orbit and spin-rotation coupling constants. The coupled system
has also been evaluated by applying a potential function instead of
rovibrational molecular parameters for the state . An improved
analytic potential function of the state is derived, due to
the extension of the observed rotational ladder.Comment: 15 pages, 4 figures 2 supplement file
Collisional stability of fermionic Feshbach molecules
Using a Feshbach resonance, we create ultracold fermionic molecules starting
from a Bose-Fermi atom gas mixture. The resulting mixture of atoms and weakly
bound molecules provides a rich system for studying few-body collisions because
of the variety of atomic collision partners for molecules; either bosonic,
fermionic, or distinguishable atoms. Inelastic loss of the molecules near the
Feshbach resonance is dramatically affected by the quantum statistics of the
colliding particles and the scattering length. In particular, we observe a
molecule lifetime as long as 100 ms near the Feshbach resonance.Comment: 4 pages, 4 figures, 1 tabl
Controlling the quantum stereodynamics of ultracold bimolecular reactions
Chemical reaction rates often depend strongly on stereodynamics, namely the
orientation and movement of molecules in three-dimensional space. An ultracold
molecular gas, with a temperature below 1 uK, provides a highly unusual regime
for chemistry, where polar molecules can easily be oriented using an external
electric field and where, moreover, the motion of two colliding molecules is
strictly quantized. Recently, atom-exchange reactions were observed in a
trapped ultracold gas of KRb molecules. In an external electric field, these
exothermic and barrierless bimolecular reactions, KRb+KRb -> K2+Rb2, occur at a
rate that rises steeply with increasing dipole moment. Here we show that the
quantum stereodynamics of the ultracold collisions can be exploited to suppress
the bimolecular chemical reaction rate by nearly two orders of magnitude. We
use an optical lattice trap to confine the fermionic polar molecules in a
quasi-two-dimensional, pancake-like geometry, with the dipoles oriented along
the tight confinement direction. With the combination of sufficiently tight
confinement and Fermi statistics of the molecules, two polar molecules can
approach each other only in a "side-by-side" collision, where the chemical
reaction rate is suppressed by the repulsive dipole-dipole interaction. We show
that the suppression of the bimolecular reaction rate requires quantum-state
control of both the internal and external degrees of freedom of the molecules.
The suppression of chemical reactions for polar molecules in a
quasi-two-dimensional trap opens the way for investigation of a dipolar
molecular quantum gas. Because of the strong, long-range character of the
dipole-dipole interactions, such a gas brings fundamentally new abilities to
quantum-gas-based studies of strongly correlated many-body physics, where
quantum phase transitions and new states of matter can emerge.Comment: 19 pages, 4 figure
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