308 research outputs found
Heteronuclear molecules in an optical lattice: Theory and experiment
We study properties of two different atoms at a single optical lattice site
at a heteronuclear atomic Feshbach resonance. We calculate the energy spectrum,
the efficiency of rf association and the lifetime as a function of magnetic
field and compare the results with the experimental data obtained for K-40 and
Rb-87 [C. Ospelkaus et al., Phys. Rev. Lett. 97, 120402 (2006)]. We treat the
interaction in terms of a regularized delta function pseudopotential and
consider the general case of particles with different trap frequencies, where
the usual approach of separating center-of-mass and relative motion fails. We
develop an exact diagonalization approach to the coupling between
center-of-mass and relative motion and numerically determine the spectrum of
the system. At the same time, our approach allows us to treat the anharmonicity
of the lattice potential exactly. Within the pseudopotential model, the center
of the Feshbach resonance can be precisely determined from the experimental
data.Comment: 9 pages, 7 figures, revised discussion of transfer efficienc
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
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
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
Thermal-assisted Anisotropy and Thermal-driven Instability in the Superfluidity state of Two-Species Polar Fermi Gas
We study the superfluid state of two-species heteronuclear Fermi gases with
isotropic contact and anisotropic long-range dipolar interactions. By
explicitly taking account of Fock exchange contribution, we derive
self-consistent equations describing the pairing states in the system.
Exploiting the symmetry of the system, we developed an efficient way of solving
the self-consistent equations by exploiting the symmetries. We find that the
temperature tends to increase the anisotropy of the pairing state, which is
rather counterintuitive. We study the anisotropic properties of the system by
examining the angular dependence of the number density distribution, the
excitation spectrum and the pair correlation function. The competing effects of
the contact interaction and the dipolar interaction upon the anisotropy are
revealed. We derive and compute the superfluid mass density for the
system. Astonishingly, we find that becomes negative above some
certain temperature (), signaling some instability of the system.
This suggests that the elusive FFLO state may be observed in experiments, due
to an anisotropic state with a spontaneously generated superflow.Comment: 7 pages, 5 figure
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
- …