Degenerate Bose-Fermi Mixture Research Articles

Transition from a polaronic condensate to a degenerate Fermi gas of heteronuclear molecules

The interplay of quantum statistics and interactions in atomic Bose–Fermi mixtures leads to a phase diagram markedly different from pure fermionic or bosonic systems. However, investigating this phase diagram remains challenging when bosons condense due to the resulting fast interspecies loss. Here we report observations consistent with a phase transition from a polaronic to a molecular phase in a density-matched degenerate Bose–Fermi mixture. The condensate fraction, representing the order parameter of the transition, is depleted by interactions, and the build-up of strong correlations results in the emergence of a molecular Fermi gas. The features of the underlying quantum phase transition represent a new phenomenon complementary to the paradigmatic Bose–Einstein condensate/Bardeen–Cooper–Schrieffer crossover observed in Fermi systems. By driving the system through the transition, we produce a sample of sodium–potassium molecules exhibiting a large molecule-frame dipole moment in the quantum-degenerate regime.

Suppression of Unitary Three-Body Loss in a Degenerate Bose-Fermi Mixture.

We study three-body loss in an ultracold mixture of a thermal Bose gas and a degenerate Fermi gas. We find that at unitarity, where the interspecies scattering length diverges, the usual inverse-square temperature scaling of the three-body loss found in nondegenerate systems is strongly modified and reduced with the increasing degeneracy of the Fermi gas. While the reduction of loss is qualitatively explained within the few-body scattering framework, a remaining suppression provides evidence for the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions mediated by fermions between bosons. Our model based on RKKY interactions quantitatively reproduces the data without free parameters, and predicts one order of magnitude reduction of the three-body loss coefficient in the deeply Fermi-degenerate regime.

Bose-Einstein condensate immersed in a Fermi sea: Theory of static and dynamic behavior across phase separation

We theoretically study the static and dynamic behavior of a BEC immersed in a large Fermi sea of ultracold atoms under conditions of tunable interspecies interaction. The degenerate Bose-Fermi mixture is kept in an elongated trap, typical for a single-beam optical dipole trap. We focus on the case of repulsive Bose-Fermi interaction and develop mean-field models to simulate the system over a wide range of repulsion strength. We further get analytical solutions in the regimes of phase separation and weak interaction. We obtain static density profiles and the frequency of the radial breathing mode, which is an elementary dynamic phenomenon of the mixture. Our results unveil the structure of the Bose-Fermi interface and describe the origin of the frequency shift of the breathing mode when the components become phase-separated at strong repulsion. We show that the mediated interaction between bosons induced by the Fermi sea can be understood as an adiabatic second-order mean-field effect, which is valid also beyond the weak-interaction regime for relevant experimental conditions. These results are consistent with our recent observations in a mixture of $^{41}$K and $^6$Li.

A quantum degenerate Bose–Fermi mixture of 41K and 6Li

We report a new apparatus for the study of two-species quantum degenerate mixture of 41K and 6Li atoms. We develop and combine several advanced cooling techniques to achieve both a large atom number and high phase space density of the two-species atom clouds. Furthermore, we build a high efficiency two-species magnetic transport system to transfer atom clouds from the 3D magneto-optical-trap chamber to a full glass science chamber of an extreme high vacuum environment and good optical access. We perform a forced radio-frequency evaporative cooling for 41K atoms while the 6Li atoms are sympathetically cooled in an optically plugged magnetic trap. Finally, we achieve the simultaneous quantum degeneracy for the 41K and 6Li atoms. The Bose–Einstein condensate of 41K has 1.4 × 105 atoms with a condensate fraction of about 62%, while the degenerate Fermi gas of 6Li has a total atom number of 5.4 × 105 at 0.25 Fermi temperature.

Optimized Degenerate Bose—Fermi Mixture in Microgravity: DSMC Simulation of Sympathetic Cooling

Applying the direct simulation Monte Carlo (DSMC) method developed for the cold atom system, we explore the possibility of a high-efficiency sympathetic cooling process between 87Rb and 40K without gravitational force in an optical trap. The relation between the pre-cooling of Bosons and the sympathetic cooling efficiency is also studied. We find that the absence of gravitational force is beneficial to the process of sympathetic cooling. Furthermore, an inefficient pre-cooling process will in fact hamper the creation of Fermi degenerate gases. This suggests the advantages of sympathetic cooling in microgravity.

Quantum degenerate Bose-Fermi mixture of chemically different atomic species with widely tunable interactions

We have created a quantum degenerate Bose-Fermi mixture of ${}^{23}$Na and ${}^{40}$K with widely tunable interactions via broad interspecies Feshbach resonances. Over 30 Feshbach resonances between ${}^{23}$Na and ${}^{40}$K were identified, including $p$-wave multiplet resonances. The large and negative triplet background scattering length between ${}^{23}$Na and ${}^{40}$K causes a sharp enhancement of the fermion density in the presence of a Bose condensate. As explained via the asymptotic bound-state model, this strong background scattering leads to wide Feshbach resonances observed at low magnetic fields. Our work opens up the prospect to create chemically stable, fermionic ground-state molecules of ${}^{23}$Na-${}^{40}$K, where strong, long-range dipolar interactions would set the dominant energy scale.

Position swapping and pinching in Bose-Fermi mixtures with two-color optical Feshbach resonances

We examine the density profiles of the quantum degenerate Bose-Fermi mixture of $^{174}\mathrm{Yb}$-$^{173}\mathrm{Yb}$, experimentally observed recently, in the mean-field regime. In this mixture there is a possibility of tuning the Bose-Bose and Bose-Fermi interactions simultaneously using two well-separated optical Feshbach resonances, and it is a good candidate to explore phase separation in Bose-Fermi mixtures. Depending on the Bose-Bose scattering length ${a}_{\mathrm{BB}}$, as the Bose-Fermi interaction is tuned the density of the fermions is pinched or swapping with bosons occurs.

Quadrupole oscillations in a quantum degenerate Bose–Fermi mixture

We study the collective dynamics in a degenerate Bose–Fermi mixture of 174Yb and 173Yb atoms. We excite collective oscillations by a sudden reduction of the trapping confinement and observe low m=0 quadrupole oscillations of condensates in 174Yb. First the oscillations in 174Yb atoms alone are investigated, and they are well described by the time-dependent Gross–Pitaevskii equation in the Thomas–Fermi approximation. Using the same procedure the quadrupole oscillations are excited for a 174Yb–173Yb Bose–Fermi mixture. In comparing data taken with and without fermionic 173Yb atoms, the oscillation frequency of the quadrupole mode in the condensate decreases with the presence of 173Yb atoms.

Phonon spectrum and damping of a dilute quantum degenerate Bose–Fermi mixture at finite temperature

In this Letter we investigated Bose–Fermi gas mixture at finite temperature by using Green's function method. First for spin-polarized system the temperature effects on the phonon spectrum have been taken into account. Second we consider a dilute mixture of a Bose gas and a two components Fermi gas when both bosons and fermions have undergone superfluid transitions. In this case the interaction between two hyperfine Fermi components is important. In addition we calculate the dispersion of phonon excitations, including correction to the sound velocity and Landau damping. The damping rate varies as T 2 in low temperatures limit. There is also damping at absolute zero temperature due to the interaction between bosons and fermions.

New directions in degenerate dipolar molecules via collective association

We survey results on the creation of heteronuclear Fermi molecules by tuning a degenerate Bose-Fermi mixture into the neighborhood of an association resonance, either photoassociation or Feshbach, as well as the subsequent prospects for Cooper-like pairing between atoms and molecules. In the simplest case of only one molecular state, corresponding to either a Feshbach resonance or one-color photoassociation, the system displays Rabi oscillations and rapid adiabatic passage between a Bose-Fermi mixture of atoms and fermionic molecules. For two-color photoassociation, the system admits stimulated Raman adiabatic passage (STIRAP) from a Bose-Fermi mixture of atoms to stable Fermi molecules, even in the presence of particle-particle interactions. By tailoring the STIRAP sequence it is possible to deliberately convert only a fraction of the initial atoms, leaving a finite fraction of bosons behind to induce atom-molecule Cooper pairing via density fluctuations; unfortunately, this enhancement is insufficient to achieve a superfluid transition with present ultracold technology. We therefore propose the use of an association resonance that converts atoms and diatomic molecules (dimers) into triatomic molecules (trimers), which leads to a crossover from a Bose-Einstein condensate of trimers to atom-dimer Cooper pairs. Because heteronuclear dimers may possess a permanent electric dipole moment, this overall system presents an opportunity to investigate novel microscopic physics.

Raman photoassociation of Bose-Fermi mixtures and the subsequent prospects for atom-molecule Cooper pairing

We theoretically investigate Raman photoassociation of a degenerate Bose-Fermi mixture of atoms and the subsequent prospect for anomalous (Cooper) pairing between atoms and molecules. Stable fermionic molecules are created via free-bound-bound stimulated Raman adiabatic passage which, in contrast to purely bosonic systems, can occur in spite of collisions. With the leftover atomic condensate to enhance intrafermion interactions, the superfluid transition to atom-molecule Cooper pairs occurs at a temperature that is roughly an order of magnitude below what is currently feasible.

Shortcut to a Fermi-degenerate gas of molecules via cooperative association.

We theoretically examine the creation of a Fermi-degenerate gas of molecules by considering a photoassociation or Feshbach resonance applied to a degenerate Bose-Fermi mixture of atoms. This problem raises interest because, unlike bosons, fermions in general do not behave cooperatively, so that the collective conversion of a degenerate gas atoms into a macroscopic number of diatomic molecules is not to be expected. Nevertheless, we find that the coupled Fermi system displays collective Rabi-like oscillations and a rapid adiabatic passage between atoms and molecules, thereby mimicking Bose-Einstein statistics. Cooperative association of a degenerate mixture of Bose and Fermi gases could therefore serve as a shortcut to a degenerate gas of Fermi molecules.

Phonon spectrum and dynamical stability of a dilute quantum degenerate Bose-Fermi mixture.

We calculate the phonon excitation spectrum in a zero-temperature dilute boson-fermion gaseous mixture. We show how the sound velocity changes due to the boson-fermion interaction, and we determine the dynamical stability regime of a homogeneous mixture. We identify a resonant phonon-exchange interaction between the fermions as the physical mechanism leading to the instability.