Coupled dipole oscillations of a mass-imbalanced Bose-Fermi superfluid mixture

Abstract

Recent experimental realizations of superfluid mixtures of Bose and Fermi quantum gases provide a unique platform for exploring diverse superfluid phenomena. We study dipole oscillations of a double superfluid in a cigar-shaped optical dipole trap, consisting of 41 K and 6 Li atoms with a large mass imbalance, where the oscillations of the bosonic and fermionic components are coupled via the Bose-Fermi interaction. In our high-precision measurements, the frequencies of both components are observed to be shifted from the single-species ones, and exhibit unusual features. The frequency shifts of the 41 K component are upward (downward) in the radial (axial) direction, whereas the 6 Li component has down-shifted frequencies in both directions. Most strikingly, as the interaction strength is varied, the frequency shifts display a resonant-like behavior in both directions, for both species, and around a similar location at the BCS side of fermionic superfluid. These rich phenomena challenge theoretical understanding of superfluids.

Collective dipole oscillations in a mixture of Bose and Fermi superfluids in the BCS–BEC crossover

“…It is also observed that the frequency of the Bose superfluid is downshifted (upshifted) in the axial (radial) direction, while the frequencies of the Fermi superfliud are downshifted in the both directions. The variation of the frequency shift shows a nonmonotonic behavior around the BCS side [35], differently from a monotonic increase in the 7 Li- 6 Li superfluid mixture.…”

“…In the ENS experiment of 7 Li- 6 Li superfluid mixture [1], the frequency of the Bose superfluid oscillating along the axial direction is found to be downshifted, and the value of the downshift increases monotonically from the BCS side to BEC side. Subsequently, the USTC group studied both axial and radial dipole oscillations in the 41 K- 6 Li superfluid mixture which has a large mass imbalance [6]. The frequency shifts of the Bose and Fermi superfluids are both measured in high precision, and the frequency shift of the Fermi superfluid is showed to be much smaller than the bosonic counterpart.…”

“…The recent experimental realization of mixtures of Bose and Fermi superfluids in ultra-cold atoms advances an essential step to explore double superfluidity belonging to different statistics [1,2], which is a long-sought goal in liquid helium, and opens a new scenario for the physics of superfluid mixtures [3][4][5][6]. Motivated by the breakthrough experiments [1][2][3][4][5][6], many theoretical studies have been done, such as Chandrasekhar-Clogston critical polarization [7], atom-dimer scattering properties [8,9], quasiparticle excitation [10,11], counterflow instability [12], dark-bright soliton [13], and vortex [14][15][16].…”

“…Since the realization of a mixture of the Bose and Fermi superfluids in the BCS-BEC crossover [1], the experiments first concentrate on the studies of the simplest collective excitation, i.e. dipole mode [1,2,5,6], which are quite useful to obtain deep insight in physical systems [28][29][30]. For a single component, the Kohn theorem [31] states that the center-of-mass of the entire cloud oscillates back and forth in a harmonic trap with the natural frequency of the trap independent of oscillation amplitude and interatomic interaction.…”

“…For a single component, the Kohn theorem [31] states that the center-of-mass of the entire cloud oscillates back and forth in a harmonic trap with the natural frequency of the trap independent of oscillation amplitude and interatomic interaction. However, in coupled systems inter-species interaction modifies the dipole oscillations and gives rise to a rich set of new phenomena [1,2,5,6,[28][29][30][32][33][34]. A long-time dipole oscillation is first observed in a mixture of Bose and Fermi superfluids.…”