Two-Element Mixture of Bose and Fermi Superfluids

Abstract

We report on the production of a stable mixture of bosonic and fermionic superfluids composed of the elements 174 Yb and 6 Li which feature a strong mismatch in mass and distinct electronic properties. We demonstrate elastic coupling between the superfluids by observing the shift in dipole oscillation frequency of the bosonic component due to the presence of the fermions. The measured magnitude of the shift is consistent with a mean-field model and its direction determines the previously unknown sign of the interspecies scattering length to be positive. We also observe the exchange of angular momentum between the superfluids from the excitation of a scissors mode in the bosonic component through interspecies interactions. We explain this observation using an analytical model based on superfluid hydrodynamics.Ultracold atomic gases offer excellent opportunities to produce and investigate quantum matter, allowing fundamental studies in many-body physics and quantum simulation [1]. Beginning with the cooling of atoms to BoseEinstein condensation [2,3] and later followed by the achievement of superfluidity in atomic Fermi gases [4], such studies have found many parallels with the analogous superfluids of bosonic 4 He and fermionic 3 He as well as superconductors, familiar from condensed matter physics. While the goal of simultaneous superfluidity in mixtures of 4 He-3 He still remains elusive due to strong inter-isotope interactions [5,6], Bose-Fermi superfluidity in an atomic gas isotopic mixture of 7 Li-6 Li has recently been realized [7]. The original interest [8,9] in such dual superfluid systems is now strongly intensified [10][11][12][13][14][15][16].The extension of Bose-Fermi superfluidity to mixtures of different elements is experimentally challenging, but holds the potential to open a larger arena of scientific studies. A large mass ratio between the components is predicted to alter the interaction energy between the superfluids [12] as well as the character of excitations across the Bose-Einstein condensate to Bardeen-CooperSchreiffer (BEC-BCS) crossover for the fermion pairs [15]. Specific interaction strengths and mass ratios can aid the detection of exotic states such as the FuldeFerrell-Larkin-Ovchinnikov (FFLO) phase [10] and darkbright solitons [16]. Furthermore, species-selective potentials for relative positioning and selective addressing make two-element systems more amenable for systematic studies.In this paper, we report on the realization of a twoelement Bose-Fermi superfluid mixture of 174 Yb-6 Li. We measure their coupling by observing the interactioninduced frequency shift of the bosonic dipole mode. We also detect transfer of angular momentum between the superfluids through the excitation of a scissors mode in the bosonic component. The scattering length a F of the two-spin alkali 6 Li fermionic system is tunable across the BEC-BCS crossover through a Feshbach resonance centered at 832 G, while the scattering length a B of the alkaline-earth-like bosonic Yb remains constant throughout. The...

Thermometry of a deeply degenerate Fermi gas with a Bose-Einstein condensate

“…Extreme mass ratios [58][59][60][61] are of particular interest for pushing the accessible regime further down to temperatures on the order of 0.01 T F . However, at such ultralow temperatures, the larger number of collisions required for thermalization and the increasing Pauli blocking effect will increase the thermalization time, which will make it more difficult to reach thermal equilibrium on a realistic experimental time scale.…”

A Dual-Species Bose-Einstein Condensate with Attractive Interspecies Interactions

“…Ultracold atomic mixtures, formed by atoms of the same species in different spin states, or as different isotopes or elements, enable us to address a wide variety of problems in many-body physics. They have been largely used to investigate phase-transitions [1][2][3][4], multi-component superfluidity [5][6][7], topological defects [8][9][10], magnetism [11][12][13], ultracold chemistry [14] and impurity and polaron physics [15][16][17]. Most of these phenomena are accessible thanks to the ability to control the sign and strength of interactions, opening the way to the study of unconventional matter phases.…”

Observation of a Degenerate Fermi Gas Trapped by a Bose-Einstein Condensate

“…Mixtures of atomic quantum gases serve as an exciting platform to study a rich variety of physics, such as the observation of heteronuclear molecules [1][2][3][4][5][6][7], Bose and Fermi polarons [8][9][10][11][12], and superfluid mixtures [13][14][15]. Novel quantum phases have be suggested theoretically [16][17][18][19] and probed experimentally [20][21][22][23].…”