Probing the Interface of a Phase-Separated State in a Repulsive Bose-Fermi Mixture

Abstract

We probe the interface between a phase-separated Bose-Fermi mixture consisting of a small Bose-Einstein condensate of ^{41}K residing in a large Fermi sea of ^{6}Li. We quantify the residual spatial overlap between the two components by measuring three-body recombination losses for variable strength of the interspecies repulsion. A comparison with a numerical mean-field model highlights the importance of the kinetic energy term for the condensed bosons in maintaining the thin interface far into the phase-separated regime. Our results demonstrate a corresponding smoothing of the phase transition in a system of finite size.

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

“…This reduces thermal decoherence effects as observed in studies of impurities coupled to the Fermi sea [41] and opens up the possibility of observing new phenomena [29], such as multiple particle-hole excitations and the onset of the orthogonality catastrophe [62]. Moreover, we are interested in the collective zero-temperature dynamics of bosonic impurities in the Fermi sea close to an interspecies Feshbach resonance [63].…”

Breathing mode of a Bose-Einstein condensate repulsively interacting with a fermionic reservoir

“…Our system, realized with optically trapped 41 K bosons and 6 Li fermions (see Fig. 1), offers tunable interspecies interaction and allows us to explore the regime of strong repulsion, where the BEC phase separates from the surrounding fermions [41]. As a dynamic phenomenon resulting from phase separation, we demonstrate the emergence of a drastic up-shift of the BEC's breathing mode frequency.…”

“…and m f are the boson and the fermion masses respectively, k F = 2m f E F / 2 is the Fermi wave number, and a bb is the boson-boson scattering length. We note that, under the realistic experimental conditions of a system of finite size, the phase transition is smoothed by the kinetic energy of the BEC [41].…”

“…As a basic example, the center-of-mass oscillations of different components (their so-called dipole modes), which can experience frequency shifts and damping [26][27][28], have been utilized in recent experiments to study coupling effects in mixed superfluids [29][30][31][32] and to investigate mediated interactions [33]. In the case of strong interspecies interactions, instabilities (collapse [34,35] or phase separation [36][37][38][39][40][41]) render the collective dynamics even more complex. Although the understanding of collective behavior near instabilities is essential in view of proposed fermionic superfluids based on mediated pairing [20,[42][43][44][45][46][47], corresponding experimental information is rather scarce.…”

“…We define a depletion scattering length a d as the value at which the fermion density drops to zero in the center of the trap, where one finds the highest boson density. The value of a d is trap specific and depends on the densities of the components, so we obtain it numerically [41]. We also define a critical scattering length…”