Observation of a Smooth Polaron-Molecule Transition in a Degenerate Fermi Gas

Abstract

Understanding the behavior of an impurity strongly interacting with a Fermi sea is a long-standing challenge in many-body physics. When the interactions are short ranged, two vastly different ground states exist: a polaron quasiparticle and a molecule dressed by the majority atoms. In the single-impurity limit, it is predicted that at a critical interaction strength, a first-order transition occurs between these two states. Experiments, however, are always conducted in the finite temperature and impurity density regime. The fate of the polaron-to-molecule transition under these conditions, where the statistics of quantum impurities and thermal effects become relevant, is still unknown. Here, we address this question experimentally and theoretically. Our experiments are performed with a spin-imbalanced ultracold Fermi gas with tunable interactions. Utilizing a novel Raman spectroscopy combined with a high-sensitivity fluorescence detection technique, we isolate the quasiparticle contribution and extract the polaron energy, spectral weight, and the contact parameter. As the interaction strength is increased, we observe a continuous variation of all observables, in particular a smooth reduction of the quasiparticle weight as it goes to zero beyond the transition point. Our observation is in good agreement with a theoretical model where polaron and molecule quasiparticle states are thermally occupied according to their quantum statistics. At the experimental conditions, polaron states are hence populated even at interactions where the molecule is the ground state and vice versa. The emerging physical picture is thus that of a smooth transition between polarons and molecules and a coexistence of both in the region around the expected transition. Our findings establish Raman spectroscopy as a powerful experimental tool for probing the physics of mobile quantum impurities and shed new light on the competition between emerging fermionic and bosonic quasiparticles in non-Fermi-liquid phases.

Impurities in systems of noninteracting trapped fermions

“…We restrict ourselves here to the case of immobile impurities. The case of mobile impurities has been extensively studied for fermionic systems both theoretically [30][31][32][33][34][35][36] and experimentally [37]. The set up of immobile impurities that we study in this paper is more difficult to access experimentally, however it has been suggested that impurities could be introduced by superimposing an optical lattice on an overall trapping potential [38].…”

Ultra-Dilute Gas of Polarons in a Bose–Einstein Condensate

“…Finite temperature effect may favor the stabilization of the system against collapse [57]. In addition, both a non-negligible concentration of impurities and finite temperature effects may combine, changing the polaron properties drastically in comparison with the single impurity case at T = 0, as recently revealed for Fermi polarons [58]. Finally, another avenue is studying the role of the direct impurity-impurity interaction and its influence on forming a gas or liquid of polarons and the role of bosonic quasi-particles in the formation of self-bound structures [59][60][61].…”

Topological transport of mobile impurities

“…In the present polaron context, this measurement would have to be performed in a state-dependent manner to extract the Hall drag. In addition, one could conduct either a state-dependent time-of-flight measurement [39,40], or Raman spectroscopy (as recently implemented for polarons [41]), to infer the in-trap momentum distribution of the impurity, in view of evaluating the current response of the impurity to an applied force.…”