6 research outputs found
Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture
We investigate the properties of a strongly interacting imbalanced mixture of
bosonic K impurities immersed in a Fermi sea of ultracold Li atoms.
This enables us to explore the Fermi polaron for large impurity concentrations
including the case where they form a Bose-Einstein condensate. The system is
characterized by means of radio-frequency injection spectroscopy for tunable
interactions using an interspecies Feshbach resonance. We find that the energy
of the Fermi polarons formed in the thermal fraction of the impurity cloud
remains rather insensitive to the impurity concentration, even as we approach
equal densities for both species. The apparent insensitivity to high
concentration is consistent with the theoretical prediction, based on Landau's
quasiparticle theory, of a weak effective interaction between the polarons. The
condensed fraction of the bosonic K gas is much denser than its thermal
component, which leads to a break-down of the Fermi polaron description.
Instead, we observe a new branch in the radio-frequency spectrum with a small
energy shift, which is consistent with the presence of Bose polarons formed by
Li fermions inside the K condensate. A closer investigation of the
behavior of the condensate by means of Rabi oscillation measurements support
this observation, indicating that we have realized Fermi and Bose polarons, two
fundamentally different quasiparticles, in one cloud.Comment: 16 pages, 10 figure
Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture
We investigate the properties of a strongly interacting imbalanced mixture of bosonic 41K impurities immersed in a Fermi sea of ultracold 6 Li atoms. This enables us to explore the Fermi polaron scenario for large impurity concentrations including the case where they form a Bose-Einstein condensate. The system is characterized by means of radio-frequency injection spectroscopy and interspecies interactions are widely tunable by means of a well-characterized Feshbach resonance. We find that the energy of the Fermi polarons formed in the thermal
fraction of the impurity cloud remains rather insensitive to the impurity concentration, even as we approach
equal densities for both speciesPostprint (published version
Mediated interactions between Fermi polarons and the role of impurity quantum statistics
The notion of quasi-particles is essential for understanding the behaviour of
complex many-body systems. A prototypical example of a quasi-particle, a
polaron, is an impurity strongly interacting with a surrounding medium. Fermi
polarons, created in a Fermi sea, provide a paradigmatic realization of this
concept. As an inherent and important property such quasi-particles interact
with each other via modulation of the medium. While quantum simulation
experiments with ultracold atoms have significantly improved our understanding
of individual polarons, the detection of their interactions has remained
elusive in these systems. Here, we report the unambiguous observation of
mediated interactions between Fermi polarons consisting of K impurities
embedded in a Fermi sea of Li atoms. Our results confirm two landmark
predictions of Landau's Fermi-liquid theory: the shift of the polaron energy
due to mediated interactions, linear in the concentration of impurities, and
its sign inversion with impurity quantum statistics. For weak to moderate
interactions between the impurities and the medium, we find excellent agreement
with the static (zero-momentum and energy) predictions of Fermi-liquid theory.
For stronger impurity-medium interactions, we show that the observed behaviour
at negative energies can be explained by a more refined many-body treatment
including retardation and molecule formatio