Crossover polarons in an interacting Fermi superfluid: how to prevent a catastrophe predicted by Anderson
Time: 1pm AEST - Melbourne, Sydney, Canberra, Brisbane; 3pm NZST
Speaker: Dr Jia Wang
Affiliation: Swinburne University of Technology
Abstract
We investigate the system of a heavy impurity embedded in a paired two-component Fermi gas at the crossover from a Bose-Einstein condensate (BEC) to a Bardeen-Cooper-Schrieffer (BCS) superfluid. We describe the Fermi superfluid through the conventional Bardeen-Cooper-Schrieffer (BCS) theory and investigate the role of the pairing gap. We calculate the system's Ramsey response and radio-frequency spectroscopy by extending the functional determinant approach (FDA), an in-principal exact numerical approach.
In a non-interacting ideal Fermi gas, a mobile impurity typically couples with a few particle-hole pairs and forms a celebrated quasiparticle: a polaron. In contrast, an infinitely heavy (or static) impurity can excite many particle-hole pairs close to the Fermi surfaces without costing recoil energy, leading to the occurrence of the well-known phenomenon: Anderson's orthogonality catastrophe (OC). OC means that the many-particle states with and without impurity become orthogonal, giving vanishing quasiparticle residues.
In a BCS superfluid, the pairing gap plays the same role as recoiling energy to suppress the excitation of multiple particle-hole pairs and prevent the OC. Consequently, using FDA, we can rigorously confirm the remarkable features such as dark continuum, molecule-hole continuum, and repulsive polaron. In addition, for a magnetic impurity scattering at finite temperature, we predict additional resonances related to the subgap Yu-Shiba-Rusinov bound state, whose positions can be used to measure the superfluid pairing gap. We surprisingly find undamped repulsive polarons for a nonmagnetic scattering at zero temperature. These exact results might be readily observed in quantum gas experiments with mixtures of two different species of atoms that have a large-mass ratio.
About Australia - New Zealand Cold Atom Seminars
This new initiative hosts a seminar once a month for the Australia – New Zealand cold atom research community. The purpose is to encourage and facilitate ongoing discussions and collaboration in the local community. Talks should be less than 40 minutes in length to allow ample time for questions and discussion.
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