In order to investigate the low-energy physics of a system composed of an impurity immersed in a many-body medium, one may study the properties of the quasiparticle formed by the impurity dressed by excitations of the surrounding medium : the so-called polaron. This idea was first introduced in solid-state physics by Landau and Pekar in the 1950’s to study the coupling of electrons to a crystalline lattice. Decades after these first theoretical studies, the development of the field of ultracold atoms provided a variety of experimental setups for the study of polaron physics : an impurity in a weakly interacting Bose-Einstein Condensate (BEC) forms a Bose polaron, whereas a Fermi polaron is obtained when the surrounding medium is a Fermi sea. By choosing the surrounding medium to be a 2-component Fermi superfluid, we can interpolate between these two types of physics along the BEC-BCS crossover.
We present a calculation of the energy of an impurity weakly coupled to a spin-1/2 fermionic superfluid based on Random-Phase Approximation. We show that it is necessary to go beyond mean-field BCS theory in order to have a proper description of the excitations of the many-body background, allowing us to cure the divergences caused by three-body effects. RPA allows us to describe the excitations of the system along the BEC-BCS crossover and to regularize the energy of the impurity. These effects should be experimentally observable, for example in the case of a small BEC immersed in a Fermi superfluid, through a measurement of the frequency shift of the oscillations of the impurity in an external potential.
|Presenter name||Arnaud Bigué|
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