Universal van der Waals Force Between Heavy Polarons in Superfluids

TL;DR

This paper demonstrates that two heavy impurities in superfluid gases experience a universal long-range van der Waals-like attraction mediated by phonons, with temperature-dependent corrections analyzed via effective field theory.

Contribution

The study introduces a universal EFT-based framework for the long-range impurity interactions in superfluids, including finite temperature effects and applicability to various superfluid types.

Findings

The induced potential scales as 1/r^7 at zero temperature.

Finite temperature introduces a T^6/r correction for short distances.

At large distances, the potential behaves as T/r^6, showing a nonrelativistic van der Waals form.

Abstract

We investigate the long-range behavior of the induced Casimir interaction between two spinless heavy impurities, or polarons, in superfluid cold atomic gases. With the help of effective field theory (EFT) of a Galilean invariant superfluid, we show that the induced impurity-impurity potential at long distance universally shows a relativistic van der Waals-like attraction ($\sim 1/r^7$) resulting from the exchange of two superfluid phonons. We also clarify finite temperature effects from the same two-phonon exchange process. The temperature $T$ introduces the additional length scale $c_s/T$ with the speed of sound $c_s$. Leading corrections at finite temperature scale as $T^6/r$ for distances $r \ll c_s/T$ smaller than the thermal length. For larger distances the potential shows a nonrelativistic van der Waals behavior ($\sim T/r^6$) instead of the relativistic one. Our EFT formulation applies not only to weakly coupled Bose or Fermi superfluids but also to that composed of strongly-coupled unitary fermions with a weakly coupled impurity. The sound velocity controls the magnitude of the van der Waals potential, which we evaluate for the fermionic superfluid in the BCS-BEC crossover.