Energy, decay rate, and effective masses for a moving polaron in a Fermi sea: Explicit results in the weakly attractive limit

Laboratoire Kastler Brossel, École Normale Supérieure and CNRS, UPMC - 24 rue Lhomond, 75231 Paris, France

Received: 29 October 2013
Accepted: 29 November 2013

Abstract

We study the properties of an impurity of mass M moving through a spatially homogeneous three-dimensional fully polarized Fermi gas of particles of mass m. In the weakly attractive limit, where the effective coupling constant and perturbation theory can be used, both for a broad and a narrow Feshbach resonance, we obtain an explicit analytical expression for the complex energy of the moving impurity up to order two included in g. This also gives access to its longitudinal and transverse effective masses , , as functions of the impurity wave vector K. Depending on the modulus of K and on the impurity-to-fermion mass ratio M/m we identify four regions separated by singularities in derivatives with respect to K of the second-order term of , and we discuss the physical origin of these regions. Remarkably, the second-order term of presents points of non-differentiability, replaced by a logarithmic divergence for M = m, when K is on the Fermi surface of the fermions. We also discuss the third-order contribution and relevance for cold-atom experiments.