Polarons and dressed molecules near narrow Feshbach resonances

The properties of impurities immersed in a large Fermi sea are naturally described in terms of dressed quasiparticles: attractive and repulsive polarons, and dressed molecules. Motivated by recent experiments on narrow Feshbach resonances, we analyze here how the quasiparticle properties are affected by a non-zero resonance range. We find two interesting analytic results. For large range, the ground state energy close to resonance is shown to become perturbative in the inverse range. In the limit of broad resonance instead, we provide a new Tan's relation linking the impurity ground state energy $E_\downarrow$ to the number of atoms in its dressing cloud $ΔN$. As a corollary, at unitarity one finds $ΔN=-E_\downarrow/ε_F $, with $ε_F$ the Fermi energy of the bath.