Impurity-induced trimer formation across the BCS-BEC crossover

Abstract

By means of diffusion Monte Carlo calculations, we study the behavior of a single impurity embedded in two balanced Fermi baths across the BCS-BEC crossover. The impurity interacts attractively with both baths through Pöschl-Teller potentials of different strengths. That interaction is such that does not support an impurity--bath two-body bound state. When that attraction is sufficiently strong, we observe the formation of trimers composed of the impurity and one atom from each bath. These clusters are Borromean on the BCS side, where no two-body subsystem is bound, and evolve continuously into tango-like trimers on the BEC side. Subtracting from the total energy the binding energy of one trimer and that of the remaining bath-bath dimers restores the equation of state of the impurity-free system, supporting a dilute-cluster interpretation. The analysis of interparticle distances shows that the embedded trimer is not simply a dimer dressed by the impurity, but a genuine three-body cluster formed inside the two Fermi baths.