Xiaoling Cui‬ (Institute of Physics, Chinese Academy of Sciences)

Nature of polaron-molecule transition in Fermi polarons

For a single impurity moving in the background of Fermi-sea atoms, whether there will be a first-order transition from polaron to molecule as the impurity-fermion attraction increases is a fundamental question. Many theories have supported the existence of such transition based on a separate treatment of polaron and molecule states. On the other hand, the experiments have never observed the signature of such an abrupt transition. In this talk, I will introduce our recent work on this problem by utilizing a unified variational ansatz on both states. The existence of polaron-molecule transition is confirmed in 3D and 2D impurity systems, and more importantly, the nature of such transition is identified as an energy competition between systems with different total momenta. Near the transition, both momenta states appear as local minima in the dispersion curve, providing the underlying mechanism for polaron and molecule coexistence in realistic experiments with a finite impurity density and at finite temperature. This theory produces quantitatively good fits to recent experimental data of 3D Fermi polarons in unitary regime.