Observation of a smooth polaron-molecule transition in a degenerate Fermi gas.

Understanding the behavior of a spin impurity strongly-interacting with a Fermi sea is a long-standing challenge in many-body physics. A central question is under what conditions the impurity can be described as a quasiparticle, called polaron. For short-range interactions and zero temperature, most theories predict that beyond a critical interaction strength, a first-order phase transition will occur from a polaronic ground state to a molecular one. We study this question with a spin-imbalanced ultracold Fermi gas with tunable interactions, utilizing a novel Raman spectroscopy probing technique that allows us to isolate the quasiparticle contribution. We find that for increasing interactions, there is a smooth transition from a polaronic to a molecular response, with no evidence of a first-order phase transition. Nonetheless, the polaron contribution almost vanishes above the expected transition point. From the Raman spectra, we determine the polaron energy, molecule binding energy, and the contact parameter. The energies agree well with theoretical predictions and connect smoothly around the vanishing point of the polaron. The contact parameter follows the molecular branch, in contrast to the prediction that it will have a clear change of behavior as the ground state changes its nature. The overall emerging physical picture based on our measurements is of a smooth transition between polarons and molecules and coexistence of both in the region around the expected phase-transition.