The fate of the polaron in a strongly-interacting Fermi gas

To analyze the motion of an electron through an ionic lattice, L. Landau suggested treating the electron and the phonons that accompany its movement as a new quasi-particle named “polaron”. The concept of polaron was found to be applicable in many other systems, including semiconductors, high-temperature superconductors, alkali halides insulators, transition metal oxides, and 2D materials embedded in a cavity. One of the most straightforward scenarios in which polarons naturally emerge is when a single spin impurity is weakly-interacting with a sea of opposite spins. The polaron ground state persists even as the interaction increases, but beyond a critical interaction strength, a first-order phase transition to a molecular ground state is predicted to occur. We study the impurity problem with an ultracold Fermi gas, which is ideally suitable to this end thanks to extremely long spin-relaxation times and tunability of the interaction. Experimentally, the impurity problem poses a challenge: the signals are naturally very weak. To overcome this difficulty, we have developed novel sensitive rf and Raman spectroscopic techniques, which are based on fluorescence detection. The physical picture that arises from our measurements is quite intriguing. We do not observe a signature of a first-order phase transition, but instead, the behavior seems to support a co-existence of both polarons and molecules near the expected transition. I will present our experiments and place them in the broader context of quantum simulations with ultracold atoms.