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

Abstract: 

To analyze the motion of an electron through an ionic lattice, Lev 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 simplest 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 between the majority and minority particles increases. However, most theories predict a first-order phase transition to a molecular ground state beyond a critical interaction strength. We study this scenario 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 from the minority atoms are inherently 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 first-order phase transition, but rather a smooth transition. Our measurements seems to favor a physical picture of co-existence of both polarons and molecules near the expected transition.

​

Event Organizer: Dr. Iair Arcavi