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Unveiling the polaron-to-molecule transition

P. Pieri

 

At low enough temperature, matter reveals its quantum nature. Particles having bosonic character organize their motion in a coherent way, while particles with fermionic character like to keep their individuality and stay apart.
What happens when a few bosons are immersed in a Fermi sea? For strong attraction they bind to fermions forming molecules. These particles have fermionic character, so that the coherent behavior disappears from the system and a quantum phase transition to a normal state occurs. In the limit of a single impurity this transition reduces to the polaron-to-molecule transition, which recently has been highly debated in the context of ultracold Fermi gases. In this case, however, the transition is masked by a large region of phase separation.
 
 
In the present work, G. Bertaina et al. show that for Bose-Fermi mixtures the case is dramatically different. The region of phase separation becomes very narrow at low concentration of bosons, and reduces to zero in the single impurity limit. The polaron-to-molecule transition is then finally uveiled, and its presence is shown to fully determine the equations of state of the  mixture at low concentration of bosons.