Ultracold Fermions With Strong Interactions
We have studied ultracold fermions with both attractive and repulsive interactions. Fermions with attractive interactions undergo a phase transition to superfluidity. For repulsive interactions, a transition to a ferromagnetic phase has been predicted – the so-called Stoner model for itinerant ferromagnetism. However, any strong short-range repulsive potential necessarily features a weakly bound paired state. Our experiments show that the ferromagnetic transition does not take place, but is rather preempted by rapid pair formation. Our experimental study can be regarded as a quantum simulation of a Hamiltonian, for which even the qualitative phase diagram cannot be reliably obtained by computational methods.