The Feshbach Insulator: a New State of Quantum Matter
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Feshbach resonances - namely resonances between an unbound two-body state
(atomic state) and a bound (molecular) state - are a unique tool to tune the
interaction properties of ultracold atoms. In this talk, I will show that the
coherent coupling of the atomic and molecular state, can lead to a novel
insulating phase - the Feshbach insulator - for bosons in an optical lattice
close to a narrow Feshbach resonance.  This new state of quantum matter appears
around the resonance, preventing the system from collapsing when the effective
atomic scattering length becomes negative.
Surprisingly enough, the transition from condensate to Feshbach insulator has a
characteristic first-order nature, due to the simultaneous loss of coherence in
the atomic and molecular components.
Our realistic study shows that these features appear clearly in the ground-state
phase diagram of e.g. rubidium 87 around the 414 G resonance, and they are
therefore directly amenable to experimental observation.