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Abstract

An impurity interacting with an ultracold Fermi gas can form either a polaron
state or a dressed molecular state in which the impurity forms a bound state
with one gas particle. This molecular state features rich physics, including a
first-order transition to the polaron state and a negative effective mass at
small interactions. However, these features have remained so far experimentally
inaccessible. In this work we show theoretically how the molecular state can be
directly prepared experimentally even in its excited state using
state-of-the-art cold atom Raman spectroscopy techniques. Initializing the
system in the ultra-strong coupling limit, where the binding energy of the
molaron is much larger than the Fermi energy, our protocol maps out the
momentum-dependent spectral function of the molecule. Using a diagrammatic
approach we furthermore show that the molecular spectral function serves as a
direct precursor of the elusive Fulde-Ferell-Larkin-Ovchinnikov phase, which is
realized for a finite density of fermionic impurity particles. Our results pave
the way to a systematic understanding of how composite particles form in
quantum many-body environments and provide a basis to develop new schemes for
the observation of exotic phases of quantum many-body systems.