Bound states of two bosons in an optical lattice near an association resonance

We model two bosons in an optical lattice near a Feshbach or photoassociation resonance, focusing on the Bose-Hubbard model in one dimension. Whereas the usual atoms-only theory with a tunable scattering length yields one bound state for a molecular dimer for either attractive or repulsive atom-atom interaction, an atom-molecule theory gives two bound states that may represent attractively and repulsively bound dimers occurring simultaneously. Such unusual molecular physics should be observable for an atom-molecule coupling strength comparable to the width of the dissociation continuum of the lattice dimer, for example, using narrow Feshbach resonances in Na, $^{87}$Rb, and $^{133}$Cs or low-intensity photoassociation in $^{174}$Yb.