  The ground state and spectral properties of Bose gases in double-well
potentials are studied in two different scenarios: i) an interacting atomic
Bose gas, and ii) a mixture of an atomic gas interacting with diatomic
molecules. A ground state second-order quantum phase transition (QPT) is
observed in both scenarios. For large attractive values of the atom-atom
interaction, the ground-state is degenerate. For repulsive and small attractive
interaction, the ground-state is not degenerate and is well approximated by a
boson coherent state. Both systems depict an excited state quantum phase
transition (ESQPT). For the mixed atom-molecule system the critical point of
the ESQPT displays a discontinuity in the first derivative of the density of
states.
