  We study the effect of atom-molecule internal tunneling on the ground state
of atom-molecule Bose-Einstein condensates in a double-well potential. In the
absence of internal tunneling between atomic and molecular states, the ground
state is symmetric, which has equal-particle populations in two wells. From the
linear stability analysis, we show that the symmetric stationary state becomes
dynamically unstable at a certain value of the atom-molecule internal tunneling
strength. Above the critical value of the internal tunneling strength, the
ground state bifurcates to the particle-localized ground states. The origin of
this transition can be attributed to the effective attractive inter-atomic
interaction induced by the atom-molecule internal tunneling. This effective
interaction is similar to that familiar in the context of BCS-BEC crossover in
a Fermi gas with Feshbach resonance. Furthermore, we point out the possibility
of reentrant transition in the case of the large detuning between the atomic
and molecular states.
