  The generalized purity is employed for investigating the process of coherence
loss and delocalization of the Q-function in the Bloch sphere of a two-mode
Bose-Einstein condensate in a symmetrical double well with cross-collision.
Quantum phase transition of the model is signaled by the generalized purity as
a function of an appropriate parameter of the Hamiltonian and the number of
particles (N). A power law dependence of the critical parameter with N is
derived.
