  Impurity substitution of Zn in La-214 and (Y,Ca)-123 high-T_c superconductors
suppresses T_c but does not affect appreciably the onset of the pseudogap phase
in the underdoped region nor optimal doping or the position of the inferred
quantum critical point. Based on a 1/N expansion of the t-J model we explain
these findings as well as the similar dependence on a magnetic field in terms
of a quantum critical point scenario where a flux phase causes the pseudogap.
