Zn-inducedTcReduction in High-TcSuperconductors: Scattering in the Presence of a Pseudogap

The depression of $T_c$ with fraction $y$ of Zn substitution is studied across the underdoped and overdoped regions in La-214 and (Y,Ca)-123. The overdoped region is well described by elastic scattering at the unitarity limit with a $d$-wave strong-coupling superconducting order parameter. The underdoped region shows a marked increase in ${\mathrm{dT}}_c/dy$, consistent with unitary scattering in the presence of an anisotropic normal-state pseudogap which depletes the density of states about $E_F$.