Universal Superconductor-Insulator Transition and T_c Depression in Zn-Substituted High- T_c Cuprates in the Underdoped Regime

The experimental results are presented on the in-plane resistivity for Zn-substituted single crystals of YBa₂Cu₃O_7-y and La_2-xSr_xCuO₄ with various hole densities. The primary effect of Zn is to produce a large residual resistivity ( ρ₀) as a potential scatterer in the unitarity limit. In the underdoped regime, due also to low carrier density in the CuO₂ plane, only a few percent Zn is sufficient for ρ₀ to reach the critical value near the universal two-dimensional resistance h/4e² and to induce a superconductor-insulator transition. By contrast, the universal behavior is not seen in the highly doped regime, suggestive of a radical change in the electronic state.