Survival of the d-Wave Superconducting State near the Edge of Antiferromagnetism in the Cuprate Phase Diagram

In the cuprate superconductor YBa₂Cu₃O_6+x, hole doping in the CuO₂ layers is controlled by both oxygen content and the degree of oxygen ordering. At the composition YBa₂Cu₃O_6.35, the ordering can occur at room temperature, thereby tuning the hole doping so that the superconducting critical temperature gradually rises from 0 to 20 K. Here we exploit this to study the ĉ-axis penetration depth as a function of temperature and doping. The temperature dependence shows the d-wave superconductor surviving to very low doping, with no sign of another ordered phase interfering with the nodal quasiparticles. The only apparent doping dependence is a smooth decline of superfluid density as T_c decreases.