Evidence for d-wave to s-wave gap symmetry conversion in Ca and Ce doped YBCO

We have synthesized YBCO samples with Ca and Ce atoms substituted at the Y sites to form (Y_1-X-YCa_XCe_Y)Ba₂Cu₃O_7-d for (X,Y) = (0, 0), (0.13, 0), (0.13, 0.13), (0.26, 0.13), (0.32, 0.16), (0.26, 0.26), (0.29, 0.29), and (0.32, 0.32). All samples were found to be superconducting with a Tc that decreased from the undoped (0, 0) value of ~91K down to ~70K. There is a sharp drop of Tc from (0, 0) doping to (0.13, 0.13) doping. Above (0.13, 0.13) doping, the Tc is found to saturate at ~70K for all samples. This Tc saturation is unexpected because the nonmagnetic Ca and Ce dopants are pair-breaking for a d-wave gap symmetry. To better characterize this Tc saturation with doping, we measured the temperature dependence of the penetration depth from 3K up to 26K. We find a linear in temperature T penetration depth for the undoped (0, 0) sample, compatible with a d-wave gap symmetry, that changes to a T² penetration depth at doping (0.26, 0.13) and greater. As shown by Hirschfeld and Goldenfeld [PRB vol 48, 4219 (1993)], a T² penetration depth can arise from nonmagnetic impurity doping in a d-wave superconductor with resonant scattering. We show that our measurements are incompatible with this scenario, suggesting that the gap symmetry of highly doped Ca/Ce YBCO is s-wave.