Nuclear spin-lattice relaxation rate and nonmagnetic pair-breaking effect in electron-doped Pr_{0.91}LaCe_{0.09}CuO_{4-y}: Signature of highly anisotropic s-wave gap

摘要

We numerically calculate the nuclear spin-lattice relaxation rate (Rs) in thesuperconducting state in terms of anisotropic s-wave gaps. By taking intoaccount electron-phonon coupling, our calculated Rs for a conventional s-wavesuperconductor, indium, is in quantitative agreement with the experimental datawith a clear Hebel-Slichter peak. In contrast, by using the highly anisotropics-wave gaps inferred from the magnetic penetration depth and scanning tunnelingmicroscopy, our calculated Rs curves for electron-dopedPr_{0.91}LaCe_{0.09}CuO_{4-y} show no Hebel-Slichter peak, in agreement withthe experimental data. Finally, the observed weak nonmagnetic pair-breakingeffect provides unambiguous evidence for a highly anisotropic s-wave gap inthis underdoped cuprate.