Nuclear spin-lattice relaxation rate in electron-doped Pr0.91LaCe0.09CuO4 − y: Constraint on the gap symmetry

We present numerical calculations of the nuclear spin-lattice relaxation (Rs) rate in the superconducting state of pure indium and slightly underdoped n-type cuprate Pr_0.91LaCe_0.09CuO_4 − y. By properly taking into account electron-phonon coupling, our calculated Rs for the conventional s-wave superconductor, indium, is in quantitative agreement the experimental data with a clear Hebel-Slichter peak. In contrast, the absence of the Hebel-Slichter peak in the Rs data of Pr_0.91LaCe_0.09CuO_4 − y can be explained by either d-wave or highly anisotropic s-wave gap symmetry. Thus, the absence of the Hebel-Slichter peak does not necessarily argue against an s-wave gap symmetry in this electron-doped cuprate.