Evidence for nodeless gap in superconducting Nd1.85Ce0.15CuO4-y:

The pairing symmetry in a single crystal of Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ is studied by measuring the point-contact spectroscopy along nodal and anti-nodal directions. For comparison the same measurements on a hole-doped cuprate single crystal of La$_{1.89}$Sr$_{0.11}$CuO$_{4}$ is also presented. A nearly identical spectrum is obtained in Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ for both directions along Cu-O bond and Cu-Cu bond and no any zero bias conductance peak is observed. This is in contrast to the results of La$_{1.89}$Sr$_{0.11}$CuO$_{4}$, in which an angular dependent spectrum is observed with a remarkable zero bias conductance peak in the nodal direction. Our results support an s-wave like symmetry in optimally electron-doped cuprate Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ other than the d-wave dominant symmetry as demonstrated in hole-doped cuprates.