Distinct pairing symmetries in Nd 1.85 Ce 0.15 CuO 4-y and La 1.89 Sr 0.11 CuO 4 single crystals: Evidence from comparative tunneling measurements

We used point-contact tunneling spectroscopy to study the superconducting pairing symmetry of electron-doped Nd1.85Ce0.15CuO4-y (NCCO) and hole-doped La1.89Sr0.11CuO4 (LSCO). Nearly identical spectra without zero bias conductance peak (ZBCP) were obtained on the (110) and (100) oriented surfaces (the so-called nodal and anti-nodal directions) of NCCO. In contrast, LSCO showed a remarkable ZBCP in the nodal direction as expected from a d-wave superconductor. Detailed analysis reveals an s-wave component in the pairing symmetry of the NCCO sample with Delta/k(B)T(c)=1.66, a value remarkably close to that of a weakly coupled Bardeen-Cooper-Schriffer (BCS) superconductor. We argue that this s-wave component is formed at the Fermi surface pockets centered at (+/-pi,0) and (0,+/-pi) although a d-wave component may also exist.