Abstract
Andreev-reflection and tunneling studies have been carried outon a single crystal of underdoped Nd1.85Ce0.15CuO4 − δ (NCCO) byusing Ag, Pt-Ir and Nb tips to clarify the symmetry of its order parameter.Surprisingly, we observed in the NCCO, on the one hand, the Josephsoncurrent (with Nb tip) which indicates the presence of s-wave orderparameter, on the other hand, a zero-bias conductance peak (with Ag tip)which is a manifestation of d-wave order parameter. In addition, we findthe presence of two distinct energy scales in the underdoped NCCO: thesmallest energy scale is associated with the Andreev gap and c-axistunneling gap, and the second scale corresponds to the maximum in-plane tunneling gap. Itseems that the phenomenon of superconductivity in electron- and hole-doped cuprates can beunderstood within a common scheme.
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