Local and macroscopic tunneling spectroscopy of Y 1− x Ca x Ba 2 Cu 3 O 7− δ films: Evidence for a doping-dependent is or id xy component in the order parameter

Tunneling spectroscopy of epitaxial (110) Y1−xCaxBa2Cu3O7−δ films reveals a doping-dependent transition from a pure dx2−y2 to dx2−y2+is or dx2−y2+idxy order parameter. The subdominant (is or idxy) component manifests itself in a splitting of the zero-bias conductance peak and the appearance of subgap structures. The splitting is seen in the overdoped samples, increases systematically with doping, and is found to be an inherent property of the overdoped films. It was observed in both local tunnel junctions, using scanning tunneling microscopy (STM), and in macroscopic planar junctions, for films prepared by either sputtering or laser ablation. The STM measurements exhibit a fairly uniform splitting size in [110] oriented areas on the order of 10nm2 but vary from area to area, indicating some doping inhomogeneity. U- and V-shaped gaps were also observed, with good correspondence to the local faceting, a manifestation of the dominant d-wave order parameter.