Low-Temperature Scanning Tunneling Spectroscopy of a-Axis-Oriented PrBa2Cu3Oy Films on YBa2Cu3Ox

In order to elucidate the mechanism of the long-range proximity effect, a-axis-oriented PrBa2Cu3Oy (PBCO) thin films with thickness up to 50 nm deposited on a-axis-oriented YBa2Cu3Ox (YBCO) films were studied by low-temperature scanning tunneling spectroscopy at 4.2 K. Stable- and unstable-STM-image areas were observed at the PBCO surface. The stable-STM-image areas decreased when the thickness of PBCO increased. Gap structures were also observed in conductance spectra in these stable-STM-image areas. These results and the long-range proximity effect in YBCO/PBCO/YBCO junctions are discussed in terms of the model in which PBCO consists of small-scale conductive and nonconductive regions.