Superconducting Current-Phase Relation and Andreev Bound States in Nb/Au/YBa2Cu3Ox Josephson Heterojunctions

Nb/Au/YBa2Cu3Ox heterojunctions in (001) and (1 1 20)-oriented YBa2Cu3Ox films have been fabricated. I-V curves of both kinds of heterojunctions show pronounced superconducting current. As affected by microwave radiation Shapiro steps at voltages hf/4e arise in the I-V curves of Nb/Au/YBa2Cu3Ox (001) heterojunctions implying the double periodic (~sin2φ) superconducting current-phase relation. However, in the case of Nb/Au/YBa2Cu3Ox (1 1 20) heterojunctions Shapiro steps have been observed in the I-V curves at voltages hf/2e only supporting a sinusoidal (~sinφ) superconducting current-phase relation within measurement accuracy. A zero bias conductance peak associated with zero-energy Andreev bound states occurs in the I-V curve of the Nb/Au/YBa2Cu3Ox (1 1 20) heterojunctions. We discuss the experimental results in terms of the d-wave symmetrical superconducting order parameter in YBCO.