Modern Physics Letters B © World Scientific Publishing Company 1 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 sinusoidal (~sinφ) superconducting current-phase relation within the measurement accuracy. 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 d-wave symmetry of superconducting order parameter in YBCO. 1