Research results on electron transport in Au/YBa2Cu3Ox and Nb/Au/YBa2Cu3Ox thin-film heterojunctions are reviewed. The experimental current–phase relations of Nb/Au/YBa2Cu3Ox heterojunctions on c-oriented YBCO films exhibit a second harmonic, temperature dependence, and a phase shift that is explained in terms of a combined symmetry dx2−y2+s of the superconducting order parameter of YBa2Cu3Ox. The current–voltage characteristics of Au/YBa2Cu3Ox and Nb/Au/YBa2Cu3Ox heterojunctions on (1 1 20)YBa2Cu3Ox thin films with an inclined crystallographic c axis display an anomaly of the conductance at low voltages, the behavior of which is studied at various temperatures and magnetic fields. The experimental results are analyzed in the framework of a model for the appearance of bound states caused by multiple Andreev reflection in junctions containing a superconductor with dx2−y2 symmetry of the superconducting order parameter. Studies of the noise characteristics of Nb/Au/(1 1 20)YBa2Cu3Ox heterojunctions at T=4.2 K reveal the presence of thermal and shot components. However, enhancement of the shot noise due to multiple Andreev reflection is not observed in the experiment.
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