Tunneling spectroscopy of Fe-doped YBa2Cu3O7 single crystals

A comparison between the orthorhombic oxygen-deficient YBa2Cu3O7-δ 60-K phase and the tetragonal YBa2(Cu0.96Fe0.04)3O7 system with Tc=57 K is made by means of tunnel junctions. No relevant differences are found in the behavior of the high-voltage gaplike structure at 25 mV while a suppression of the low-voltage structure at 5 mV is observed in the Fe-doped junctions. In some cases a zero-bias conductance peak is developed; this anomaly is analyzed in the framework of the Appelbaum-Kondo model for electron magnetic interactions. © 1992 The American Physical Society