Evidence of s-wave subdominant order parameter in YBa2Cu3O7-? from break-junction tunneling spectra

The tunneling spectra of YBa2Cu3O7- break junctions have been investigated for tunneling close to the node direction. The behavior of the zero-bias conductance peak ZBCP and the Josephson current have been studied as a function of the temperature and magnetic field. The deep splitting of the ZBCP is observed starting at TS20-30 K in agreement with the dx2-y2is order parameter theory Y. Tanuma, Y. Tanaka, S. Kashiwaya, Phys. Rev. B 64, 214519 2001. We have observed that a low 0.04 T magnetic field significantly suppresses such splitting. The 1/T temperature dependence of the maximum Josephson current going to saturation at TTS also confirms mixed order parameter formation.