Evidence of a s-Wave Pairing in Heavily Zn-doped $YBa_2Cu_3O_{7-\delta}$ from Andreev Reflection Spectra

Abstract

When the $d_{x^2-y^2}$-wave pairing is suppressed by Zn-doping in $YBa_2Cu_3O_{7-\delta}$ some of the Andreev reflection spectra were found to be similar to the s-wave spectra of conventional superconductors. The energy gap is rather reproducible (2.3-3.0meV). It is suppressed by low magnetic field ($H_C^{PC}=120-270mT$) in great contrast to the d-wave spectra ($H_C^{PC}>3T$) with the similar order of the gap value. We suppose that the s-wave pairing occurs near the Zn-impurities.