Tunneling spectroscopy of the quasiparticle Andreev bound state in ion-irradiated ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$/Pb junctions

Effects of disorder on the in-plane density of states of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ are investigated by planar tunneling spectroscopy. Low-fluence 1-MeV ${\mathrm{He}}^{+}$ irradiation reduces the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ $T_c$ and mean-free path without altering the junction quality. As the dirty limit is approached, the zero-bias conductance peak (ZBCP) disappears, as expected for a quasiparticle Andreev bound state. Thus, the quasiparticle scattering rate may be determined by the ZBCP magnitude. This rate is a direct measure of the pair-breaking strength and accounts for the $T_c$ suppression.