Spectroscopic Signatures of Defect-Induced Pair Breaking inBi2Sr2CaCu2O8+x

We investigated the effect of disorder on the spectral properties of the high temperature superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$. We find that small defect densities, in the low ${10}^{-3\mathrm{}}$ range, already suppress the characteristic spectral signature of the superconducting state, while new excitations appear within the gap. We conclude that, due to defect-induced pair breaking, superconducting pairs and normal carriers coexist below $T_c$. At higher levels of disorder the normal state is also strongly affected, and the quasiparticle features progressively smeared out.