Spectroscopic signatures of defect-induced pair breaking in Bi2Sr2CaCu2O8+x

We investigated the effect of disorder on the spectral properties of the hi gh temperature superconductor Bi2Sr2CaCu2O8+x. We find that small defect de nsities, in the low 10(-3) range, already suppress the characteristic spect ral signature of the superconducting state, while new excitations appear wi thin the gap. We conclude that, due to defect-induced pair breaking, superc onducting 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.