Phenomenological and spectroscopic evidence for defect-induced high-temperature superconductivity

Two models can provide the strong coupling required to explain high-temperature superconductivity with the established Fr\"ohlich-Bardeen electron-phonon interaction. One utilizes quasi-two-dimensional energy bands with EF pinned by the logarithmic saddle-point singularity in N(E), while the other uses defect enhancement of N(EF). Two kind of composition-dependent experimental data---Tc itself, and spectroscopic measurements of N(EF) and E(k)---can be used to distinguish these models. Overall both kinds of data favor defect enhancement.