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 Fröhlich-Bardeen electron-phonon interaction. One utilizes quasi-two-dimensional energy bands with ${\mathit{E}}_{\mathit{F}}$ pinned by the logarithmic saddle-point singularity in N(E), while the other uses defect enhancement of N(${\mathit{E}}_{\mathit{F}}$). Two kind of composition-dependent experimental data—${\mathit{T}}_{\mathit{c}}$ itself, and spectroscopic measurements of N(${\mathit{E}}_{\mathit{F}}$) and E(k)—can be used to distinguish these models. Overall both kinds of data favor defect enhancement.