Giant defect-enhanced electron-phonon interactions in ternary copper oxide superconductors

Abstract

A microscopic but schematic model is discussed for defects (such as oxygen vacancies ${\mathrm{O}}^{\mathrm{\square }}$) in compounds such as ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7+\mathrm{\delta }}$ ${\mathrm{O}}_{2\mathrm{-}\mathrm{\delta }}^{\mathrm{\square }}$ and ${\mathrm{La}}_{3\mathrm{-}\mathit{x}}$ ${\mathrm{Ba}}_{3+\mathit{x}}$ ${\mathrm{Cu}}_6$ ${\mathrm{O}}_{14\mathrm{-}\mathrm{\delta }}$ ${\mathrm{O}}_{4\mathrm{-}\mathrm{\delta }}^{\mathrm{\square }}$ in relation to the superconductive properties of these materials. The discussion shows in detail how $T_c$ can reach its maximum value at or near the metal-semiconductor transition $T_{\mathrm{ms}}$. It introduces a new mechanism for enhancing of electron-phonon interactions that is separate from, and empirically superior to, Fermi-surface nesting.