ELECTRON-PHONON COUPLING IN HIGH-TEMPERATURE SUPERCONDUCTORS - THE CONNECTION TO SUPERCONDUCTING BEHAVIOR

The results of recent calculations of the frequency, linewidths, and coupling strengths to carriers on the Fermi surface for several phonons in La1.85Sr0.15CuO4 (LSCO) [our work] and YBa2Cu3O7 (YBCO) [by the Stuttgart group] are discussed and related to experiment. The estimates of the electron-phonon coupling constant are lambda = 1.35 for LSCO based on 11 phonons, and lambda = 1.7 for YBCO based on 17 phonons. These coupling strengths are large enough to make the electron-phonon interaction a central feature in normal state transport properties and a likely candidate for the pairing mechanism for high temperature superconductivity. The strong coupling arises from features that are generic in the high temperature superconducting cuprates: a modest (certainly not large) density of states at the Fermi level which results in weak screening; a confinement of the metallic electrons to layers separated by dielectric regions, which further weakens screening and results in Madelung-like contributions to electron-phonon matrix elements; the site energies of the Cu d states and the O p states are roughly equal, leading to strong hybridization and highly metallic bands at tbe Fermi level.