HIGH-TEMPERATURE SUPERCONDUCTIVITY - THE ROLES OF SOFT-PHONON DRIVEN STRUCTURAL PHASE-TRANSITIONS AND BIPHONON-ELECTRON COUPLING

For the first time, fully self-consistent soft phonon data, associated with the tetragonal-orthorhombic structural transition in the high-temperature superconductor La2-xSrxCuO4, are used to solve Eliashberg equations. It is found that the electron-phonon coupling constant is strongly x- and temperature-dependent. The superconducting transition temperature T(c) follows the experimental x-dependence but does not exceed 28 K. The higher T(c) values observed are attributed to nonlinear electron-phonon interactions, due to high oxygen ion polarizability, which produce biphonon resonances and bound states and significantly increase T(c).