ANHARMONICITY-INDUCED MULTIPHONON PROCESSES IN HIGH-TEMPERATURE SUPERCONDUCTORS

Electron-two-phonon interactions are considered on the basis of an extended BCS-type Hamiltonian. An equation for the superconducting transition temperature T(c) and the energy gap 2-DELTA-0 is derived in the weak-coupling limit. Instead of a Debye frequency a reduced optic-acoustic mode frequency, together with a coupled effective electron-phonon interaction, determines the magnitude of T(c). This leads to a distinctly different isotope effect from BCS, which is compared with data on La(2-x)Sr(x)CuO4. The energy gap is predicted to be anisotropic and 2-DELTA-0/kT(c) to be compound dependent.