PHONON SOFTENING IN BILAYER CUPRATE SUPERCONDUCTORS DUE TO INTERLAYERPAIR TUNNELING

The anomalous phonon mode softening observed in the superconducting ph ase in inelastic neutron scattering experiments is shown to occur due to the interlayer pair tunnelling mechanism of Wheatley et al. [Phys. Rev. B 37 (1988) 5897]. We show the emergence of a novel supercurrent phonon coupling in Josephson coupled superconducting layers. Differenc e in phonon frequency shifts for the A(g) and A(u) Raman active modes are shown to have a natural explanation within this mechanism. Frequen cy softening mechanisms for Raman and infrared phonons, in both normal and superconducting state are discussed. (C) 1998 Elsevier Science B. V.