RAMAN-STUDY OF THE ANOMALOUS BEHAVIOR OF THE 340-CM(-1)CUO2 PLANAR-OXYGEN PHONON IN NI-DOPED YBA2CU3O7-DELTA SUPERCONDUCTING FILMS

The unusual softening of the B1g-like phonon of YBa2Cu3O7 is studied a s a function of T(c) by substituting up to 6% nickel for Cu. The abrup t onset and small temperature range over which the softening occurs in undoped YBa2Cu3O7 are modified upon doping with the softening occurri ng well above T(c) and continuing smoothly to 10 K when T(c) is reduce d to 71 K. The phonon linewidth in the doped films shows no anomalies, regardless of Ni concentration. In contrast, the self-energies of the A(g) modes associated with the plane copper [Cu(2)] and bridging oxyg en (O4) atoms reveal normal thermal behavior for all films. We conclud e that an additional mechanism, besides strong coupling of phonons to superconducting electrons, contributes to the B1g-phonon anomalies.