Anomalous behavior of the lattice vibration of Bi₂Sr₂Ca₁Cu_1.96Fe_0.04O_y and its possible correlation with T_c

The vibrational motion of Fe atoms in Bi₂Sr₂Ca₁Cu_1.96Fe_0.04O_y studied by means of Mossbauer spectroscopy was found to be very anharmonic. This anharmonic behaviour can be explained by local structural excitations between two or more states supposedly existing in the material rather than by the usual phonon-phonon interactions. The increase in T_c for the annealed Bi₂Sr₂Ca₁Cu_1.96Fe_0.04O_y is attributed to the increase in local structural instabilities which enhances the effective electron-electron pairing.