Study of Low Frequency Phonon Modes in YBCO System

The phonon modes in YBa2Cu3O7-δ and YBa2-xLaxCu3O7-δ systems have been systematically studied by Raman spectroscopy. The new phonon modes of 104 cm-1, 94 cm-1, and 89 cm-1 were found in all these samples. A crude estimate about the wavenumber of the collective vibration of the stable CuO2 plane was given in this paper. The standard deviations of the new phonons in YBa2Cu3O7-δ and YBa2-xLaxCu3O7-δ systems were discussed. The results of the calculation indicated that the 104 cm-1 mode probably stands the c-direction collective vibration of the stable CuO2 plane, the 94 cm-1 mode stands the a-direction vibration, and the 89 cm-1 mode stands b-direction vibration. The relevance between these phonons and the superconductivity was discussed. It is found that, as the Tc decreased, the 104 cm-1 mode and the 94 cm-1 mode softened, and the 89 cm-1 mode hardened slightly.