Evidence of strong electron-phonon coupling from infrared excited Raman scattering in the YBa₂Cu₃O_7-y superconducting system

Abstract

Infrared excited Raman spectra at 1064 nm (1.16 eV) of the YBa₂Cu₃O_7-y superconducting ceramic system were measured by means of Fourier transform interferometry (FT-Raman) at different oxygen stoichiometries. The standard Raman scattering spectrum excited with visible light was measured for comparison. The FT-Raman spectrum of YBa₂Cu₃O_7-y (y≥0.7) is dominated by the multiple order scattering of the 505 cm^-1 Ag Cu(1)-O(4) axial stretching vibration. This effect is accounted for by a Frank-Condon Raman scattering due to the strong electron-phonon coupling between the axial mode and the local electronic transitions that contribute to the low energy absorption tail of YBa₂Cu₃O_7-y. In view of this we tentatively assign the interband gap to a perpendicular charge transfer transition.