Resonant Raman Scattering of the Normal and Superconducting State of A3C60 Films (A=Rb,K)

Significant spectral changes upon crossing superconducting Tc have been observed by resonant Raman scattering (RRS) (excited at 1.16 eV) in Rb3C60 and K3C60 thin films. In RRS spectra Hg (l) and Hg (2) modes clearly show Fano-lineshapes due fo resonance with electronic continuum. The electronic Raman scattering is clearly observed as a broad background (fwhm =450 cm-1) at low energy. The large linewidth observed in Hg (2) (fwhm = 60 cm-1) indicates a sizable electron-phonon coupling. Below Tc only Hg (2) mode changes dramatically its asymmetric lineshape becoming more symmetric in the superconducting phase. The fitting of experimentally observed Hg(2) lineshapes shows the abrupt change of the Fano parameters at Tc. Interband t lu tlu * transitions are suggested as a possible origin for low energy electronic excitations.