RAMAN-SCATTERING STUDY OF THE ELECTRON-PHONON INTERACTION IN M3C60 (M = K, RB)

Temperature (T)-dependent Raman scattering from C-60(3-) intramolecular modes and simultaneous electrical resistivity measurements were made on Rb3C60 films which exhibited a superconducting transition temperature T(c) = 28 K. K3C60 films were studied for comparison, but at T = 300 K only. Weak, or no T dependence in the intramolecular modes is observed through T(c), consistent with the fact that the mode frequencies omega > 2DELTA (superconducting gap). By comparing the linewidths of the modes in M3C60 to values in pristine C60 we have estimated their contributions to the electron-phonon coupling constant lambda. Values of lambda have been found to be 0.6 and 0.5 for K3C60 and Rb3C60, respectively, in good agreement with the theoretical value lambda=0.6 obtained previously by Schluter and co-workers. The H(g)(2)-derived mode is found to make the dominant contribution to lambda. A line-shape analysis of the H(g)(1)-derived mode suggests that this mode is a Breit-Wigner-Fano resonance. The radial A(g)(1)-derived mode linewidth in Rb3C60 is anomalously narrow (approximately 0.1 cm-1), and found to be very sensitive to sample quality, i.e., strongly correlated with the width of the superconducting transition.