Enhanced isotope effect in 13C-rich superconducting M xC 60 ( M= K, Rb): support for vibronic pairing

Abstract

By means of non-resonant microwave absorption in low magnetic fields it is found that the superconducting T c significantly decreases in 60% 13C isotope substituted K 3C 60 and Rb 3C 60. The exponent α of the isotope effect, T crmc 1/m α, is estimated roughly to be α=1.2–2.25, much higher than predicted by the BCS theory. The intrinsic disorder is observed both in the wide-mass spectrum and the dramatically changed IR spectrum reflecting the inhomogeneous distribution of 13C isotopes in 13C 60− n 12 C 60 molecules. The large positive α supports the intramolecular vibronic pairing, and the decrease of the electron-vibron coupling λ due to the disturbed symmetry of H g modes in mixed 13C 60− n 12 C n molecules is proposed to contribute to the enhanced α, in addition to the conventional isotope frequency shift.