Charge transfer degree and superconductivity of the incommensurate organic superconductor (MDT-TSF) (I3) 0.422

The influence of a small change of carrier number on the superconducting transition temperature Tc in the incommensurate organic superconductors is investigated for (MDT-TSF) (I3) 0.422 (MDT-TSF: methylenedithio- tetraselenafulvalene, Tc =4.9 K) in comparison with (MDT-TSF) (AuI2) 0.436 (Tc =4.5 K). Careful estimation of the degree of charge transfer by means of the Raman spectra as well as the Shubnikov-de Haas (SdH) oscillations indicates that the charge-transfer degree 0.422 for the I3 salt is obviously smaller than 0.436 in the AuI2 salt. According to the band calculation, the former salt has smaller density of states. However, the former salt exhibits higher Tc than the latter compounds, and this disagrees with the naive prediction of the BCS theory. The former salt shows considerably large effective cyclotron mass extracted from the SdH oscillations. These observations demonstrate that the strength of the many-body effect is the major factor that determines Tc in these organic superconductors.