Near constancy of the pressure dependence of T-c across families of organic and fullerene superconductors

Pressure strongly depresses the superconducting transition temperature T-c of both the doped fullerenes and the beta-ET[(BEDT-TTF)(n)X(m), (BEDT-TTF)=bis(ethylenedithio)tetrathiofulvalene)] organic superconductors. An outstanding puzzle in these materials is the weak variation dT(c)/dP across families of compounds with large variations in T-c. This property has been used to argue against phonon-induced pairing. However, closer examination of the volume dependence of T-c reveals that the weak variation of dT(c)/dP is consistent with moderate (but not weak) electron-phonon coupling.