Pressure-induced shift of the photoluminescence band in single crystals of buckminster fullerene C60 and its implications for superconductivity in doped samples

We report the first observation of a large pressure-induced red-shift of the photoluminescence band in single crystals of C60 and the associated colour change of the sample from red to black at 3.2 GPa. Interpreting the red shift of the luminescence band as a reduction of the bandgap Eg of the crystal, we get the pressure derivative ∂E g /∂P = −0.138 eV/GPa. This result taken in conjunction with the known compressibility, gives the deformation potential ∂E g /∂(ln a) = 6.0 eV, where ‘a’ is the lattice constant. Taking the same value for the alkali-doped fullerene, and combining it with the known pressure dependence of the transition temperature in the context of a single BCS expression for Tc we estimate the electron-phonon coupling parameter λ - 0.3.