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

Abstract

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 E_g 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 T_c we estimate the electron-phonon coupling parameter λ-0.3.