Controlling the gap of fullerene microcrystals by applying pressure: the role of many-body effects

We studied theoretically the optical properties of C$_{60}$ fullerene microcrystals as a function of hydrostatic pressure with first-principles many-body theories. Calculations of the electronic properties were done in the GW approximation. We computed electronic excited states in the crystal by diagonalizing the Bethe-Salpeter equation (BSE). Our results confirmed the existence of bound excitons in the crystal. Both the electronic gap and optical gap decrease continuously and non-linearly as pressure of up to 6 GPa is applied. As a result, the absorption spectrum shows strong redshift. We also obtained that "negative" pressure shows the opposite behavior: the gaps increase and the optical spectrum shifts toward the blue end of the spectrum. Negative pressure can be realized by adding cubane (C$_8$H$_8$) or other molecules with similar size to the interstitials of the microcrystal. For the moderate lattice distortions studied here, we found that the optical properties of fullerene microcrystals with intercalated cubane are similar to the ones of an expanded undoped microcrystal. Based on these findings, we propose doped C60 as active element in piezo-optical devices.