Instability of Pa3 Cs_3C_60 at ambient pressure and superconducting state of the FCC phase

The alkali-doped fulleride Cs_3C_60, crystallized in the space group Fm3m or Pm3n, exhibits unconventional s-wave superconductivity under pressure with a maximum T_c∼ 38 K. Recently, a new primitive-cubic-structured Cs_3C_60 phase corresponding to the space group Pa3 has been reported (arXiv:2208.09429) and the authors observed superconductivity at ambient pressure. Using density-functional theory (DFT) calculations, we show that the proposed Pa3 structure is not stable under ionic relaxation, but transforms into the FCC structure. We study the normal and superconducting state of the stable FCC phase at different temperatures and volumes using DFT plus dynamical mean-field theory (DFT+DMFT) in the Nambu formalism. As temperature increases, the transition between superconductor and normal metal (Mott insulator) at small (big) volume is found to be second (first) order. The recently developed maximum entropy analytic continuation method for the anomalous-self-energy is used to study the momentum-resolved spectra and optical conductivity.