Instability of Pa$\overline 3$ Cs$_{3}$C$_{60}$ at ambient pressure and superconducting state of the FCC phase

TL;DR

This study investigates the stability and superconducting properties of Cs$_{3}$C$_{60}$, revealing that the proposed Pa$ar{3}$ structure is unstable and that the FCC phase exhibits temperature-dependent transitions between superconducting and insulating states.

Contribution

The paper demonstrates through DFT calculations that the Pa$ar{3}$ structure of Cs$_{3}$C$_{60}$ is unstable and provides a detailed analysis of the stable FCC phase's superconducting and normal states using advanced theoretical methods.

Findings

Pa$ar{3}$ structure is unstable and transforms into FCC.

Superconducting transition is second order at small volume.

Transition to normal metal or Mott insulator depends on temperature and volume.

Abstract

The alkali-doped fulleride Cs$_{3}$C$_{60}$, crystallized in the space group Fm$\overline 3$m or Pm$\overline 3$n, exhibits unconventional $s$-wave superconductivity under pressure with a maximum $T_c\sim 38$ K. Recently, a new primitive-cubic-structured Cs$_{3}$C$_{60}$ phase corresponding to the space group Pa$\overline 3$ 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 Pa$\overline 3$ 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.