Stability and superconductivity of compressed superhydride CeH₉ with Dirac-nodal-line fermions

The experimental realization of high-temperature superconductivity (SC) in compressed hydrides under megabar pressures has aroused many efforts to search for hydrides stabilized at modest pressures. For cerium hydride CeH₉ recently synthesized at 80-100 GPa, our density-functional theory calculations with the Migdal-Eliashberg formalism show the presence of large anisotropic SC with a single s-wave gap spreading in the range of ~3-18 meV at T = 10 K, giving rise to a superconducting temperature of ~80 K. It is revealed that, although the Fermi surface consists of three separate sheets, their electronic states are mostly characterized by strong hybridization between Ce 4f and H 1s orbitals. All the Fermi surface sheets possess the large anisotropy of superconducting gap, which is induced by anisotropic electron-phonon coupling due to the H clathrate structure comprising three different species of H atoms. Further, we find that the nonsymmorphic S_2z symmetry protects Dirac nodal surface around the Fermi energy, which is converted into Dirac nodal lines through spin-orbit coupling. Our findings offer a new platform to investigate the intriguing interplay between compressed hydrides, SC, and Dirac fermions.