Soft modes and superconductivity in the layered hexagonal carbides V2CAs, Nb2CAs, and Nb2CS -: art. no. 174519

The electronic structure, selected phonon frequencies, and electron phonon coupling of the hexagonal layered compounds Nb2CS, Nb2CAs, and V2CAs are elucidated using density functional calculations. All materials are good three-dimensional metals with moderate values of the density of states, N(E-F). The electronic structure of Nb2CS, the only superconductor within this family of materials, consists of well-hybridized Nb d-S p-C p derived bands characteristic around the Fermi energy. The S is more ionic in this compound than the As in the related arsenides Nb2CAs or V2CAs. This results in a relative softness for phonon modes involving S. Rigid muffin tin approximation calculations show moderate electron phonon couplings consistent with low-temperature superconductivity. The key difference from the carbo-arsenides is the presence of soft moderately coupled S modes.