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Soft modes and superconductivity in the layered hexagonal carbides V 2 CAs , Nb </…

2002; American Physical Society; Volume: 65; Issue: 17 Linguagem: Inglês

10.1103/physrevb.65.174519

1095-3795

S. V. Halilov, David J. Singh, D. A. Papaconstantopoulos,

Iron-based superconductors research

The electronic structure, selected phonon frequencies, and electron phonon coupling of the hexagonal layered compounds ${\mathrm{Nb}}_{2}\mathrm{CS},$ ${\mathrm{Nb}}_{2}\mathrm{CAs},$ and ${\mathrm{V}}_{2}\mathrm{CAs}$ 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 ${\mathrm{Nb}}_{2}\mathrm{CS},$ the only superconductor within this family of materials, consists of well-hybridized Nb $d--\mathrm{S} p--\mathrm{C} p$ derived bands characteristic around the Fermi energy. The S is more ionic in this compound than the As in the related arsenides ${\mathrm{Nb}}_{2}\mathrm{CAs}$ or ${\mathrm{V}}_{2}\mathrm{CAs}.$ 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.