Electronic instability and phonon softening in YS

The study of the electronic structure of YS by means of the augmented-plane-wave method reveals important differences with the isoelectronic superconductor NbC. Contrary to NbC, the hybridized $3p$ bands of sulfur are found below the metal d bands and do not overlap with them; this results in a much smaller metal d-ligand p hybridization at the Fermi energy in YS as compared to the strongly hybridized 9-valence-electron carbides. In spite of these differences, the geometry of the Fermi surface of YS is very similar to that of NbC; the presence of strong nesting features in the Fermi surface lead through an enhanced intraband scattering to large resonances in the bare susceptibility χ(→q). The maxima in χ(→q) occur in the [100], [110], and [111] directions at the →q values where anomalously soft phonons have recently been observed by Roedhammer et al. It is remarkable that in spite of important differences in the electronic structure, a similar correlation exists consistently for the 9-valence-electron superconductors such as NbC, TaC, TiN, and YS; this one-to-one correspondence between the resonant screening and the soft modes shows the importance of the Fermi-surface-induced electronic instability in bringing about the phonon anomalies in these compounds.