Kohn anomaly and van Hove singularity in IV^B and V^B group transition metals nitrides and carbides

The superconducting behavior in IVB-VB group transition metal nitrides and carbides has generally been associated with the phonon anomaly and Fermi surface nesting. However, the origin of phonon anomaly has remained ambiguous (i.e. longitudinal acoustic or transverse acoustic modes). We performed first-principles calculations to investigate the phononic properties of these materials and theoretically confirmed that the Kohn anomaly originates from the lower transverse acoustic mode along the ГX direction, thereby revealing the frequency derivative discontinuity of the mode. In particular, the Kohn anomaly region is found to move from the interior to the boundary X point of the Brillouin zone with increasing number of valence electrons. We deduced that the Kohn anomaly originated from the electrons of the filled energy level near the van Hove singularity. These results suggest that the screening of the ionic electric field decreases, while the coupling of conduction electrons with the highly degenerate modes between the TA_∥ and LA via Umklapp scattering process increases. The Fermi surface nesting also plays a role in enhancing the superconductivity. The electronic excitation effect induces a stabilization of the V² group transition metal nitrides. (paper)