Spin and charge fluctuations in α-structure layered nitride superconductors

To explore conditions underlying the superconductivity in electron-doped TiNCl where T-c = 16 K, we calculate the electronic structure, Wannier functions, and spin and charge susceptibilities using first-principles density functional theory. TiNCl is the first high-temperature superconductor discovered in the alpha structure of the layered transition-metal nitride family MNCl (M = Ti,Zr,Hf). We construct a tight-binding model based on Wannier functions derived from the band structure, and consider explicit electronic interactions in a multiband Hubbard Hamiltonian, where the interactions are treated within the random phase approximation to calculate spin and charge susceptibility. The results show that, consistent with TiNCl being a nonmagnetic material, spin fluctuations do not dominate over charge fluctuations and both may have comparable impact on the properties of the doped system.