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Spin and Charge Fluctuations in the $α$-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 $α$-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 multi-band Hubbard Hamiltonian, where the interactions are treated with the random phase approximation (RPA) 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.