Role of Electron-Lattice Interaction in Layered Transition Metal Dichalcogenide 2H–NbS₂. I. Phonon Anomaly and Superconductivity

Abstract

Lattice dynamics of 2H–NbX 2 (X=S, Se) is studied by taking account of electron-lattice interaction derived microscopically on the basis of the realistic tight-binding bands fitted to the first-principles bands of 2H–NbX 2 . Remarkable frequency renormalization of Σ 1 phonon mode around q =(2/3)ΓM is caused due to the characteristic wavevector and mode dependences of the electron-lattice interaction as well as the effect of Fermi surface nesting. It is shown that the short range force constant for neighboring X ions on different X-layers in the same X–Nb–X sandwich determines primarily whether lattice instability occurs (NbSe 2 case) or not (NbS 2 case). By using the electron-lattice interaction and the renormalized phonon frequencies obtained for 2H–NbS 2 , the spectral function α 2 F (ω) is calculated and the superconducting transition temperature T c is evaluated by solving the linearized Eliashberg equation. Renormalization of phonon frequencies due to the electron-lattice interaction raises T c considerably and the obtained value of T c agrees in order of magnitude with the experimental data. Effects of intercalation on T c are also discussed.