Lattice dynamics of 2H-NbX2 (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-NbX2. Remarkable frequency renormalization of ∑1 phonon mode around q=(2/3)┌ M 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 (NbSe2 case) or not (NbS2 case). By using the electron-lattice interaction and the renormalized phonon frequencies obtained for 2H-NbS2, the spectral function α2 F(ω) is calculated and the superconducting transition temperature Tc is evaluated by solving the linearized Eliashberg equation. renormalization of phonon frequencies due to the electron-lattice interaction raises Tc considerably and the obtained value of Tc agrees in order of magnitude with the experimental data. Effects of intercalation on Tc are also discussed.
ASJC Scopus subject areas
- Physics and Astronomy(all)