ROLE OF ELECTRON-LATTICE INTERACTION IN LAYERED TRANSITION-METAL DICHALCOGENIDE 2H-NBS2 .1. PHONON ANOMALY AND SUPERCONDUCTIVITY

Lattice dynamics of 2H-NbX(2) (X=S, Se) is studied by taking account o f electron-lattice interaction derived microscopically on the basis of the realistic tight-binding bands fitted to the first-principles band s of 2H-NbX(2). Remarkable frequency renormalization of Sigma(1) phono n mode around q=(2/3)Gamma M is caused due to the characteristic wavev ector and mode dependences of the electron-lattice interaction as well as the effect of Fermi surface nesting. It is shown that the short ra nge 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-N bS2, the spectral function alpha(2)F (omega) is calculated and the sup erconducting transition temperature T-c is evaluated by solving the li nearized Eliashberg equation. Renormalization of phonon frequencies du e to the electron-lattice interaction raises T-c considerably and the obtained value of T-c agrees in order of magnitude with the experiment al data. Effects of intercalation on T-c are also discussed.