First-principles study of Na-intercalated bilayer NbSe2: Suppressed charge-density wave and strain-enhanced superconductivity

Abstract

Layered NbSe2 is a metallic layered transition metal dichalcogenide (TMD) that has attracted much interest for the coexistence of superconductivity and charge-density wave (CDW) down to the monolayer limit. Here we report first-principles calculations of the lattice dynamics, electronic structure, and electron-phonon coupling of Na-intercalated bilayer NbSe2. We show that upon the Na atom intercalation, the CDW instability in the bilayer NbSe2 can be effectively suppressed, accompanied by the removal of the soft phonon modes at qCDW=23ΓM. The underlying mechanism for this phenomenon is that a large electron doping from the intercalated Na contracts the Fermi surface of bilayer NbSe2 and reduces the electron-phonon coupling at qCDW. In spite of the disappearance of CDW, the superconductivity still survives in the NbSe2 intercalate, with a predicted superconducting transition temperature Tc of 3 K. Moreover, we find that the biaxial compressive strain can greatly increase density of states near the Fermi surface and soften characteristic phonons contributing to superconductivity, leading to an increase of Tc by more than 100% at a low strain level of 3%. Our results would have significant implications for tuning CDW and superconductivity in NbSe2 and other metallic TMD materials.

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