Suppressed Charge Density Wave and Strain-Enhanced Superconductivity in Na-Intercalated Bilayer NbSe₂

Atomically thin NbSe₂ is a metallic layered transition metal dichalcogenide (TMD) with coexisting superconducting and charge-density-wave (CDW) orders. Using first-principles calculations, we have systematically studied the lattice dynamics, electronic structure and electron-phonon coupling of Na-intercalated bilayer NbSe₂. The results show that Na intercalation can effectively suppress the CDW instability of the bilayer NbSe₂. In this process, a large electron doping from the intercalated Na contracts the Fermi surface of bilayer NbSe₂, causing the reduction of electron-phonon coupling at q_CDW. We further find that despite the disappearance of CDW, the superconductivity is still preserved in the NbSe₂ intercalate with a predicted transition temperature T_c of 3 K. More interestingly, the biaxial compressive strain is found to largely increase electronic density of states at the Fermi level, soften phonon modes and improve the superconducting T_c of this system by more than 100% at a low strain level of 3%. The present work will stimulate future experimental studies of the synergistic effects of electron doping and strain on CDW and superconductivity in 2D metallic TMD materials.