Emergence of topological states in a hybrid superconductor of monolayer Pb grown on NbSe2

Topological superconductors have attracted tremendous interest in recent years because of their fundamental importance and potential applications in topological quantum computing. Here, using a monolayered Pb(111) film grown on a NbSe₂ substrate as an example, we show that it is possible to render a hybrid superconducting system nontrivial topology by combining two conventional s-wave superconductors with sufficiently strong spin-orbit coupling (SOC). We first demonstrate, based on first-principles calculations, that the monolayered Pb(111) film is stable on NbSe₂, and the system possesses a nonzero Z₂ topological invariant as well as Dirac-like edge states. Such a nontrivial topology originates from hybridization of the Pb and NbSe₂ bands, associated with a gap closure and reopening process in the presence of SOC. We further reveal that the (3×3)-type charge density wave (CDW) on the NbSe₂ surface is converted into a (2×2)-type CDW for the heterosystem, and the nontrivial topology is inherited in the CDW phase. Moreover, the CDW phase is still superconducting, albeit with a reduced transition temperature, exhibiting delicate competitions between the CDW and superconducting orders. Our findings offer opportunities for realizing topological superconductivity based on conventional s-wave superconductors.