Anisotropic band splitting in monolayer NbSe2: implications for superconductivity and charge density wave

Realization of unconventional physical properties in two-dimensional (2D) transition-metal dichalcogenides (TMDs) is currently one of the key challenges in condensed-matter systems. However, the electronic properties of 2D TMDs remain largely unexplored compared to those of their bulk counterparts. Here, we report the fabrication of a high-quality monolayer NbSe 2 film with a trigonal prismatic structure by molecular beam epitaxy, and the study of its electronic properties by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and electrical transport measurements, together with first-principles band-structure calculations. In addition to a charge density wave (CDW) with 3 × 3 periodicity and superconductivity below 1.5 K, we observed sizable (~0.1 eV) band splitting along the Γ-K cut in the Brillouin zone due to inversion symmetry breaking in the monolayer crystal. This splitting is highly anisotropic in k space, leading to a spin-split van-Hove singularity in the band structure. The present results suggest the importance of spin–orbit coupling and symmetry breaking for unconventional superconductivity and CDW properties in monolayer TMDs. Superconductivity: monolayer NbSe2 undergoes anisotropic band splitting The interplay between symmetry breaking, superconductivity and charge density wave in monolayer NbSe 2 is unveiled by spectroscopic techniques. A team led by Takafumi Sato at Tohoku University used molecular beam epitaxy to fabricate atomically thin NbSe 2 films on bilayer graphene, and investigated their electronic properties by a combination of electrical transport measurements, scanning tunneling microscopy, and angle-resolved photoemission spectroscopy. A charge density wave transition with a strong periodic charge modulation was detected below 1.5 K, accompanied by the emergence of superconductivity. Owing to the inversion symmetry breaking occurring in monolayer NbSe 2 , a band splitting developed along the Γ-Κ direction of the Brillouin zone. Such band splitting was found to be related to the robustness of the observed superconductivity, as well as the formation of the charge density wave.