Emergence of monolayer electron behavior in bulk van der Waals superlattice

Monolayer transition metal dichalcogenides (TMDs) have a simple crystal structure, but they exhibit intricate physical phenomena that differ from their bulk counterparts. Recently, there has been significant interest in the electronic behavior of monolayer TMDs hosted in a natural van der Waals superlattice material, Ba₆Nb₁₁S₂₈, consisting of alternating NbS₂ monolayers and block layers. Here, we report the electronic structure study of Ba₆Nb₁₁S₂₈ and Ba₆Ta₁₁S₂₈. Using angle-resolved photoemission spectroscopy and density functional theory calculation, we show that the electronic structures of the superlattices are similar to those of monolayer TMDs. The two-dimensional characteristics indicate that the interlayer coupling of adjacent TMD layers is suppressed by the intercalation of the Ba₃NbS₅ or Ba₃TaS₅ block layer. A clear band splitting due to spin-orbital coupling is observed in Ba₆Ta₁₁S₂₈, while no obvious splitting is found in Ba₆Nb₁₁S₂₈. These observations are in qualitative agreement with the observation on monolayer films of NbS₂ and TaS₂. Based on our findings, these natural superlattices can serve as an effective model system for studying monolayer materials and their potential applications.