Realization of strongly correlated 2D honeycomb boron.

Borophene, a two-dimensional (2D) boron sheet, has recently gained considerable attention as a postgraphene material owing to its unique properties, such as intrinsic metallic character, lightest element monolayer structure, and high-frequency phonons. These features, together with its theoretically predicted high density of states, make borophene an excellent platform for investigating various quantum phenomena. However, the realization of emergent phenomena is hindered by fabrication challenges and inherently weak electronic correlations within the s and p electron system. Here, we demonstrate a strongly correlated 2D electronic state derived from a honeycomb boron lattice exposed at the surface of ternary boride LaRh3B2. Using angle-resolved photoemission spectroscopy, we uncover the extended saddle-point van Hove singularity near the Fermi level, which is linked to the substantial lattice expansion and many-body interaction. Moreover, scanning tunneling microscopy reveals the electronic nematicity, which likely originates from the saddle-point instability. Our findings open a pathway to exploring the exotic correlated phenomena in boron-based 2D systems.