Direct evidence of two-dimensional electron gas-like band structures in hafnene

Two-dimensional (2D) honeycomb-like materials have been widely studied due to their fascinating properties. In particular, 2D honeycomb-like transition metal monolayers, which are good 2D ferromagnet candidates, have attracted intense research interest. The honeycomb-like structure of hafnium, hafnene, has been successfully fabricated on the Ir(111) substrate. However, its electronic structure has not yet been directly elucidated. Here, we report the electronic structure of hafnene grown on the Ir(111) substrate using angle-resolved photoemission spectroscopy (ARPES). Our results indicate that the presence of spin-orbit coupling and Hubbard interaction suppresses the earlier predicted Dirac cones at the K points of the Brillouin zone. The observed band structure of hafnene near the Fermi level is very simple: an electron pocket centered at the Γ point of the Brillouin zone. This electron pocket shows typical parabolic dispersion, and its estimated electron effective mass and electron density are approximately 1.8 m_e and 7 × 10¹⁴ cm⁻², respectively. Our results demonstrate the existence of 2D electron gas in hafnene grown on the Ir(111) substrate and therefore provide key information for potential hafnene-based device applications.