Magnetic Insulator Stabilized $\alpha$-Bismuthene with a Large Room-Temperature Gap Consistent with a Quantum Spin Hall State

Abstract

Interfacing two-dimensional bismuth with a magnetic substrate has been proposed as a promising route for exploring magnetically tunable topological states. Here, we report the experimental realization of epitaxial bismuthene-magnetic insulator heterostructure by growing ultrathin Bi films on ferromagnetic EuO(111). Tunneling spectroscopy exhibits a robust energy gap of about 400 meV, consistent with a quantum spin Hall insulating phase persisting at room temperature. Spatially resolved spectroscopy on the $\alpha$-phase bismuthene islands further reveals enhanced edge-localised states at the island boundaries. To elucidate the measured electronic states, we construct a sixteen-band tight-binding model based on the experimentally determined Bi(012) lattice, incorporating Slater-Koster hopping and atomic spin-orbit coupling. The calculation reproduces a gapped $Z_2$-nontrivial phase with helical edge modes, providing a microscopic basis for the quantum spin Hall interpretation. Our results provides a promising platform for future experimental investigations of magnetism-enabled higher-order topology.