An ideal two-dimensional nodal-ring semimetal in tetragonal borophene oxide

Abstract

Free-standing stable two-dimensional (2D) boron monolayers, i.e., borophenes, usually settle into triangular lattices with different ratios of monoatomic vacancies. However, a stable polymorph can be drastically distinct from a free-standing one upon charge doping or on a substrate, as evidenced by the free-standing unstable hexagonal borophene that was prepared on the Al(111) substrate [Sci. Bull., 2018, 63, 282]. Moreover, 2D borophenes prefer to be oxidized to form more stable borophene oxides under ambient conditions. In this work, with the help of first-principles calculations, we propose a stable borophene oxide (t-B₂O) through oxidizing the free-standing unstable T-borophene. More interestingly, t-B₂O is a topological nodal-ring semimetal protected by in-plane mirror symmetry and characterized by a topological index. The energy fluctuation of the nodal ring is small and no extraneous bands are entangled with the nodal ring around the Fermi level. Two tight-binding models are developed to elucidate the orbital interactions and the formation of the nodal ring. Our work not only discovers a new ideal 2D topological nodal-ring semimetal, but the method used here also provides a fresh view in the search for 2D materials.