Ideal Nodal Line Semimetal in a Two-Dimensional Boron Bilayer

The successful experimental syntheses of two-dimensional (2D) boron allotropes with intriguing properties have stimulated great interest in searching for novel low-dimensional boron. By using high-throughput first-principles calculations, we proposed a new stable 2D boron with a bilayer structure (P6̅-boron), composed of the building blocks of buckled B12 cluster. We showed the possibility of experimental syntheses of P6̅-boron on metal substrates and proposed a strategy to pursue for P6̅-boron by the B12 clusters self-assembly. Specifically, P6̅-boron possesses a topologically nontrivial Dirac nodal line, which is protected by the mirror-reflection symmetry. Furthermore, we employed a low-energy effective k·p model to prove the existence of the nodal line solution. In addition, the topological analysis of bonding suggests that the chemical bonds of P6̅-boron are all covalent rather than ionic bonds found in bilayer P6/mmm boron. We expect that our findings can favor the low-dissipation high-speed nanoelectronic devices based on 2D boron sheets.