We predict a new three-dimensional (3D) boron allotrope based on systematic first-principles electronic structure calculations. This allotrope can be derived by substituting each carbon atom in a hexagonal diamond lattice with a B4 tetrahedron and possesses the same space group P63/mmc as hexagonal diamond, hence it is termed as H-boron. We show that H-boron has good stability and excellent mechanical property. Remarkably, we find that H-boron is a topological metal with rich types of spin-orbit-free emergent fermions, including semi-Dirac fermion, quadratic and linear triple-point fermion, nodal-line fermion, and nodal-surface fermion. We clarify their symmetry protections and characterize them by constructing the corresponding low-energy effective models. Our work not only discovers a new boron allotrope with excellent properties, it also offers a platform to explore interesting physics of new kinds of emergent fermions.
@article{arxiv.1901.08413,
title = {Hexagonal supertetrahedral boron: A topological metal with multiple spin-orbit-free emergent fermions},
author = {Yan Gao and Weikang Wu and Peng-Jie Guo and Chengyong Zhong and Shengyuan A. Yang and Kai Liu and Zhong-Yi Lu},
journal= {arXiv preprint arXiv:1901.08413},
year = {2019}
}