# Hexagonal supertetrahedral boron: A topological metal with multiple spin-orbit-free emergent fermions

## TL;DR

This paper predicts a new 3D boron allotrope, H-boron, which is a topological metal hosting multiple types of spin-orbit-free emergent fermions, with potential for exploring novel physics.

## Contribution

It introduces a novel boron allotrope, H-boron, with unique topological and mechanical properties, and characterizes its rich emergent fermion spectrum.

## Key findings

- H-boron is a stable, mechanically robust 3D boron allotrope.
- H-boron hosts various spin-orbit-free emergent fermions including semi-Dirac and triple-point fermions.
- The study provides symmetry-based explanations and effective models for these fermions.

## Abstract

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 B$_{4}$ tetrahedron and possesses the same space group $P6_{3}/mmc$ 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.