Spindle Nodal Chain in Three-Dimensional alpha' Boron

TL;DR

This paper reports the discovery of a new 3D boron allotrope, alpha' boron, which exhibits a spindle nodal chain topological semimetal state, expanding the understanding of topological materials in boron structures.

Contribution

The study identifies a stable 3D boron structure, alpha' boron, as a novel topological nodal-chain semimetal, constructed from stacked 2D sheets and characterized by unique topological features.

Findings

3D-alpha' boron is a stable nodal-chain semimetal.

It features six nodal lines and rings forming a spindle nodal chain.

Topological properties are linked to pi bonds between boron atoms.

Abstract

Topological metal/semimetals (TMs) have emerged as a new frontier in the field of quantum materials. A few two-dimensional (2D) boron sheets have been suggested as Dirac materials, however, to date TMs made of three-dimensional (3D) boron structures have not been found. Herein, by means of systematic first principles computations, we discovered that a rather stable 3D boron allotrope, namely 3D-alpha' boron, is a nodal-chain semimetal. In the momentum space, six nodal lines and rings contact each other and form a novel spindle nodal chain. This 3D-alpha' boron can be formed by stacking 2D wiggle alpha' boron sheets, which are also nodal-ring semimetals. In addition, our chemical bond analysis revealed that the topological properties of the 3D and 2D boron structures are related to the pi bonds between boron atoms, however, the bonding characteristics are different from those in the 2D and 3D carbon structures.