Type-III Weyl Semimetals: (TaSe4)2I

TL;DR

This paper introduces a new class of Weyl semimetals called type-III, characterized by unique Fermi surfaces, and identifies (TaSe4)2I as a representative material with experimental and theoretical evidence supporting its classification.

Contribution

The work proposes the concept of type-III Weyl semimetals, identifies (TaSe4)2I as a candidate, and demonstrates its properties through first-principles calculations and experimental measurements.

Findings

(TaSe4)2I is a type-III Weyl semimetal with large chiral charges.

(TaSe4)2I exhibits four-fold helicoidal surface states and long Fermi arcs.

External strain can induce topological phase transitions among Weyl semimetal types.

Abstract

Weyl semimetals have been classified into type-I and type-II with respect to the geometry of their Fermi surfaces at the Weyl points. Here, we propose a new class of Weyl semimetal, whose unique Fermi surface contains two electron or two hole pockets touching at a multi-Weyl point, dubbed as type-III Weyl semimetal. Based on first-principles calculations, we first show that quasi-one-dimensional compound (TaSe4)2I is a type-III Weyl semimetal with larger chiral charges. (TaSe4)2I can support four-fold helicoidal surface states with remarkably long Fermi arcs on the (001) surface. Angle-resolved photoemission spectroscopy measurements are in agreement with the gapless nature of (TaSe4)2I at room temperature and reveal its characteristic dispersion. In addition, our calculations show that external strain could induce topological phase transitions in (TaSe4)2I among the type-III, type-II, and type-I Weyl semimetals, accompanied with the Lifshitz transitions of the Fermi surfaces. Therefore, our work first experimentally indicates (TaSe4)2I as a type-III Weyl semimetal and provides a promising platform to further investigate the novel physics of type-III Weyl fermions.