Addressing Shape and Extent of Weyl cones in TaAs by Landau level spectroscopy

TL;DR

This study uses Landau level spectroscopy to investigate Weyl cones in TaAs, revealing the isotropic W2 nodes' wide energy cone and the hidden, less accessible W1 cones, clarifying their distinct electronic signatures.

Contribution

It provides direct spectroscopic evidence of the shape and extent of Weyl cones in TaAs, distinguishing between W1 and W2 nodes and their electronic behaviors.

Findings

W2 Weyl cones are nearly isotropic and extend over a wide energy range.

W1 Weyl cones are more anisotropic and located deep below the Fermi level.

Weyl cones' signatures can be distinguished via Landau level spectroscopy.

Abstract

Tantalum arsenide, TaAs, is a prime example of a topological semimetal with two types of Weyl nodes, W1 and W2, whose bulk signatures have proven elusive. We apply Landau level spectroscopy to crystals with multiple facets and identify -- among other low-energy excitations between parabolic bands -- the response of a cone extending over a wide energy range. Comparison with density functional theory studies allows us to associate this conical band with nearly isotropic W2 nodes. In contrast, W1 cones, which are more anisotropic and less extended in energy, appear to be buried too deep beneath the Fermi level. They cannot be accessed directly. Instead, the excitations in their vicinity give rise to an optical response typical of a narrow-gap semiconductor rather than a Weyl semimetal.