Weyl Fermion Magneto-Electrodynamics and Ultra-low Field Quantum Limit in TaAs

TL;DR

This study uses magneto-infrared reflection spectroscopy to observe Weyl fermions in TaAs, revealing a low-field quantum limit and isolating Weyl contributions, enabling exploration of exotic quantum phenomena.

Contribution

It demonstrates the first direct observation of Weyl fermions in TaAs at ultra-low magnetic fields using Landau level spectroscopy.

Findings

Weyl fermions exhibit sharp inter-Landau level transitions at fields as low as 0.4 Tesla.

The W2 Weyl point is 8.3 meV below the Fermi energy, with a quantum limit at 0.8 Tesla.

Weyl fermions can be isolated from other carriers in TaAs using Landau level spectroscopy.

Abstract

Topological semimetals are predicted to exhibit unconventional electrodynamics, but a central experimental challenge is singling out the contributions from the topological bands. TaAs is the prototypical example, where 24 Weyl points and 8 trivial Fermi surfaces make the interpretation of any experiment in terms of band topology ambiguous. We report magneto-infrared reflection spectroscopy measurements on TaAs. We observed sharp inter-Landau level transitions from a single pocket of Weyl Fermions in magnetic fields as low as 0.4 tesla. We determine the W2 Weyl point to be 8.3 meV below the Fermi energy, corresponding to a quantum limit - the field required to reach the lowest LL - of 0.8 Tesla - unprecedentedly low for Weyl Fermions. LL spectroscopy allows us to isolate these Weyl Fermions from all other carriers in TaAs and our result provides a new way for directly exploring the more exotic quantum phenomena in Weyl semimetals, such as the chiral anomaly.