Magnetic torque anomaly in the quantum limit of the Weyl semi-metal NbAs

TL;DR

This paper reports a significant magnetic torque anomaly in NbAs at the quantum limit, revealing topological effects of Weyl fermions and proposing torque measurements as a straightforward method to identify Weyl and Dirac semimetals.

Contribution

It demonstrates a large torque anomaly in NbAs at the quantum limit, linking it to the topological properties of Weyl semimetals and introducing a new experimental identification technique.

Findings

Large torque anomaly observed in NbAs at high magnetic fields

Sign reversal of magnetic anisotropy linked to topological effects

Quantum limit torque measurements can identify Weyl semimetals

Abstract

Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue physicists due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we report a large anomaly in the magnetic torque of the Weyl semi-metal NbAs upon entering the "quantum limit" state in high magnetic fields, where topological corrections to the energy spectrum become dominant. The quantum limit torque displays a striking change in sign, signaling a reversal of the magnetic anisotropy that can be directly attributed to the topological properties of the Weyl semi-metal. Our results establish that anomalous quantum limit torque measurements provide a simple experimental method to identify Weyl- and Dirac- semi-metals.