Chiral Landau levels in Weyl semimetal NbAs with multiple topological carriers

TL;DR

This study uses magneto-optical techniques to observe chiral zeroth Landau levels in NbAs Weyl semimetal, revealing complex topological features and multiple carriers, including massive Dirac fermions, under high magnetic fields.

Contribution

It provides the first direct optical evidence of zeroth chiral Landau levels in NbAs and uncovers the coexistence of multiple topological carriers in this material.

Findings

Observation of zeroth chiral Landau levels in NbAs

Verification of particle-hole asymmetry in Weyl cones

Detection of multiple carriers including massive Dirac fermions

Abstract

Recently, Weyl semimetals have been experimentally discovered in both inversion-symmetry-breaking and time-reversal-symmetry-breaking crystals. The non-trivial topology in Weyl semimetals can manifest itself with exotic phenomena which have been extensively investigated by photoemission and transport measurements. Despite the numerous experimental efforts on Fermi arcs and chiral anomaly, the existence of unconventional zeroth Landau levels, as a unique hallmark of Weyl fermions which is highly related to chiral anomaly, remains elusive owing to the stringent experimental requirements. Here, we report the magneto-optical study of Landau quantization in Weyl semimetal NbAs. High magnetic fields drive the system towards the quantum limit which leads to the observation of zeroth chiral Landau levels in two inequivalent Weyl nodes. As compared to other Landau levels, the zeroth chiral Landau level exhibits a distinct linear dispersion in z momentum direction and allows the optical transitions without the limitation of zero z momentum or square root of magnetic field evolution. The magnetic field dependence of the zeroth Landau levels further verifies the predicted particle-hole asymmetry of the Weyl cones. Meanwhile, the optical transitions from the normal Landau levels exhibit the coexistence of multiple carriers including an unexpected massive Dirac fermion, pointing to a more complex topological nature in inversion-symmetry-breaking Weyl semimetals. Our results provide insights into the Landau quantization of Weyl fermions and demonstrate an effective tool for studying complex topological systems.