Chiral Landau levels in Weyl semimetal NbAs with multiple topological carriers

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 toward 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 magnetic field direction and allows the optical transitions without the limitation of zero z momentum or 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.

Complete optical valley polarization in Weyl semimetals in strong magnetic fields

“…We conclude this section by commenting on a number of recent Landau level spectroscopy measurements in WSM. 17,18 Reference [17] has reported splitting features in the optical transitions involving the chiral Landau levels of W2 nodes in NbP, which are reminiscent to the ones predicted in this work. In NbP, the separation between Weyl nodes is small and thus the authors use a model of two coupled nodes to interpret their data.…”

The discovery of dynamic chiral anomaly in a Weyl semimetal NbAs

“…Owing to the larger gaps between the Landau levels at higher fields, the frequency of the absorbed photon increases with magnetic fields. The Landau level spectra of Weyl semimetal NbAs have been studied thoroughly in previous reports 32,33 . We now focus on the feature X, which shows a completely different behavior.…”

Nodal lines in momentum space: topological invariants and recent realizations in photonic and other systems

“…Two years after this theoretical work, Weyl points were experimentally observed in the microwave frequency range [8]. Following this first experimental demonstration, the realization of Weyl points has been achieved using photonic crystals [9,10,43,44,[55][56][57][58][59][60], phononic crystals [3,14,[61][62][63][64][65][66], metals [67][68][69][70] and semimetals [71][72][73][74][75][76][77].…”