Optical evidence of chiral Landau level in Weyl semimetal

Weyl semimetal has been extensively studied with exotic phenomena such as chiral anomaly. Despite the numerous experimental efforts to explore the chiral anomaly in different materials, electrical transport is the main approach where negative magneto-resistivity works as a criterion. More evidence of chiral anomaly, such as the existence of chiral Landau level, is desired for further study. Here we report a magneto-optical study in Weyl semimetal. Allowed optical transition under magnetic field can reveal the Landau level structure. The Chiral Landau levels are experimentally found in typical Weyl semimetals NbAs while the system is driven close to the quantum limit. The related optical transition exhibits distinct difference from other normal Landau levels. The z momentum can be finite with unique magnetic field evolution. Meanwhile, multiple carriers with different topological origins are found, indicate complex band structure of the crystal. Our results demonstrate an effective tool for studying complex topological systems. Further evidence of chiral magnetic effect will also be discussed.