# Direct optical detection of Weyl fermion chirality in a topological semimetal

## Abstract

A Weyl semimetal is a novel topological phase of matter1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16, in which Weyl fermions arise as pseudo-magnetic monopoles in its momentum space. The chirality of the Weyl fermions, given by the sign of the monopole charge, is central to the Weyl physics, since it directly serves as the sign of the topological number5,15 and gives rise to exotic properties such as Fermi arcs5,9,12 and the chiral anomaly15,16,17,18,19. Here, we directly detect the chirality of the Weyl fermions by measuring the photocurrent in response to circularly polarized mid-infrared light. The resulting photocurrent is determined by both the chirality of Weyl fermions and that of the photons. Our results pave the way for realizing a wide range of theoretical proposals15,16,20,21,22,23,24,25,26,27,28,29,30 for studying and controlling the Weyl fermions and their associated quantum anomalies by optical and electrical means. More broadly, the two chiralities, analogous to the two valleys in two-dimensional materials31,32, lead to a new degree of freedom in a three-dimensional crystal with potential novel pathways to store and carry information.

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## Acknowledgements

We thank L. Ye and J. Checkelsky for the help with sample preparation. N.G. and S.-Y.X. acknowledge support from US Department of Energy, BES DMSE, Award number DE-FG02-08ER46521 (initial planning), the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4540 (data analysis), and in part from the MRSEC Program of the National Science Foundation under award number DMR-1419807 (data taking, manuscript writing, and using shared experimental facilities). Work in the P.J.-H. group was partly supported by the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Award Number DESC0001088 (fabrication and measurement) and partly through AFOSR grant FA9550-16-1-0382 (data analysis), as well as the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4541 to P.J.-H. P.A.L. acknowledges the support by DOE under grant DE-FG02-03-ER46076 (theoretical analyses). T.P. and Y.L. acknowledge partial funding support from the ONR PECASE project (Award No. 021302-001) and the MIT/Army Institute for Soldier Nanotechnologies (Award No. 023674) (experimental setup). G.C. and H.L. were supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its NRF fellowship NRF Award No. NRF-NRFF2013-03 (first-principles band structure calculations). C.-L.Z. and S.J. were supported by National Basic Research Program of China (grant Nos. 2013CB921901 and 2014CB239302) (single crystal growth). W.X. was supported by the start-up funding through LSU College of Science (single crystal XRD measurements).

## Author information

N.G., P.J.-H., S.-Y.X. and Q.M. designed the experiment. N.G. and P.J.-H. supervised the project. Q.M. and S.-Y.X. performed the measurements and analysed the data. Y.L. and T.P. assisted with the measurements. C.-L.Z. and S.J. grew the single crystal. G.C. and H.L. provided the first-principles band structures. C.-K.C. and P.A.L. provided theoretical analysis and calculated the photocurrents. W.X. performed the single-crystal XRD measurement. S.-Y.X. and Q.M. wrote the manuscript with input from all authors.

Correspondence to Pablo Jarillo-Herrero or Nuh Gedik.

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### Competing interests

The authors declare no competing financial interests.

## Supplementary information

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Ma, Q., Xu, S., Chan, C. et al. Direct optical detection of Weyl fermion chirality in a topological semimetal. Nature Phys 13, 842–847 (2017) doi:10.1038/nphys4146

• #### DOI

https://doi.org/10.1038/nphys4146

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