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Direct optical detection of Weyl fermion chirality in a topological semimetal

Nature Physics volume 13, pages 842–847 (2017)Cite this article

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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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Figure 1: Chirality-dependent optical transition of Weyl fermions in TaAs.
Figure 2: Observation of chirality-dependent photocurrent in TaAs.
Figure 3: Control of photocurrent by varying the Weyl fermion chirality configuration with respect to the light.
Figure 4: Detection and manipulation of chiral Weyl fermions by optical means.

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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).

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Author notes

  1. Qiong Ma and Su-Yang Xu: These authors contributed equally to this work.

Authors and Affiliations

  1. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA

    Qiong Ma, Su-Yang Xu, Ching-Kit Chan, Patrick A. Lee, Pablo Jarillo-Herrero & Nuh Gedik

  2. Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California, 90095, USA

    Ching-Kit Chan

  3. International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China

    Cheng-Long Zhang & Shuang Jia

  4. Collaborative Innovation Center of Quantum Matter, Beijing 100871, China

    Cheng-Long Zhang & Shuang Jia

  5. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore

    Guoqing Chang & Hsin Lin

  6. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore

    Guoqing Chang & Hsin Lin

  7. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA

    Yuxuan Lin & Tomás Palacios

  8. Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1804, USA

    Weiwei Xie

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Contributions

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.

Corresponding authors

Correspondence to Pablo Jarillo-Herrero or Nuh Gedik.

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The authors declare no competing financial interests.

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

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