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Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe2

Abstract

Recent experimental evidence for the quantum spin Hall (QSH) state in monolayer WTe2 has linked the fields of two-dimensional materials and topological physics1,2,3,4,5,6,7. This two-dimensional topological crystal also displays unconventional spin–torque8 and gate-tunable superconductivity7. Whereas the realization of the QSH has demonstrated the nontrivial topology of the electron wavefunctions of monolayer WTe2, the geometrical properties of the wavefunction, such as the Berry curvature9, remain unstudied. Here we utilize mid-infrared optoelectronic microscopy to investigate the Berry curvature in monolayer WTe2. By optically exciting electrons across the inverted QSH gap, we observe an in-plane circular photogalvanic current even under normal incidence. The application of an out-of-plane displacement field allows further control of the direction and magnitude of the photocurrent. The observed photocurrent reveals a Berry curvature dipole that arises from the nontrivial wavefunctions near the inverted gap edge. The Berry curvature dipole and strong electric field effect are enabled by the inverted band structure and tilted crystal lattice of monolayer WTe2. Such an electrically switchable Berry curvature dipole may facilitate the observation of a wide range of quantum geometrical phenomena such as the quantum nonlinear Hall10,11, orbital-Edelstein12 and chiral polaritonic effects13,14.

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Fig. 1: Crystal and electronic structures of monolayer WTe2.
Fig. 2: Observation of circular photogalvanic effect in monolayer WTe2.
Fig. 3: Systematic control of the circular photogalvanic current by displacement fields.
Fig. 4: Berry curvature dipole and its control via a topological field effect.

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Acknowledgements

We acknowledge Y. Lin and T. Palacios for their assistance on measurements. 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 (BES) 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. This work made use of the Materials Research Science and Engineering Center Shared Experimental Facilities supported by the National Science Foundation (NSF) (Grant No. DMR-0819762). N.G. and S.Y.X. acknowledge support from DOE, BES DMSE (data taking and analysis), and the Gordon and Betty Moore Foundations EPiQS Initiative through Grant GBMF4540 (manuscript writing). The WTe2 crystal growth performed at Princeton University was supported by an NSF MRSEC grant, DMR-1420541 (Q.D.G. and R.J.C.). K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and JSPS KAKENHI Grant Numbers JP15K21722. H.S. and L.F. acknowledge support from NSF Science and Technology Center for Integrated Quantum Materials grant DMR-1231319 (theory). The WTe2 film growth performed at Nanyang Technological University was supported by MOE Tier 2 grants MOE2016-T2-2-153 and MOE2015-T2-2-007 and the Singapore National Research Foundation under NRF award number NRF-NRFF2013-08. T.R.C. was supported by the Ministry of Science and Technology under the MOST Grant for the Columbus Program NO. 107-2636-M-006-004, National Cheng Kung University, Taiwan, and National Center for Theoretical Sciences (NCTS), Taiwan.

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Contributions

Q.M. and S.Y.X. performed the measurements and analysed the data with help from A.M.M.V. V.F. and S.W. fabricated the devices. Q.D.G. and R.J.C. grew the bulk WTe2 single crystals. K.W. and T.T. grew the bulk hBN single crystals. J.Z. and Z.L. grew the WTe2 CVD films. T.R.C., G.C. and H.L. calculated the first-principles band structures. S.Y.X., Q.M., H.S. and L.F. did theoretical analysis of the Berry curvature dipole with help from C.K.C. H.S. and L.F. performed analysis of the kp model. S.Y.X. and Q.M. wrote the manuscript with input from all authors. P.J.-H. and N.G. supervised the project.

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Correspondence to Nuh Gedik or Pablo Jarillo-Herrero.

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Supplementary Information

Supplementary figures S1 to S31, and Supplementary tables S1 to S5

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Xu, SY., Ma, Q., Shen, H. et al. Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe2. Nature Phys 14, 900–906 (2018). https://doi.org/10.1038/s41567-018-0189-6

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