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Dual quantum spin Hall insulator by density-tuned correlations in TaIrTe4

Nature volume 628, pages 515–521 (2024)Cite this article

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Abstract

The convergence of topology and correlations represents a highly coveted realm in the pursuit of new quantum states of matter1. Introducing electron correlations to a quantum spin Hall (QSH) insulator can lead to the emergence of a fractional topological insulator and other exotic time-reversal-symmetric topological order2,3,4,5,6,7,8, not possible in quantum Hall and Chern insulator systems. Here we report a new dual QSH insulator within the intrinsic monolayer crystal of TaIrTe4, arising from the interplay of its single-particle topology and density-tuned electron correlations. At charge neutrality, monolayer TaIrTe4 demonstrates the QSH insulator, manifesting enhanced nonlocal transport and quantized helical edge conductance. After introducing electrons from charge neutrality, TaIrTe4 shows metallic behaviour in only a small range of charge densities but quickly goes into a new insulating state, entirely unexpected on the basis of the single-particle band structure of TaIrTe4. This insulating state could arise from a strong electronic instability near the van Hove singularities, probably leading to a charge density wave (CDW). Remarkably, within this correlated insulating gap, we observe a resurgence of the QSH state. The observation of helical edge conduction in a CDW gap could bridge spin physics and charge orders. The discovery of a dual QSH insulator introduces a new method for creating topological flat minibands through CDW superlattices, which offer a promising platform for exploring time-reversal-symmetric fractional phases and electromagnetism2,3,4,9,10.

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Fig. 1: Electronic band structure, lattice structure and basic electrical characterization of monolayer TaIrTe4.
Fig. 2: QSH edge conduction at the CNP in monolayer TaIrTe4.
Fig. 3: Emergent insulating state at finite doping.
Fig. 4: The dual QSH effect in monolayer TaIrTe4.
Fig. 5: Theoretical investigation of the dual QSH state in monolayer TaIrTe4.

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All data that support the findings of this study are available from the corresponding authors on request. Source data are provided with this paper.

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Acknowledgements

We thank J. Cano, V. Fatemi, R. Fernando, L. Liu, H. Lu, X.-B. Qiang, Y. Ran, B. Skinner, A. Uri, I. Zeljkovic, F. Zhang and Y. Zhang for fruitful discussions. Q.M. acknowledges support from the Air Force Office of Scientific Research grant FA9550-22-1-0270 (transport measurements and data analysis). Q.M. and S.-Y.X. acknowledge support from the Center for the Advancement of Topological Semimetals, an Energy Frontier Research Center funded by the US Department of Energy Office of Science, through the Ames Laboratory under contract DEAC02-07CH11358 (device fabrication). Q.M. also acknowledges support from the National Science Foundation (NSF) CAREER award DMR-2143426 (manuscript writing), the Canadian Institute for Advanced Research (CIFAR) Azrieli Global Scholar Program and the Alfred P. Sloan Foundation. G.C. acknowledges support from the National Research Foundation, Singapore under its Fellowship Award (NRF-NRFF13-2021-0010) and the Nanyang Assistant Professorship grant. N.N. acknowledges support from the US Department of Energy, Office of Science, under award number DE-SC0021117 (single-crystal growth and characterization of TaIrTe4). Y.S. acknowledges support from the Strategic Priority Research Program of the Chinese Academy of Sciences (grant number XDB33030000) and the Informatization Plan of the Chinese Academy of Sciences (CAS-WX2021SF-0102). K.S.B. and Y.W. acknowledge support from the Air Force Office of Scientific Research under award number FA9550-20-1-0246. X.Q. acknowledges support from the NSF under award number DMR-1753054 and from the donors of the American Chemical Society Petroleum Research Fund under grant number 65502-ND10. D.C.B. acknowledges support from the Harvard University Center for Nanoscale Systems, a member of the National Nanotechnology Coordinated Infrastructure Network, under NSF award number ECCS-2025158, and the STC Center for Integrated Quantum Materials, NSF grant number DMR-1231319. Z.S. acknowledges support from Swiss National Science Foundation under grant number P500PT-206914. M.G. acknowledges the support of the NSF Electronics, Photonics and Magnetic Devices programme through grant 2211334. A.S. acknowledges support from DMR-2103842. Portions of this research were conducted with the advanced computing resources provided by Texas A&M High Performance Research Computing. J.L. and Y.Z. are partly supported by the NSF Materials Research Science and Engineering Center programme through the UT Knoxville Center for Advanced Materials and Manufacturing (grant number DMR-2309083). L.F. and Q.M. acknowledge support from the National Science Foundation Convergence Program under grant number ITE-2235945. Ziqiang Wang is supported by the US Department of Energy, Basic Energy Sciences grant number DE-FG02-99ER45747. K.W. and T.T. acknowledge support from the Japan Society for the Promotion of Science KAKENHI (grant numbers 21H05233 and 23H02052) and World Premier International Research Center Initiative, MEXT, Japan. We also acknowledge that some of the work was carried out in the Boston College cleanroom and nanotechnology facilities.

Author information

Author notes

  1. These authors contributed equally: Jian Tang, Thomas Siyuan Ding, Hongyu Chen

Authors and Affiliations

  1. Department of Physics, Boston College, Chestnut Hill, MA, USA

    Jian Tang, Thomas Siyuan Ding, Zumeng Huang, Zhe Sun, Michael Geiwitz, Mohamed Shehabeldin, Vsevolod Belosevich, Zihan Wang, Yiping Wang, Ziqiang Wang, Kenneth S. Burch & Qiong Ma

  2. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, Singapore

    Hongyu Chen & Guoqing Chang

  3. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA

    Anyuan Gao, Zhe Sun & Su-Yang Xu

  4. Department of Physics and Astronomy and California NanoSystems Institute, University of California Los Angeles, Los Angeles, CA, USA

    Tiema Qian & Ni Ni

  5. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing, China

    Xin Han & Youguo Shi

  6. Department of Materials Science and Engineering, Texas A&M University, College Station, TX, USA

    Alex Strasser & Xiaofeng Qian

  7. Department of Physics and Astronomy, University of Tennessee, Knoxville, TN, USA

    Jiangxu Li & Yang Zhang

  8. Min H. Kao Department of Electrical Engineering and Computer Science, University of Tennessee, Knoxville, TN, USA

    Jiangxu Li & Yang Zhang

  9. Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Japan

    Kenji Watanabe

  10. Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan

    Takashi Taniguchi

  11. Harvard John A. Paulson School of Engineering and Applied Sciences and The Center for Nanoscale Systems, Harvard University, Cambridge, MA, USA

    David C. Bell

  12. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA

    Liang Fu

  13. CIFAR Azrieli Global Scholars program, CIFAR, Toronto, Ontario, Canada

    Qiong Ma

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  1. Jian Tang
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Contributions

Q.M. conceived the experiments and supervised the project. J.T. fabricated the devices with the help of T.S.D., A.G., M.G., S.-Y.X. and K.S.B. J.T. carried out the electrical measurements and analysed data with the assistance from T.S.D., A.G., Z.H., Z.S., M.S., V.B. and Zihan Wang. A.S. and X.Q. carried out the first-principles calculations of the single-particle band; H.C. and G.C. carried out the calculations for different edge terminations, electronic susceptibility, CDW band structures and topology; J.L. and Y.Z. carried out the phonon dispersion calculation. L.F. and Ziqiang Wang provided the theory inputs. Y.W. and K.S.B. carried out the Raman measurements. D.C.B. carried out the scanning transmission electron microscopy characterization. T.Q., X.H., Y.S. and N.N. grew the TaIrTe4 bulk crystals. K.W. and T.T. grew the BN bulk crystals. Q.M., S.-Y.X. and J.T. wrote the manuscript with input from all authors.

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Correspondence to Guoqing Chang or Qiong Ma.

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Extended data figures and tables

Extended Data Fig. 1 Basic characterizations of TaIrTe4 flakes.

a, Optical image displaying exfoliated TaIrTe4 flakes on a Si/SiO2 substrate. b, Atomic force microscopy (AFM) image of a TaIrTe4 flake. Inset exhibits the linecut profile, indicating a step thickness of approximately 0.8 nm. c, Raman spectrum of TaIrTe4 flakes ranging from monolayers (1L) to three layers (3L). With increasing layer numbers, the intensity ratio of A2 (@125.7 cm−1 for 1L)/A2 (@137.3 cm−1 for 1L) rises, and the A1 mode (@231.8 cm−1 for 1L) shifts toward lower wavenumbers. d, Cross-sectional scanning transmission electron microscopy (STEM) images of monolayer, bilayer, and trilayer flakes.

Extended Data Fig. 2 Fabrication processes of TaIrTe4 devices.

a,b, Schematic and corresponding fabrication processes for Design-I (a) and Design-II (b) bottom structures. c, Fabrication processes of the top structure (involving TaIrTe4 and top gate) conducted inside an argon-glovebox. Scale bars: 10 μm.

Extended Data Fig. 3 Four-probe resistance versus carrier density and temperature.

a, Device D17; b, Device D1; c, Device D2.

Extended Data Fig. 4 Temperature dependence of charge neutrality point (CNP) conductance and thermal activation fitting (device D2).

The four-probe conductance (Gxx) versus carrier density (n) is shown with temperatures ranging from 250 K to 4 K, along with corresponding thermal activation gap fits. Channel lengths: a,b, Lch = 220 nm; c,d, Lch = 270 nm; e,f, Lch = 550 nm; g,h, Lch = 850 nm; i,j, Lch = 950 nm; k,l, Lch = 1100 nm.

Extended Data Fig. 5 Nonlocal measurements without edge contribution.

a, An optical image of device D3 with half of its boundaries covered by BN. Scale bar: 10 μm. b, Device schematic and contact labelling. c, Plots of local and nonlocal voltages versus carrier density n. The current was injected from Contact 10 to 7 (Ixx = 100 nA), and the voltages were measured between Contact 9 and 8 (VL) and between Contact 11 and 12 (VNL).

Extended Data Fig. 6 Channel length dependence of the resistance at the CNP gap (QSH-I) as well as the correlated gap (QSH-II).

Displayed here are individual curves for devices D2, D5, D7, D16, as referenced in Fig. 2g and Fig. 4e.

Extended Data Fig. 7 Conductance at CNP under in-plane and out-of-plane magnetic fields.

When a magnetic field is applied, a Zeeman gap is induced at the Dirac point of the edge states, influencing the edge conductance, which can be described by a thermal activation behaviour: \(G={G}_{0}{e}^{-g{\mu }_{{\rm{B}}}| B| /2{k}_{{\rm{B}}}T}\). Here, G0 is the conductance at B = 0, g is the effective g-factor, μB is the Bohr magneton, and kB is the Boltzmann constant. a,b, Raw data G versus B for both in-plane and out-of-plane magnetic fields. c,d, Corresponding \({\rm{ln}}(G/{G}_{0})\) versus μBB/kBT, from the slope of which the g-factor can be directly extracted. The data was collected at T = 1.7 K.

Extended Data Fig. 8 Enhanced nonlocal transport in both the CNP and second insulating gaps.

a, An optical image of device D1. Scale bar: 10 μm. b, The local (VL) and nonlocal (VNL) voltages measured with Ixx = 100 nA.

Extended Data Fig. 9 Band structure and its orbital decomposition for monolayer TaIrTe4.

a–c, The orbital distribution of Ta (a), Ir (b) and Te (c), showing that the lowest energy bands originate primarily from Ta orbitals. d, The red-shaded region indicates the gate-tunable range within our experiment.

Extended Data Fig. 10 Formation of QSH bands in TaIrTe4.

a, Graphene lattice structure with two distinct atoms A and B per unit cell. b, Brillouin zone of graphene with K, \({K}^{{\prime} }\) and Γ points labelled. c, Existence of Dirac cones at K and \({K}^{{\prime} }\) points, where gap openings occur upon the introduction of spin-orbit coupling, leading to QSH edge states. d, Lattice structure of monolayer TaIrTe4 with two distinct Ta atoms (Ta1 and Ta2) within each unit cell. e, Brillouin zone of TaIrTe4 with Λ1, Λ2, and Γ points labelled. f, Existence of Dirac cones at Λ1 and Λ2 points, where gap openings occur due to spin-orbit coupling, leading to QSH edge states.

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Tang, J., Ding, T.S., Chen, H. et al. Dual quantum spin Hall insulator by density-tuned correlations in TaIrTe4. Nature 628, 515–521 (2024). https://doi.org/10.1038/s41586-024-07211-8

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