Imaging emergent heavy Dirac fermions of a topological Kondo insulator


The interplay between strong electron interactions and band topology is a new frontier in the search for exotic quantum phases. The Kondo insulator SmB6 has emerged as a promising platform because its correlation-driven bulk gap is predicted to host topological surface modes entangled with f electrons, spawning heavy Dirac fermions1,2,3,4. Unlike the conventional surface states of non-interacting topological insulators, heavy Dirac fermions are expected to harbour spontaneously generated quantum anomalous Hall states5, non-Abelian quantum statistics6,7, fractionalization8 and topological order6,7,8. However, the small energy scales required to probe heavy Dirac fermions have complicated their experimental realization. Here we use high-energy-resolution spectroscopic imaging in real and momentum space on SmB6. On cooling below 35 K, we observe the opening of an insulating gap that expands to 14 meV at 2 K. Within the gap, we image the formation of linearly dispersing surface states with effective masses reaching 410 ± 20 me (where me is the mass of the electron). Our results demonstrate the presence of correlation-driven heavy surface states in SmB6, in agreement with theoretical predictions1,2,3,4. Their high effective mass translates to a large density of states near zero energy, which magnifies their susceptibility to the anticipated novel orders and their potential utility.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Anticipated topological Kondo insulator electronic structure of SmB6.
Fig. 2: Imaging QPI on the (2 × 1) surface of SmB6.
Fig. 3: Raw QPI reveals heavy Dirac surface states.
Fig. 4: Concomitant evolution of Dirac states and the KI gap.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

Code availability

The code that supports the findings of this study is available from the corresponding author on reasonable request.


  1. 1.

    Dzero, M., Sun, K., Galitski, V. & Coleman, P. Topological Kondo insulators. Phys. Rev. Lett. 104, 106408 (2010).

  2. 2.

    Alexandrov, V., Dzero, M. & Coleman, P. Cubic topological Kondo insulators. Phys. Rev. Lett. 111, 226403 (2013).

  3. 3.

    Takimoto, T. SmB6: a promising candidate for a topological insulator. J. Phys. Soc. Jpn 80, 123710 (2011).

  4. 4.

    Lu, F., Zhao, J., Weng, H., Fang, Z. & Dai, X. Correlated topological insulators with mixed valence. Phys. Rev. Lett. 110, 096401 (2013).

  5. 5.

    Efimkin, D. K. & Galitski, V. Strongly interacting Dirac liquid on the surface of a topological Kondo insulator. Phys. Rev. B 90, 081113 (2014).

  6. 6.

    Wang, C., Potter, A. C. & Senthil, T. Gapped symmetry preserving surface state for the electron topological insulator. Phys. Rev. B 88, 115137 (2013).

  7. 7.

    Chen, X., Fidkowski, L. & Vishwanath, A. Symmetry enforced non-Abelian topological order at the surface of a topological insulator. Phys. Rev. B 89, 165132 (2014).

  8. 8.

    Thomson, A. & Sachdev, S. Fractionalized Fermi liquid on the surface of a topological Kondo insulator. Phys. Rev. B 93, 125103 (2016).

  9. 9.

    Dzero, M., Xia, J., Galitski, V. & Coleman, P. Topological Kondo insulators. Annu. Rev. Condens. Matter Phys. 7, 249–280 (2016).

  10. 10.

    Allen, J. W., Batlogg, B. & Wachter, P. Large low-temperature Hall effect and resistivity in mixed-valent SmB6. Phys. Rev. B 20, 4807–4813 (1979).

  11. 11.

    Kim, D. J., Xia, J. & Fisk, Z. Topological surface state in the Kondo insulator samarium hexaboride. Nat. Mater. 13, 466–470 (2014).

  12. 12.

    Zhang, X. et al. Hybridization, inter-ion correlation, and surface states in the Kondo insulator SmB6. Phys. Rev. X 3, 011011 (2013).

  13. 13.

    Syers, P., Kim, D., Fuhrer, M. S. & Paglione, J. Tuning bulk and surface conduction in the proposed topological Kondo insulator SmB6. Phys. Rev. Lett. 114, 096601 (2015).

  14. 14.

    Luo, Y., Chen, H., Dai, J., Xu, Z.-A. & Thompson, J. D. Heavy surface state in a possible topological Kondo insulator: magnetothermoelectric transport on the (011) plane of SmB6. Phys. Rev. B 91, 075130 (2015).

  15. 15.

    Li, G. et al. Two-dimensional Fermi surfaces in Kondo insulator SmB6. Science 346, 1208–1212 (2014).

  16. 16.

    Tan, B. S. et al. Unconventional Fermi surface in an insulating state. Science 349, 287–290 (2015).

  17. 17.

    Xu, N. et al. Exotic Kondo crossover in a wide temperature region in the topological Kondo insulator SmB6 revealed by high-resolution ARPES. Phys. Rev. B 90, 085148 (2014).

  18. 18.

    Jiang, J. et al. Observation of possible topological in-gap surface states in the Kondo insulator SmB6 by photoemission. Nat. Commun. 4, 3010 (2013).

  19. 19.

    Neupane, M. et al. Surface electronic structure of the topological Kondo-insulator candidate correlated electron system SmB6. Nat. Commun. 4, 2991 (2013).

  20. 20.

    Gorshunov, B. et al. Low-energy electrodynamics of SmB6. Phys. Rev. B 59, 1808–1814 (1999).

  21. 21.

    Frantzeskakis, E. et al. Kondo Hybridization and the origin of metallic states at the (001) surface of SmB6. Phys. Rev. X 3, 041024 (2013).

  22. 22.

    Hlawenka, P. et al. Samarium hexaboride is a trivial surface conductor. Nat. Commun. 9, 517 (2018).

  23. 23.

    Matt, C. E. et al. Consistency between ARPES and STM measurements on SmB6. Preprint at (2018).

  24. 24.

    Ruan, W. et al. Emergence of a coherent in-gap state in the SmB6 Kondo insulator revealed by scanning tunneling spectroscopy. Phys. Rev. Lett. 112, 136401 (2014).

  25. 25.

    Rößler, S. et al. Hybridization gap and Fano resonance in SmB6. Proc. Natl Acad. Sci. USA 111, 4798–4802 (2014).

  26. 26.

    Zhu, Z.-H. et al. Polarity-driven surface metallicity in SmB6. Phys. Rev. Lett. 111, 216402 (2013).

  27. 27.

    Stroscio, J. A., Feenstra, R. M. & Fein, A. P. Electronic structure of the Si(111)2 × 1 surface by scanning-tunneling microscopy. Phys. Rev. Lett. 57, 2579–2582 (1986).

  28. 28.

    Schmidt, A. R. et al. Imaging the Fano lattice to ‘hidden order’ transition in URu2Si2. Nature 465, 570–576 (2010).

  29. 29.

    Aynajian, P. et al. Visualizing heavy fermions emerging in a quantum critical Kondo lattice. Nature 486, 201–206 (2012).

  30. 30.

    Allan, M. P. et al. Imaging cooper pairing of heavy fermions in CeCoIn5. Nat. Phys. 9, 468–473 (2013).

  31. 31.

    Jiao, L. et al. Additional energy scale in SmB6 at low-temperature. Nat. Commun. 7, 13762 (2016).

  32. 32.

    Guo, H.-M. & Franz, M. Theory of quasiparticle interference on the surface of a strong topological insulator. Phys. Rev. B 81, 041102 (2010).

  33. 33.

    Nyberg, R. H., Rossi, E. & Morr, D. K. Identifying collective modes through impurity pinning in cuprate superconductors. Phys. Rev. B 78, 054504 (2008).

  34. 34.

    Baum, Y. & Stern, A. Magnetic instability on the surface of topological insulators. Phys. Rev. B 85, 121105 (2012).

  35. 35.

    Nakajima, Y., Syers, P., Wang, X., Wang, R. & Paglione, J. One-dimensional edge state transport in a topological Kondo insulator. Nat. Phys. 12, 213–217 (2015).

  36. 36.

    Figgins, J. & Morr, D. K. Defects in heavy-fermion materials: unveiling strong correlations in real space. Phys. Rev. Lett. 107, 066401 (2011).

  37. 37.

    Okada, Y. et al. Direct observation of broken time-reversal symmetry on the surface of a magnetically doped topological insulator. Phys. Rev. Lett. 106, 206805 (2011).

  38. 38.

    Maltseva, M., Dzero, M. & Coleman, P. Electron cotunneling into a Kondo lattice. Phys. Rev. Lett. 103, 206402 (2009).

  39. 39.

    Figgins, J. & Morr, D. K. Differential conductance and quantum interference in Kondo systems. Phys. Rev. Lett. 104, 187202 (2010).

  40. 40.

    Akintola, K. et al. Quantum spin fluctuations in the bulk insulating state of pure and Fe-doped SmB6. Phys. Rev. B 95, 245107 (2017).

  41. 41.

    Kim, D. J. et al. Surface Hall effect and nonlocal transport in SmB6: evidence for surface conduction. Sci. Rep. 3, 3150 (2013).

Download references


The work at Harvard was supported by the US National Science Foundation under grant nos. DMR-1106023 and DMR-1410480. The work at UC Irvine was supported by US National Science Foundation under grant no. 1708199. D.K.M. acknowledges support by the US Department of Energy, Office of Science, Basic Energy Sciences, under award no. DE-FG02-05ER46225. Work at Los Alamos was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering. Work at the University of Maryland was funded by AFOSR grant no. FA9550-14-1-0332 and by the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant no. GBMF4419.

Author information

H.P., Y.L., A.S., P.C., Y.H. and M.M.Y. performed the STM experiments. X.W., J.P., P.F.S.R., D.J.-K. and Z.F. synthesized and characterized the samples. P.F.S.R. performed X-ray measurements. J.D.T. performed magnetic susceptibility measurements. H.P., A.S., Y.H., M.M.Y., M.H.H. and J.E.H. developed and carried out analyses. D.K.M. provided theoretical guidance. M.H.H. and J.E.H. supervised the project. H.P. and M.H.H. wrote the paper with key contributions from D.K.M. and J.E.H. The manuscript reflects the contributions and ideas of all authors.

Correspondence to M. H. Hamidian or Jennifer E. Hoffman.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figs. 1–17, Tables 1 and 2, text and references.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Pirie, H., Liu, Y., Soumyanarayanan, A. et al. Imaging emergent heavy Dirac fermions of a topological Kondo insulator. Nat. Phys. 16, 52–56 (2020).

Download citation

Further reading