# Imaging emergent heavy Dirac fermions of a topological Kondo insulator

## Abstract

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.

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## 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.

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## Acknowledgements

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.

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### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

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

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Pirie, H., Liu, Y., Soumyanarayanan, A. et al. Imaging emergent heavy Dirac fermions of a topological Kondo insulator. Nat. Phys. 16, 52–56 (2020). https://doi.org/10.1038/s41567-019-0700-8

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