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# Dirac fermions and flat bands in the ideal kagome metal FeSn

## Abstract

A kagome lattice of 3d transition metal ions is a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic ground states. However, the paradigmatic states of the idealized two-dimensional kagome lattice—Dirac fermions and flat bands—have not been simultaneously observed. Here, we use angle-resolved photoemission spectroscopy and de Haas–van Alphen quantum oscillations to reveal coexisting surface and bulk Dirac fermions as well as flat bands in the antiferromagnetic kagome metal FeSn, which has spatially decoupled kagome planes. Our band structure calculations and matrix element simulations demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals, and evidence that the coexisting Dirac surface state realizes a rare example of fully spin-polarized two-dimensional Dirac fermions due to spin-layer locking in FeSn. The prospect to harness these prototypical excitations in a kagome lattice is a frontier of great promise at the confluence of topology, magnetism and strongly correlated physics.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

## Code availability

The codes used for the DFT and tight-binding calculations in this study are available from the corresponding authors upon reasonable request.

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## Acknowledgements

We are grateful to C. Felser, S. Borisenko, M. Knupfer, K. Koepernik, L. Levitov and A. Fahimniya for fruitful discussions. M.P.G., J.-S.Y., J.I.F., S.F., M.R. and J.v.d.B. thank U. Nitzsche for technical assistance in maintaining computing resources at IFW Dresden. R.C. acknowledges support from the Alfred P. Sloan Foundation. This research was funded, in part, by the Gordon and Betty Moore Foundation EPiQS Initiative, Grant no. GBMF3848 to J.G.C. and ARO Grant no. W911NF-16-1-0034. M.K., L.Y., S.F., E.K. and M.P.G. acknowledge support by the STC Center for Integrated Quantum Materials, NSF grant number DMR-1231319. M.K. acknowledges support from the Samsung Scholarship from the Samsung Foundation of Culture. L.Y. acknowledges support from the Tsinghua Ed fucation Foundation. The computations in this paper were run on the ITF/IFW computer clusters (Dresden, Germany) and Odyssey cluster supported by the FAS Division of Science, Research Computing Group at Harvard University. M.R and J.v.d.B. acknowledge support from the German Research Foundation (DFG) via SFB 1143, project A5. M.P.G. thanks the Alexander von Humboldt Foundation for financial support through the Georg Forster Research Fellowship Program, Germany. J.-S.Y. and J.I.F. thank the IFW excellence programme. This research used the resources of the Advanced Light Source, a US Department of Energy (DOE) Office of Science User Facility under contract no. DE-AC02-05CH11231. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement no. DMR-1644779, the State of Florida and the DOE. Pulsed magnetic field measurements were supported by the DOE BES ‘Science at 100 T’ grant. Operation of the ESM beamline at the National Synchrotron Light Source is supported by DOE Office of Science User Facility Program operated for the DOE Office of Science by Brookhaven National Laboratory under Contract no. DE-AC02-98CH10886. D.C.B. acknowledges use of the Center for Nanoscale Systems, a member of the National Nanotechnology Coordinated Infrastructure Network, which is supported by the National Science Foundation under NSF award no. 1541959. J.v.d.B. was supported by the DFG through the Wurzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter - ct.qmat (EXC 2147, project ID 39085490).

## Author information

Authors

### Contributions

M.K. performed the ARPES experiment and analysed the resulting data while A.L., C.J., A.B., E.R., K.K. and E.V. assisted. L.Y. synthesized and characterized the single crystals and performed the quantum oscillation experiments while M.K.C., R.D.M. and D.G. assisted. M.P.G. and S.F. performed the theoretical calculations while J.-S.Y., J.I.F. J.v.d.V., M.R. and E.K. assisted. M.H. and M.K. conducted the AFM measurements. D.C.B. performed the electron microscopy study. All authors contributed to writing the manuscript. J.G.C. and R.C. supervised the project.

### Corresponding authors

Correspondence to Joseph G. Checkelsky or Riccardo Comin.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figures 1–17, Notes 1–9, refs. 1–10.

## Rights and permissions

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Kang, M., Ye, L., Fang, S. et al. Dirac fermions and flat bands in the ideal kagome metal FeSn. Nat. Mater. 19, 163–169 (2020). https://doi.org/10.1038/s41563-019-0531-0

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• DOI: https://doi.org/10.1038/s41563-019-0531-0

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