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Massive Dirac fermions in a ferromagnetic kagome metal

Abstract

The kagome lattice is a two-dimensional network of corner-sharing triangles1 that is known to host exotic quantum magnetic states2,3,4. Theoretical work has predicted that kagome lattices may also host Dirac electronic states5 that could lead to topological6 and Chern7 insulating phases, but these states have so far not been detected in experiments. Here we study the d-electron kagome metal Fe3Sn2, which is designed to support bulk massive Dirac fermions in the presence of ferromagnetic order. We observe a temperature-independent intrinsic anomalous Hall conductivity that persists above room temperature, which is suggestive of prominent Berry curvature from the time-reversal-symmetry-breaking electronic bands of the kagome plane. Using angle-resolved photoemission spectroscopy, we observe a pair of quasi-two-dimensional Dirac cones near the Fermi level with a mass gap of 30 millielectronvolts, which correspond to massive Dirac fermions that generate Berry-curvature-induced Hall conductivity. We show that this behaviour is a consequence of the underlying symmetry properties of the bilayer kagome lattice in the ferromagnetic state and the atomic spin–orbit coupling. This work provides evidence for a ferromagnetic kagome metal and an example of emergent topological electronic properties in a correlated electron system. Our results provide insight into the recent discoveries of exotic electronic behaviour in kagome-lattice antiferromagnets8,9,10 and may enable lattice-model realizations of fractional topological quantum states11,12.

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Figure 1: The kagome structure and Fe3Sn2.
Figure 2: Anomalous Hall response of Fe3Sn2.
Figure 3: Massive Dirac fermion at the zone corner of Fe3Sn2.
Figure 4: Tight binding and hall conductivity of a kagome bilayer.

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Acknowledgements

We are grateful to X.-G. Wen and E. Tang for discussions. This research was funded in part by the Gordon and Betty Moore Foundation EPiQS Initiative, grant GBMF3848 to J.G.C. and NSF grant DMR-1554891. L.Y., J.L. and F.v.C. acknowledge support by the STC Center for Integrated Quantum Materials, NSF grant number DMR-1231319. L.Y. acknowledges support by the Tsinghua Education Foundation. M.K. acknowledges a Samsung Scholarship from the Samsung Foundation of Culture. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract number DE-AC02-05CH11231. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF cooperative agreement number DMR-1157490, the State of Florida and the US Department of Energy.

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Contributions

L.Y., T.S. and C.R.W. grew the single crystals. L.Y. characterized the materials, performed the transport and magnetic measurements and analysed the resultant data. M.K., C.J., A.B. and E.R. performed the ARPES experiment and analysed the resultant data. J.L. and L.Y. performed the theoretical calculations. F.v.C. and D.C.B. performed the electron microscopy study. All authors contributed to writing the manuscript. L.F., R.C. and J.G.C. supervised the project.

Corresponding authors

Correspondence to Riccardo Comin or Joseph G. Checkelsky.

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Reviewer Information Nature thanks E. Bergholtz, B. Lake and O. Rader for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Metallic transport in Fe3Sn2.

a, Resistivity ρ as a function of temperature T in the kagome plane for Fe3Sn2 sample C1. The inset shows a photograph of Fe3Sn2 single crystals. b, c, Magnetoresistance (defined as MR = [ρxx(B) − ρxx(0)]/ρxx(0)) at selected T with B applied perpendicular (b) or parallel (c) to the kagome plane and B I (schematics of the configurations are shown as insets).

Extended Data Figure 2 Extracting anomalous Hall conductivity and high-field transport.

a, In-plane Hall conductivity σxy as a function of magnetic induction B at selected temperatures. Dashed lines represent the linear fit to . The data at 2 K and 50 K have been scaled by the factors shown for clarity. b, Magnetoresistance (main panel) and Hall effect (inset) of Fe3Sn2 with applied magnetic field μ0H c up to 31 T.

Extended Data Figure 3 Momentum and energy-dependent band structure along high-symmetry directions.

a, e, Fermi surface of Fe3Sn2 obtained from different experimental geometries. bd, f, g, Band dispersion of Fe3Sn2 along high-symmetry directions. The panels correspond to the momentum directions along the red (b), orange (c), green (d), magenta (f) and purple (g) dotted lines in a and e. The inset in d shows the raw data of Fig. 3c (with the same energy and momentum range), highlighting the spectral weight distribution near the Dirac points. h, Energy distribution curves at different K points indicated in c, d, f and g. The curves are shifted along the vertical direction for clarity. The inset shows an example of Gaussian fits; the extracted gap size is Δ = 30 ± 5 meV.

Extended Data Figure 4 Photon-energy dependence of ARPES spectra.

ARPES intensity plot for Fe3Sn2 taken along the Γ–K direction as a function of binding energy k and photon energy.

Extended Data Figure 5 Berry curvature and Hall conductivity for a massive Dirac fermion.

ac, Schematics of 2D Dirac fermions and the corresponding Bloch-sphere representation of the wavefunction of filled states for the gapless case (a) and the gapped case with EF in (b) and out of (c) the gap. d, Fermi energy EF dependence of σxy for the case of a single massive Dirac fermion with gap Δ and Fermi velocity vF.

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Ye, L., Kang, M., Liu, J. et al. Massive Dirac fermions in a ferromagnetic kagome metal. Nature 555, 638–642 (2018). https://doi.org/10.1038/nature25987

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