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Articles
https://doi.org/10.1038/s41563-019-0531-0
1Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA. 2Department of Physics, Harvard University, Cambridge, MA, USA.
3Leibniz Institute for Solid State and Materials Research, IFW Dresden, Dresden, Germany. 4Advanced Light Source, E. O. Lawrence Berkeley National
Laboratory, Berkeley, CA, USA. 5National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, NM, USA. 6National High
Magnetic Field Laboratory, Tallahassee, FL, USA. 7National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY, USA. 8John A. Paulson
School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA. 9Center for Nanoscale systems, Harvard University, Cambridge,
MA, USA. 10Dresden Center for Computational Materials Science (DCMS), TU Dresden, Dresden, Germany. 11Central Department of Physics, Tribhuvan
University, Kirtipur, Kathmandu, Nepal. 12These authors contributed equally: Mingu Kang, Linda Ye. *e-mail: checkelsky@mit.edu; rcomin@mit.edu
The kagome lattice is a two-dimensional (2D) network of
corner-sharing triangles (Fig. 1a) that originally gained the
spotlight as a platform for frustration-driven exotic spin-liq-
uid phases1,2. Recent theoretical investigations have focused on the
emergent electronic excitations engendered by the special geometry
of the kagome network, whose unique combination of lattice sym-
metry, spin–orbit coupling, and unusual magnetism sets an ideal
stage for novel topological phases3–8. Viewed as an isolated layer, the
kagome lattice hosts a flat band and a pair of Dirac bands as depicted
in the nearest-neighbour tight-binding calculation in Fig. 1b
(refs. 3,4). Compounded with spin–orbit coupling and a net magne-
tization, the 2D kagome lattice realizes a 2D Chern insulator phase
with quantized anomalous Hall conductance at 1/3 and 2/3 fillings5.
When these quantum anomalous Hall layers are stacked along the
third dimension, the interlayer interaction drives the mass gap to
be closed and reopened along the stacking axis, transforming the
system into a three-dimensional (3D) magnetic Weyl semimetal7,9.
Focusing on a flat band with quenched kinetic energy, interaction-
driven many-body electronic phases ranging from density waves
to superconductivity have been theoretically investigated10. At the
same time, the flat band on the kagome lattice also carries a finite
Chern number, and mimics the phenomenology of Landau levels,
without an external magnetic field8,11. As a result, the fractional
quantum Hall state can be realized at a partial filling of these flat
bands, further enriching the spectrum of topological phases that
can be harnessed within the kagome lattice.
These promising theoretical proposals have driven and guided
recent experimental efforts toward the realization and study of
topological kagome metals based on binary and ternary interme-
tallic compounds12–23. At variance with other widely studied s or p
orbital-based toplogical systems that are close to the non-interact-
ing limit, the kagome lattice in these intermetallic materials is popu-
lated by the low-energy 3d electrons of transition metals (Fig. 1a),
and thus provide an ideal platform to study the interplay of elec-
tronic topology and strong correlations. Correspondingly, not only
topological electronic structures but also rich intrinsic magnetism
can be found in the 3d kagome metal series. The combination of
these two aspects gives rise to intrinsic anomalous Hall conductivity
via various mechanisms12,14,16,20,21.
Despite the great potential and rich phenomenology of this
family of materials, the experimental realization of the electronic
structure of an idealized 2D kagome lattice, namely the Dirac fer-
mions and topological flat bands (Fig. 1b), in bulk magnetic kagome
crystals has remained an outstanding challenge. For instance, in the
binary intermetallic TmXn kagome series (T = Mn, Fe, Co; X = Sn,
Ge; m:n = 3:1, 3:2, 1:1) with various stacking sequences of kagome
and spacer S layers (Fig. 1c–e), the quasi-2D Dirac electronic struc-
ture has been detected only in Fe3Sn2 (ref. 16) but not in Mn3Sn
(ref. 14). Rather, in Mn3Sn and ternary kagome compound Co3Sn2S2,
3D magnetic Weyl points have been identified as the potential
origin for the chiral anomaly in transport14,20, as also confirmed by
band structure calculations7,20–22. For what concerns the flat bands,
Dirac fermions and flat bands in the ideal kagome
metal FeSn
Mingu Kang 1,12, Linda Ye 1,12, Shiang Fang 2, Jhih-Shih You 3, Abe Levitan1, Minyong Han1,
Jorge I. Facio3, Chris Jozwiak 4, Aaron Bostwick4, Eli Rotenberg 4, Mun K. Chan5, Ross D. McDonald 5,
David Graf 6, Konstantine Kaznatcheev7, Elio Vescovo7, David C. Bell8,9, Efthimios Kaxiras2,8,
Jeroen van den Brink3, Manuel Richter3,10, Madhav Prasad Ghimire 3,11, Joseph G. Checkelsky 1*
and Riccardo Comin 1*
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-dimen-
sional 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-dimen-
sional 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.
NATURE MATERIALS | VOL 19 | FEBRUARY 2020 | 163–169 | www.nature.com/naturematerials 163
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