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Dirac fermions and flat bands in the ideal kagome metal FeSn

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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. A prototypical kagome metal with magnetic and topological properties is identified.
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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 phases38. 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 compounds1223. 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 calculations7,2022. 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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... The resulting interplay between magnetism, superconductivity, density-wave orders, and strong electronic correlations has been investigated by various experimental methods [11][12][13]. Recent studies have revealed the presence of Dirac/Weyl states and flat bands [14][15][16][17][18][19], chiral spin structures [20], and skyrmionic lattices [21,22] in magnetic kagome metals. In contrast, non-magnetic counterparts have shown intriguing density-wave orders and superconductivity [23][24][25][26]. ...
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The interplay between electronic correlations, density wave orders, and magnetism gives rise to several fascinating phenomena. In recent years, kagome metals have emerged as an excellent platform for investigating these unique properties, which stem from their itinerant carriers arranged in a kagome lattice. Here, we show that electronic structure of the prototypical kagome metal, Fe3_3Sn2_2, can be tailored by manipulating the breathing distortion of its kagome lattice with external pressure. The breathing distortion is suppressed around 15 GPa and reversed at higher pressures. These changes lead to a series of Lifshitz transitions that we detect using broadband and transient optical spectroscopy. Remarkably, the strength of the electronic correlations and the tendency to carrier localization are enhanced as the kagome network becomes more regular, suggesting that breathing distortion can be a unique control parameter for the microscopic regime of the kagome metals and their electron dynamics.
... The ferromagnetic coupling of Fe atoms within the kagome layers and the antiferromagnetic alignment between adjacent kagome layers contribute to its complex magnetic properties, as evidenced in neutron diffraction and Mössbauer experiments [13][14][15] . A notable feature of FeSn's band structure is the coexistence of Dirac fermions and flat bands close to the Fermi level 16 . Angle-resolved photoemission spectroscopy (ARPES) measurement on FeSn single crystals confirmed the existence of massless Dirac fermions in bulk and 2D Weyl-like states at the surface 17 . ...
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FeSn is a room-temperature antiferromagnet composed of alternating kagome layers and honeycomb Sn layers. Its distinctive lattice allows the formation of linearly dispersing Dirac bands and topological flatbands in its electronic band structure, positioning FeSn as an ideal candidate for investigating the interplay between magnetism and topology. In this study, we investigate the epitaxial growth of FeSn thin films on GaAs(111) substrates by molecular beam epitaxy. A significant challenge in this growth process is the diffusion of Ga and As from the substrate into the deposited films and the diffusion of Fe into the substrate. This diffusion complicates the formation of a pure FeSn phase. Through a comprehensive analysis—including reflection high energy electron diffraction, high-resolution x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometry—we demonstrate that the Sn evaporation temperature plays a critical role in influencing crystallinity, surface morphology, and magnetic behavior of the films. Our results show that while it is difficult to grow a single-phase FeSn film on GaAs due to diffusion, optimizing the Sn evaporation temperature can enhance the dominance of the FeSn phase, partially overcoming these challenges.
... Their kagome lattice results in Dirac cones that encode nontrivial topology, and van Hove singularities (VHSs) and flat bands, which drive electronic correlations. These electronic features have been observed in various kagome materials, such as the presence of Weyl fermions in the ferromagnet Co 3 Sn 2 S 2 [3,4], chiral spin textures, and a significant anomalous Hall effect in the noncollinear antiferromagnet Mn 3 X (X = Sn and Ge) [5,6], Dirac fermions and flat bands in FeSn and CoSn [7,8], coexisting magnetism and superconductivity in Cu 3 Sn 2 [9], and charge-density wave (CDW) in antiferromagnetic FeGe [10]. ...
... The dispersionless energy bands with the extremely large density of states near Fermi energy cause a plethora of exotic quantum states of matter, including ferromagnetism [1], unconventional superconductivity [2], heavy fermion states [3], fractional quantum Hall states [4], strange metallic states [5], and Mott insulating states [6]. These exotic states can be stabilized in various materials, such as magic-angle twisted bilayer graphene [2,[5][6][7][8], moiré transition metal dichalcogenides [9][10][11], f -electron intermetallics [3], and kagome materials [12][13][14][15][16]. ...
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