Abstract
Electronic flat bands in momentum space, arising from strong localization of electrons in real space, are an ideal stage to realize stronglycorrelated phenomena. Theoretically, the flat bands can naturally arise in certain geometrically frustrated lattices, often with nontrivial topology if combined with spinorbit coupling. Here, we report the observation of topological flat bands in frustrated kagome metal CoSn, using angleresolved photoemission spectroscopy and band structure calculations. Throughout the entire Brillouin zone, the bandwidth of the flat band is suppressed by an order of magnitude compared to the Dirac bands originating from the same orbitals. The frustrationdriven nature of the flat band is directly confirmed by the chiral dorbital texture of the corresponding realspace Wannier functions. Spinorbit coupling opens a large gap of 80 meV at the quadratic touching point between the Dirac and flat bands, endowing a nonzero Z_{2} invariant to the flat band. These findings demonstrate that kagomederived flat bands are a promising platform for novel emergent phases of matter at the confluence of strong correlation and topology.
Introduction
Electronic correlations are a hallmark of condensed matter systems with manybody character. Localizing electrons in real space is often considered as a route to enhance correlation effects and engineer emergent phases of matter. Most wellknown examples include delectron systems, where the subtle balance between kinetic energy and localizationenhanced Coulomb interaction leads to collective electron behavior and rich manybody physics encompassing unconventional superconductivity, metal–insulator transitions, densitywave instabilities, and quantum spin liquids^{1}. In bandlike systems, electrons can still be confined in real space in lattices supporting dispersionless electronic excitations (i.e., flat bands) in momentum space. Due to the prominence of the interaction energy scale over the quenched kinetic energy, flat bands represent a versatile platform to explore exotic correlated electron phenomena. Notable examples include felectron systems with Kondo physics and heavy fermions^{2}, Landau levels under high magnetic fields and the fractional quantum Hall effect^{3}, and, more recently, magicangle twisted bilayer graphene superlattices with Mottinsulating phase and unconventional superconductivity^{4,5}.
A known experimental route to engineering electronic confinement and flat bands relies on the destructive quantum phase interference of fermion hopping paths in certain networks, including the dice, Lieb, kagome, and decorated square lattices^{6,7,8,9,10,11,12}. Here we focus on the case of the kagome lattice depicted in Fig. 1a. In the simplest nearestneighbor electronic hopping model of the sorbital kagome lattice H = Σ_{<i,j>}c\({\,\!}_{i}^{\dagger}\)c_{j} + h.c., one can construct realspace eigenfunctions with alternating phases at neighboring corners of the hexagon (Fig. 1a). This electronic state is geometrically confined within the single hexagon since any hopping to neighboring cells is hindered by the destructive phase interference as shown in Fig. 1a. This realspace electronic localization translates into momentum–space (Bloch) eigenfunctions with no energy dispersion, namely flat bands (Fig. 1b). In the tightbinding model of kagome lattice, this dispersionless excitation materializes alongside a pair of Dirac bands that are protected by the lattice symmetry similar to the case of the honeycomb lattice. Both the linear band crossing at K and quadratic band touching point at Γ become gapped once spin–orbit coupling (SOC) is included, and the Dirac and flat bands become topologically nontrivial^{10,11,12,13,14,15}. This peculiar band structure of the kagome lattice has recently attracted significant interest, not only in the context of electronic topology—topological insulator, Chern insulator, and fractional quantum Hall phases^{10,11,12,13,14,15}—but also as a platform to realize manybody electronic phases—density waves, magnetism, Pomeranchuk instability, and superconductivity^{16,17,18}.
While the latticeborn flat bands have been recently observed in optical and engineered atomic Lieb lattices, their experimental realization in a solidstate system has remained elusive^{19,20}. Unlike the ideal case shown in Fig. 1b, the dispersion of the flat band in real kagome compounds is modified by additional factors, such as inplane nextnearestneighbor hopping, interlayer coupling, and multiple orbital degrees of freedom. Therefore, the experimental realization of the kagome flat band requires careful and systematic material design. Prior scanning tunneling microscopy (STM) studies on kagome compounds Fe_{3}Sn_{2} and Co_{3}Sn_{2}S_{2} reported the evidence of flat bands from the enhancement of the momentumintegrated density of states (DOS)^{21,22,23}. However, detailed band structure calculations revealed that the region of existence of these dispersionless excitations is rather limited in the momentum space^{21,24}, due to complex hopping pathways distorting the flat dispersions in these compounds. These calculations are consistent with the relatively weak enhancement of the DOS in STM measurements compared to the ideal flat band case (see the Supplementary Fig. 1 for details)^{21,22,23}. Accordingly, angleresolved photoemission spectroscopy (ARPES) experiments have been carried out on Fe_{3}Sn_{2} and Co_{3}Sn_{2}S_{2}, but no robust evidence for flat bands have been detected^{24,25}. The unambiguous momentum–space identification of the kagomebased flat band and analysis of its topological character have, therefore, remained the subject of ongoing investigations.
In the present study of kagome metal CoSn, we combine ARPES and band structure calculations to report the presence of topological flat bands with suppressed bandwidth in all three momentum–space directions. CoSn belongs to the family of binary kagome metals T_{m}X_{n} (T: 3d transition metals, X: Sn, Ge, m:n = 3:1, 3:2, 1:1), wherein the kagome network is constructed upon 3d transition metals as shown in Fig. 1c. Compounded with the variety of magnetic ground states and topological electronic structures, this material family has been recently spotlighted as a versatile platform for novel correlated topological phases^{25,26,27,28,29}. For example, previous studies on Mn_{3}(Sn/Ge), Fe_{3}Sn_{2}, and FeSn have revealed band singularities ranging from threedimensional Weyl points to twodimensional (2D) Dirac points, which, in combination with intrinsic magnetism, generate large and intrinsic anomalous Hall conductivity^{25,26,27,28,29}. Among the series, TX compounds (m:n = 1:1) have been suggested as viable hosts for the prototypical kagome electronic structure because of their spatially decoupled quasi2D kagome planes (Fig. 1d)^{26}. In FeSn, for example, the kagomederived 2D Dirac band structure has been observed by ARPES, as well as a dispersionless band below the Fermi level, whose relationship with the observed Dirac bands has not be resolved^{26}. Consequently, the presence of a gap at the quadratic band touching points between the Dirac and flat bands could not be examined, despite its central role for the topological character of the flat band. The replacement of Co at the transition metal site suppresses the formation of local moments and magnetic ordering in this compound, presumably due to a higher dorbital filling^{30}, while, at the same time, it shifts the overall band structure below the Fermi energy, so that all kagomederived electronic excitations (both Dirac the flat bands) can be accessed by ARPES. Consequently, we could directly visualize the kagomederived flat band as well as the large SOC gap at the quadratic band touching point between Dirac and flat bands, which endows nontrivial topology to the flat band as long predicted theoretically.
Results
Visualization of kagome flat bands using ARPES
Figure 2 summarizes the experimental band structure of CoSn as measured using ARPES. The data in Fig. 2a–h were acquired using 92 eV photoexcitation, which maximizes the signal from kagome band structures. Figure 2a shows the Fermi surface of CoSn and its characteristic hexagonal symmetry (the surface Brillouin zone is marked by the white dashed lines) as expected from the underlying symmetry of the kagome lattice. In Fig. 2b–f we display a series of energy–momentum dispersions measured at k_{y} = 0.0, 0.40, 0.79, and 1.19 Å^{−1} (corresponding to orange, brown, green, and cyan traces in Fig. 2a, respectively) across various highsymmetry points. As shown in Fig. 2b, c, the energy–momentum dispersion along the \({\bar{Γ}}\)–\({\bar{\mathrm{M}}}\) highsymmetry direction displays a strikingly nondispersive band near the Fermi level at −0.27 ± 0.05 eV, which manifests itself independently of photon polarization. The dispersion of the flat band in this specific direction is lower than the experimental broadening of the band, which is below 50 meV. As shown in Fig. 2d–f, the nondispersive nature of the flat band spans most of the Brillouin zone, and acquires a small dispersion only close to the \({\bar{\mathrm{K}}}\) point. We note that the acquisition of a small but finite bandwidth near the \({\bar{\mathrm{K}}}\) point is typical for realistic kagome models due to nextnearestneighbor hopping^{14,22,31}. Even using conservative estimates, the bandwidth of the flat band over the entire Brillouin zone does not exceed 150 meV, suggesting that the electron kinetic energy is strongly quenched by quantum interference effects, preventing delocalization of the wave function across the lattice.
To further examine the lack of dispersion of the flat band, we present in Fig. 2g the experimental band structure measured along the \({\bar{\Gamma}}\)–\({\bar{\mathrm{K}}}\)–\({\bar{\mathrm{M}}}\) highsymmetry direction. Near the K point, a linearly dispersing Dirac band is found (see also Fig. 2d, e and Supplementary Fig. 2 for further characterization of the Dirac bands), which is also characteristic of the kagome band structure as previously observed in Fe_{3}Sn_{2} and FeSn^{25,26}. The Dirac point is located at −0.57 ± 0.05 eV, and only one branch of the Dirac cone could be observed along \({\bar{\Gamma}}\)–\({\bar{\mathrm{K}}}\) direction due to the matrix element effect associated to the chirality of the Dirac fermion similar to graphene^{26,32}. The velocity of the Dirac band is (1.8 ± 0.1) × 10^{5} m/s, which is renormalized by 23% from the density functional theory (DFT) value (see below) similar to magnetic 3d metals Fe and Ni^{33,34}. With the yellow dashed boxes and red and blue bars of Fig. 2g, we directly compare the bandwidth of the Dirac and flat bands. The bandwidth of Dirac band extends over a range of 1.5 eV, which is typical for 3delectron systems, such as elemental transition metals and cuprates^{35,36,37}. In contrast, the reduced bandwidth of the flat band (<0.15 eV) is highly unusual, and can be regarded as a direct consequence of quantum phase interference effects in the kagome lattice as introduced in Fig. 1a.
A defining trait of the flat band is a diverging DOS, which often sets the stage for emergent electronic phases characterized by collective electronic, magnetic, and superconducting orders^{16,17,18}. The high DOS from the flat band in CoSn can be captured (if we neglect the photoemission matrix element effect) from the momentumintegrated energy distribution curves shown in Fig. 2h. Here, colored lines are the momentumintegrated spectra from the energy–momentum sections in Fig. 2c–f, while the black line is obtained by integration over the full first Brillouin zone. All integrated energy distribution curves show a sharp and intense peak near the fixed flat band energy −0.27 ± 0.05 eV), reflecting the high DOS associated to the nondispersive flat band. In contrast, the spectral weight from all other bands (including the Dirac bands) is spread out in energy and further modulated as a function of k_{y}.
In Fig. 2i–m we present the evolution of the flat band as a function of outofplane momentum k_{z} as measured using photonenergydependent ARPES. The flat band displays negligible dispersion (<50 meV) along Γ–A as shown in Fig. 2i, while fully retaining its inplane flatness as shown in the representative cuts of Fig. 2j–m, measured at k_{z} = 0, π/3, 2π/3, and π (mod 2π). The flat band can thus be considered flat also along the k_{z} direction. Even though the bandwidthcanceling mechanisms are different for the inplane and outofplane directions—the former is quenched by quantum phase interference, while the latter is suppressed by virtue of quasi2D layered structure (see Fig. 1d)—the flat dispersion is realized along all three momentum directions in CoSn. This is indeed at variance with the limited momentum range of the flat bands in previously studied kagome compounds^{21,22,23,24}. Accordingly, the kinetic energy of flat band electrons in CoSn is strongly quenched, and interactiondriven manybody ground states are naturally expected once this latticeborn flat band is tuned to the Fermi level. In other words, if we start from a simple Hubbard model H = −tΣ_{<i,j>}c\({\,\!}_{i}^{\dagger}\)c_{j} + UΣ_{i}n_{i↑}n_{i↓}, where t is the hopping integral and U is the onsite interaction, the large U/t value essential to promote strong electronic correlations can be attained even with relative small U value owing to the quenched t of the flat bands. We calculate U ≈ 5–6 eV for Co 3d electrons, based on the linear response approach^{38} (see Methods), confirming that the interaction energy scale indeed dominates the kinetic energy scale of the flat band electrons as measured by the quenched bandwidth of the flat bands (<0.2 eV). The realization of strongly correlated phases based on the flat band electrons has been recently demonstrated in magicangle twisted Moiré superlattices, whose flat minibands serve as a basis for Mottinsulating, superconducting, and magnetic ground states^{4,5,39}. Similar correlated states of matter have been also theoretically investigated based on the flat band of the kagome lattice^{16,17,40,41,42,43}. Here, we experimentally identify a candidate material to extend this research avenue to real kagome systems.
DFT calculations and Wannier function analysis
To understand the origin of the observed flat band, we use relativistic DFT to calculate the theoretical band structure of CoSn as shown in Fig. 1e. Despite the complexity inherent to the presence of multiple dorbital degrees of freedom, the calculations closely capture the experimental manifestation of the kagome band structure. For example, in the k_{z} = 0 plane, a strongly dispersing Dirac bands (with Dirac points at K) is reproduced between −0.3 and −1.8 eV with a 1.1–1.5 eV bandwidth. At the same time, two flat bands (with bandwidth quenched below 0.2 eV) appear above the Dirac bands (highlighted with orange and cyan boxes), in close correspondence to Fig. 1b. Our analysis of orbital character in Supplementary Fig. 3 reveals that the two flat bands arise from different orbital degrees of freedom: the upper (lower) flat band in the orange (cyan) box is mainly formed by d_{xy}/d_{x2 −y2} (d_{xz}/d_{yz}) orbitals. These flat bands also have suppressed dispersion along k_{z} direction (see Γ–A direction for example), consistent with experimental observations. We also found a flat band at k_{z} = π (brown box in Fig. 1e), which arises from d_{z2} orbital. However, due to its outofplane orbital character, the band is not effectively localized in the k_{z} direction, exhibiting a bandwidth >1 eV. We note that the steep k_{z} dispersive bands may account for the low outofplane resistivity and positive outofplane Hall coefficient observed in our transport measurements on CoSn (see the Supplementary Note 1 and Supplementary Fig. 4).
For a detailed comparison between experiment and theory, we tune the photon energy to 128 eV to visualize the inplane electronic structure at the k_{z} = 0 (mod 2π) plane and acquire highresolution energy–momentum maps of the flat band. As shown in Fig. 3a, c, we could directly detect the two flat bands with dispersion <0.1 eV along Γ–M and <0.2 eV along Γ–K–M. The experimental dispersion closely follows the theoretical dispersion shown in Fig. 3b, d (which are shifted up by 140 meV to match the experimental Fermi level), confirming the assignment of these features to the two flat bands arising from different dorbital degrees of freedom as discussed above. The Dirac point is again observed at K, and positioned at slightly higher binding energy 0.73 ± 0.05 eV due to small but finite dispersion of the Dirac bands along k_{z}. We note that the flat band highlighted in Fig. 2 corresponds to the lower flat band (cyan) with d_{xz}/d_{yz} orbital characters. Below, we will focus on these prototypical flat bands at k_{z} = 0 to analyze their localization and topology.
At this point, an important outstanding question is how the localization mechanism in the simple sorbital kagome tightbinding model (Fig. 1a, b) manifests in the realistic dorbital kagome lattice of CoSn. To address this aspect, we derived a DFTbased ab initio tightbinding model of CoSn (Supplementary Fig. 5), and use the k_{z} = 0 flat bands to construct the realspace effective Wannier functions on the 2D kagome plane (see the Supplementary Note 3 for details). We construct the flat band Wannier function to retain the highest degree of symmetries possible (except those abandoned by the topological obstructions associated with nontrivial Z_{2} invariant and mirror Chern number; see “Discussion” below), which include a subset of important symmetries of the kagome lattice such as C_{6} rotational symmetry, xz/yz mirror symmetry, combined inversion/timereversal symmetry, and combined xy mirror/timereversal symmetry. As such, the Wannier functions we derived could serve as a basis for future analyses of interaction effects within the flat bands. The realspace orbital textures of the constructed flat band Wannier functions are displayed in Fig. 3e, f, while the corresponding spin textures are displayed in Supplementary Fig. 6. Several important points are apparent: (1) Due to the finite dispersion of the flat bands in CoSn, we could observe a finite charge density leaking out of the central hexagon unlike the ideal case in Fig. 1a. Nonetheless, the charge density rapidly and exponentially decays away from the central hexagon (insets in Fig. 3e, f), and 85% of total charge are confined in the first to third sites from the center. This provides the length scale of the localization ≈7 Å. (2) If we focus on the states at the central hexagon, chiral orbital textures could be observed around the hexagon, in which the d orbitals at the neighboring corners are aligned antiphase toward the site outside the hexagon. This is in reminiscence of the alternating phases in the compact localized states of sorbital kagome model in Fig. 1a. In the Supplementary Note 3 and Supplementary Fig. 10, we have demonstrated that such orbital textures combined with finetuned multiorbital hopping parameters in the kagome lattice geometry suppress the charge leakage outside the hexagon and localize the realspace electronic wave functions. This finding implies that the destructive quantum phase interference from alternating phases in the swave kagome tightbinding model is transferred to the realspace chiral orbital textures in realistic dorbital kagome model. This analogy unequivocally confirms the frustrationdriven origin of the observed flat bands in CoSn. (3) The Wannier functions centered at the hexagon of the kagome lattice effectively forms a triangular lattice. We estimate the hopping t between Wannier functions at neighboring triangular lattice sites to be ≈15 meV reflecting suppressed kinetic energy of the flat band electrons (Supplementary Table 4). In this context, the exotic electronic phases recently derived from the Hubbard model on triangular lattices, including spiral magnetic orders, unconventional superconductivity, and quantum or chiral spin liquids, might be relevant to the flat bands in kagome lattice^{44,45,46}.
Nontrivial topology of the kagome flat bands
After demonstrating the realization of the kagomederived flat band in CoSn, we examined the SOCinduced gap opening at the quadratic touching point (at Γ) between the Dirac band and the flat band. In the viewpoint of the prototypical tightbinding framework for the kagome lattice, the gap at the quadratic band touching point is responsible for rendering the flat band topologically nontrivial (see Figs. 1b and 4c), endowing nonzero Chern number/Z_{2} invariant under the timereversal breaking/symmetric condition^{10,11,12,13,14,15}. Figure 4a highlights the band dispersion near the Γ point obtained by averaging three spectra taken at the first, second, and third Brillouin zone to minimize the influence of photoemission matrix element effect to the intensity distributions. The band dispersion closely follows the DFT calculation shown in Fig. 4b, and in particular, it exhibits a quadratic band that emerges from the Dirac band at K (see also Fig. 3c, d) and touches the flat band at Γ. Focusing on this quadratic band and the flat band with same orbital origin (d_{xz}/d_{yz} orbitals, cyancolored lines Fig. 4b), the detailed analysis of the energy distribution curves in Fig. 4d clearly reveals the spin–orbitinduced gap opening at the quadratic touching point. The SOC gap size could be quantified to be Δ_{ARPES} = 80 ± 20 meV (Δ_{DFT} = 57 meV), which is considerably larger than the gap observed at the linear band crossing of Fe_{3}Sn_{2} ≈30 meV^{25}. The direct observation of the SOC gap between the Dirac and the flat band strongly signals the nontrivial topology of the observed flat band at k_{z} = 0.
To support this, we use the DFTderived Wannier tightbinding model to analyze the parity eigenvalue of the flat bands at the k_{z} = 0 plane following the Fu–Kane formula^{47}. This analysis yields a topological index Z_{2} = 1 for both flat bands confirming their topological nature (Fig. 5a). We note that the nontrivial topology of the flat bands is also reflected in the spin texture of the constructed Wannier wave functions (Supplementary Fig. 6), where timereversal symmetry has to be implemented in a nontrivial/nonlocal manner due to the Z_{2} obstruction^{48}. To exemplify the manifestation of the nontrivial flat band topology on bulk properties, we also calculated the band and momentumresolved spin Hall conductivity (SHC) of CoSn (Fig. 5c, d). As displayed in Fig. 5c, the inplane momentumresolved SHC is concentrated near the lifted degeneracy point between the Dirac and flat bands, which connects to the topological nature of the latter. The k_{z}resolved SHC in Fig. 5d additionally demonstrates that, owing to the twodimensionality of the flat band in CoSn (Figs. 1e and 2i–m), its contribution to the SHC is fairly k_{z} independent for ~40% of the Brillouin zone, before it hybridizes with other k_{z}dispersive bands. When extended to the 2D limit of a single kagome sheet, the 2D topological insulator state can be realized based on the topological flat band exactly at the Fermi level (Fig. 5b), where helical edge states and quantized SHC are expected inside the SOC gap. Overall, our experimental observation of the SOC gap between the Dirac and flat bands confirms the realization of another defining properties of 2D kagome band structure in CoSn, which endows nontrivial topological character to the correlated flat band electrons as supported by our calculations.
In sum, we have successfully discovered and characterized the topological flat bands in the frustrated kagome lattice CoSn. An important future research direction is to raise the kagome flat band to the Fermi level to observe the proposed latticeborne correlated topological phases. Potential routes to this include bulk doping (e.g., by Co_{1 − x}Fe_{x}Sn, ref. ^{49}), monolayer fabrication, and application of compressive inplane strain as suggested by our DFT calculations (Supplementary Figs. 7–9). Altogether, the observation of topological flat bands in the kagome lattice opens up a new avenue to study correlationdriven emergent electronic phenomena on the background of topological nontriviality.
Methods
Synthesis of CoSn single crystals
Single crystals of CoSn were synthesized using an Sn selfflux method. Cobalt powder (Alfa Aesar, 99.998%) and tin pieces (Alfa Aesar, 99.9999%) were mixed with a molar ratio of 1:9 and loaded in an alumina crucible and sealed in a quartz tube under high vacuum. The tube was heated to 950 °C and maintained for 5 h, and was gradually cooled to 650 °C with a typical cooling rate of 2–3 °C/h. At 650 °C the tube was removed from the furnace and centrifuge dissociation was performed to separate the crystals from flux. Crystals with a hexagonal prismatic shape were obtained with the longest dimension (6–8 mm) typically along [001].
Angleresolved photoemission spectroscopy
ARPES experiments were performed at Beamline 4 (MERLIN) and Beamline 7 (MAESTRO) of the Advanced Light Source equipped with R8000 and R4000 hemispherical electron analyzers (Scienta Omicron), respectively. For ARPES measurements, surface of CoSn was prepared in two different ways, one by ex situ fine polishing followed by in situ Ar^{+} ion sputtering and annealing at 1000 °C and the other by in situ lowtemperature cleaving. The experiments were performed below ≈60 K and under the ultrahigh vacuum ≈4 × 10^{−11} torr. The photon energy was scanned in the range from 30 to 160 eV, which covers more than two full threedimensional Brillouin zone of CoSn. Corresponding k_{z} momentum was calculated by assuming nearly freeelectron final state with inner potential 5.5 eV. We used spolarized photons unless specified, which maximize the signal from the flat bands. The convoluted energy and momentum resolutions of the beamline and the analyzer were better than 30 meV and 0.01 Å^{−1}, respectively. Termination dependence or the effect of surface polarity have not been observed in our ARPES experiment.
Electronic structure calculation from the first principles
DFT calculations were performed with the Vienna Ab initio Simulation Package^{50,51}. The pseudopotentials are of the Projector Augmented Wave^{52} type with exchangecorrelation energy functional parameterized by Perdew, Burke, and Ernzerhof^{53} within the generalized gradient approximation. The bulk DFT calculations are converged with planewave energy cutoff 350 eV and a reciprocal space Monkhorst–Pack grid sampling of size 15 × 15 × 11. The SOC terms are included for the electronic band structure. The calculation of the effective interaction parameters U in CoSn is performed based on the linear response approach following ref. ^{38} (see the Supplementary Fig. 14 for details). We note that the theoretical Fermi level from the DFT needs to be shifted down by 140 meV to fit the experimental band structure, presumably due to a slight Sn offstoichiometry in our crystals. Such shifting has been applied in the calculations presented in Figs. 1e, 2h, 3b, d, 4b, and 5c, d, and Supplementary Figs. 3, 5, and 13.
Wannier tightbinding model of CoSn
To interpret the firstprinciple calculations, the postprocess Wannier90 code^{54} is used to convert the extended periodic Bloch wave function basis in the DFT into the localized Wannier functions basis in the real space^{55}. With this Wannier transformation, the effective tightbinding Hamiltonian for a selected group of bands of the material can be constructed. The construction gives both the accurate DFT bands and physically transparent pictures of localized atomic orbitals and their mutual hybridizations. Here, we have included Co 3d states, and Sn 5s and 5p states in the Wannier construction for the bands around the Fermi level. The Wannier model is first constructed without SOC. The SOC interactions are modeled with atomic terms, which capture the essential feature of the DFT bands with SOC. With these Wannier models, the symmetry properties and the topological index could be directly computed. In the Supplementary Fig. 5a, b, we show the band structure comparison between the full DFT calculations and the interpolated Wannier tightbinding model with and without SOC modeling, respectively. The extracted SOC strength \(\lambda\vec{L}\cdot\vec{S}\) has λ_{Co} ≈ 70 meV for Co atoms and λ_{Sn} ≈ 290 meV for Sn atoms. Hybridization between Co and Sn orbitals may account for the larger mass of the Dirac electrons at K (see Supplementary Figs. 3a and 5b) compared to related kagome compounds Fe_{3}Sn_{2} and FeSn^{25,26}. The constructed Wannier tightbinding model for CoSn allows us to perform ab initio calculations for the SHC and the kresolved contributions from each band, using the Kubo formula following refs. ^{56,57}.
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.
References
 1.
Maekawa, S. et al. Physics of Transition Metal Oxides (Springer Nature, Switzerland, 2004).
 2.
Si, Q. & Steglich, F. Heavy fermions and quantum phase transitions. Science329, 1161–1166 (2010).
 3.
Tsui, D. C., Stormer, H. L. & Gossard, A. C. Twodimensional magnetotransport in the extreme quantum limit. Phys. Rev. Lett.48, 1559–1562 (1982).
 4.
Cao, Y. et al. Correlated insulator behaviour at halffilling in magicangle graphene superlattices. Nature556, 80–84 (2018).
 5.
Cao, Y. et al. Unconventional superconductivity in magicangle graphene superlattices. Nature556, 43–50 (2018).
 6.
Sutherland, B. Localization of electronic wave functions due to local topology. Phys. Rev. B34, 5208–5211 (1986).
 7.
Lieb, E. H. Two theorems on the Hubbard Model. Phys. Rev. B62, 1201–1204 (1989).
 8.
Leykam, D., Andreanov, A. & Flach, S. Artificial flat band systems: from lattice models to experiments. Adv. Phys. X3, 677–701 (2018).
 9.
Wu, C., Bergman, D., Balents, L. & Das Sarma, S. Flat bands and wigner crystallization in the honeycomb optical lattice. Phys. Rev. Lett.99, 070401 (2007).
 10.
Sun, K., Gu, Z., Katsura, H. & Das Sarma, S. Nearly flatbands with nontrivial topology. Phys. Rev. Lett.106, 236803 (2011).
 11.
Tang, E., Mei, J.W. & Wen, X.G. Hightemperature fractional quantum Hall states. Phys. Rev. Lett.106, 236802 (2011).
 12.
Neupert, T., Santos, L., Chamon, C. & Mudry, C. Fractional quantum Hall states at zero magnetic field. Phys. Rev. Lett.106, 236804 (2011).
 13.
Guo, H. M. & Franz, M. Topological insulator on the kagome lattice. Phys. Rev. B80, 113102 (2009).
 14.
Xu, G., Lian, B. & Zhang, S.C. Intrinsic quantum anomalous Hall effect in the kagome lattice Cs_{2}LiMn_{3}F_{12}. Phys. Rev. Lett.115, 186802 (2015).
 15.
Bolens, A. & Nagaosa, N. Topological states on the breathing kagome lattice. Phys. Rev. B99, 165141 (2019).
 16.
Mielke, A. Exact ground states for the Hubbard model on the Kagome lattice. J. Phys. A25, 4335–4345 (1992).
 17.
Wen, J., Ruegg, A., Wang, C. C. & Fiete, G. A. Interactiondriven topological insulators on the kagome and the decorated honeycomb lattices. Phys. Rev. B82, 075125 (2010).
 18.
Kiesel, M. L., Platt, C. & Thomale, R. Unconventional fermi surface instabilities in the kagome Hubbard model. Phys. Rev. Lett.110, 126405 (2013).
 19.
Taie, S. et al. Coherent driving and freezing of bosonic matter wave in an optical Lieb lattice. Sci. Adv.1, e1500854 (2015).
 20.
Drost, R., Ojanen, T., Harju, A. & Liljeroth, P. Topological states in engineered atomic lattices. Nat. Phys.13, 668–671 (2017).
 21.
Lin, Z. et al. Flatbands and emergent ferromagnetic ordering in Fe_{3}Sn_{2} kagome lattices. Phys. Rev. Lett.121, 096401 (2018).
 22.
Yin, J.X. et al. Negative flat band magnetism in a spin–orbitcoupled correlated kagome magnet. Nat. Phys.15, 443–448 (2019).
 23.
Jiao, L. et al. Signatures for halfmetallicity and nontrivial surface states in the kagome lattice Weyl semimetal Co_{3}Sn_{2}S_{2}. Phys. Rev. B99, 245158 (2019).
 24.
Liu, D. F. et al. Magnetic Weyl semimetal phase in a Kagome crystal. Science365, 1282–1285 (2019).
 25.
Ye, L. et al. Massive Dirac fermions in a ferromagnetic kagome metal. Nature555, 638–642 (2018).
 26.
Kang, M. et al. Dirac fermions and flat bands in ideal kagome metal FeSn. Nat. Mater.19, 163–169 (2020).
 27.
Nakatsuji, S., Kiyohara, N. & Higo, T. Large anomalous Hall effect in a noncollinear antiferromagnet at room temperature. Nature527, 212–215 (2015).
 28.
Kuroda, K. et al. Evidence for magnetic Weyl fermions in a correlated metal. Nat. Mater.16, 1090–1095 (2017).
 29.
Nayak, A. K. et al. Large anomalous Hall effect driven by nonvanishing Berry curvature in noncollinear antiferromagnet Mn3Ge. Sci. Adv.2, e1501870 (2016).
 30.
Allred, J. M., Jia, S., Bremholm, M., Chan, B. C. & Cava, R. J. Ordered CoSntype ternary phases in Co 3Sn 3−xGe x. J. Alloy Compd.539, 137–143 (2012).
 31.
Mazin, I. I. et al. Theoretical prediction of a strongly correlated Dirac metal. Nat. Commun.5, 4261 (2014).
 32.
Liu, Y., Bian, G., Miller, T. & Chiang, T. C. Visualizing electronic chirality and Berry phases in graphene systems using photoemission with circularly polarized light. Phys. Rev. Lett.107, 166803 (2011).
 33.
Schafer, J., Hoinkis, M., Rotenberg, E., Blaha, P. & Claessen, R. Fermi surface and electron correlation effects of ferromagnetic iron. Phys. Rev. B72, 155115 (2005).
 34.
Eberhardt, W. & Plummer, E. W. Angleresolved photoemission determination of the band structure and multielectron excitations in Ni. Phys. Rev. B21, 3245–3255 (1980).
 35.
Courths, R., Cord, B., Wern, H. & Hufner, S. Angleresolved photoemission and band structure of copper. Phys. Scr.1983, 144–147 (1983).
 36.
Kamakura, N. et al. Bulk band structure and Fermi surface of nickel: a soft xray angleresolved photoemission study. Phys. Rev. B74, 045127 (2006).
 37.
Damascelli, A., Hussain, Z. & Shen, Z. Angleresolved photoemission studies of the cuprate superconductors. Rev. Mod. Phys.75, 473–541 (2003).
 38.
Cococcioni, M. & de Gironcoli, S. Linear reponse approach to the calculation of the effective interaction parameters in the LDA + U method. Phys. Rev. B71, 035105 (2005).
 39.
Sharpe, A. L. et al. Emergent ferromagnetism near threequarters filling in twisted bilayer graphene. Science365, 605–608 (2019).
 40.
Chen, Y. et al. Ferromagnetism and Wigner crystallization in kagome graphene and related structures. Phys. Rev. B98, 035135 (2018).
 41.
Iglovikov, V. I. et al. Superconducting transitions in flatband systems. Phys. Rev. B90, 094506 (2014).
 42.
Huhtinen, K. et al. Spinimbalanced pairing and Fermi surface deformation in flat bands. Phys. Rev. B97, 214503 (2018).
 43.
Han, W. H. et al. A metal–insulator transition via Wigner crystallization in boron triangular kagome lattice. Preprint at https://arxiv.org/abs/1902.08390 (2019).
 44.
Sahebsara, P. & Senechal, D. Hubbard model on the triangular lattice: spiral order and spin liquid. Phys. Rev. Lett.100, 136402 (2008).
 45.
Szasz, A., Motruk, J., Zaletel, M. P. & Moore, J. E. Chiral spin liquid phase of the triangular lattice Hubbard model: a density matrix renormalization group study. Phys. Rev. X10, 021042 (2020).
 46.
Venderley, J. et al. Density matrix renormalization group study of superconductivity in the triangular lattice Hubbard model. Phys. Rev. B100, 060506(R) (2019).
 47.
Fu, L. & Kane, C. L. Topological insulators with inversion symmetry. Phys. Rev. B76, 045302 (2007).
 48.
Soluyanov, A. A. & Vanderbilt, D. Wannier representation of Z2 topological insulators. Phys. Rev. B83, 035108 (2011).
 49.
Meier, W. R. et al. Reorientation of antiferromagnetism in cobalt doped FeSn. Phys. Rev. B100, 184421 (2019).
 50.
Kresse, G. & Furthmuller, J. Efficient iterative schemes for ab initio totalenergy calculations using a planewave basis set. Phys. Rev. B54, 11169–11186 (1996).
 51.
Kresse, G. & Furthmller, J. Efficiency of abinitio total energy calculations for metals and semiconductors using a planewave basis set. Comput. Mater. Sci.6, 15–50 (1996).
 52.
Blochl, P. E. Projector augmentedwave method. Phys. Rev. B50, 17953–17979 (1994).
 53.
Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett.77, 3865–3868 (1996).
 54.
Mostofi, A. A. et al. An updated version of wannier90: a tool for obtaining maximallylocalised Wannier functions. Comput. Phys. Commun.185, 2309–2310 (2014).
 55.
Marzari, N., Mostofi, A. A., Yates, J. R., Souza, I. & Vanderbilt, D. Maximally localized wannier functions: theory and applications. Rev. Mod. Phys.84, 1419–1475 (2012).
 56.
Guo, G. Y., Murakami, S., Chen, T.W. & Nagaosa., N. Intrinsic spin hall effect in platinum: firstprinciples calculations. Phys. Rev. Lett.100, 096401 (2008).
 57.
Guo, G. Y., Yao, Y. & Niu, Q. Ab initio calculation of the intrinsic spin hall effect in semiconductors. Phys. Rev. Lett.94, 226601 (2005).
Acknowledgements
We thank Paul Neves for insightful discussions and helpful feedbacks. The authors acknowledge Yang Zhang for helping the computation of spin Hall conductivity. This work was supported by the STC Center for Integrated Quantum Materials, NSF Grant No. DMR1231319. 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. W911NF1610034. This research used resources of the Advanced Light Source, a US DOE Office of Science User Facility under contract no. DEAC0205CH11231. M.K. acknowledges support from the Samsung Scholarship from the Samsung Foundation of Culture. S.F. is supported by a Rutgers Center for Material Theory Distinguished Postdoctoral Fellowship. L.Y. acknowledges support from the Tsinghua Education Foundation.
Author information
Affiliations
Contributions
M.K., J.D., C.J., A.B., and E.R. performed the ARPES experiment and analyzed the resulting data. S.F. performed the theoretical calculations with help from H.C.P and E.K. L.Y. synthesized and characterized the single crystals. J.G.C. and R.C. supervised the project. M.K., S.F., L.Y., and R.C. wrote the manuscript with input from all coauthors.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing interests.
Additional information
Peer review informationNature Communications thanks the anonymous reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Supplementary information
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Kang, M., Fang, S., Ye, L. et al. Topological flat bands in frustrated kagome lattice CoSn. Nat Commun 11, 4004 (2020). https://doi.org/10.1038/s41467020174651
Received:
Accepted:
Published:
Further reading

Dirac cone, flat band and saddle point in kagome magnet YMn6Sn6
Nature Communications (2021)

Kagome vanderWaals Pd3P2S8 with flat band
Scientific Reports (2020)

Localized spinorbit polaron in magnetic Weyl semimetal Co3Sn2S2
Nature Communications (2020)

Quantum distance and anomalous Landau levels of flat bands
Nature (2020)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.