The kagome lattice is a two-dimensional network of corner-sharing triangles that is known to host exotic quantum magnetic states. Theoretical work has predicted that kagome lattices may also host Dirac electronic states that could lead to topological and Chern insulating phases, but these states have so far not been detected in experiments. Here we study the d-electron kagome metal FeSn, 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 antiferromagnets and may enable lattice-model realizations of fractional topological quantum states.

The kagome lattice based on 3d transition metals is a versatile platform for novel topological phases hosting symmetry-protected electronic excitations and exotic magnetic ground states. However, the paradigmatic states of the idealized two-dimensional (2D) kagome lattice -Dirac fermions and topological flat bands -have not been simultaneously observed, partly owing to the complex stacking structure of the kagome compounds studied to date. Here, we take the approach of examining FeSn, an antiferromagnetic single-layer kagome metal with spatially-decoupled kagome planes. Using polarization-and termination-dependent angleresolved photoemission spectroscopy (ARPES), we detect the momentum-space signatures of coexisting flat bands and Dirac fermions in the vicinity of the Fermi energy. Intriguingly, when complemented with bulk-sensitive de Haas-van Alphen (dHvA) measurements, our data reveal an even richer electronic structure that exhibits robust surface Dirac fermions on specific crystalline terminations. Through band structure calculations and matrix element simulations, we demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals under kagome symmetry, while the surface state realizes a rare example of fully spin-polarized 2D Dirac fermions when combined with spin-layer locking in FeSn. These results highlight FeSn as a prototypical host for the emergent excitations of the kagome lattice. The prospect to harness these excitations for novel topological phases and spintronic devices is a frontier of great promise at the confluence of topology, magnetism, and strongly-correlated electron physics.

Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, epitaxial graphene/single-crystal 2D gallium, indium, and tin heterostructures. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to graphene overlayer, i.e. they are "half van der Waals" metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals open compelling opportunities for superconducting devices, topological phenomena, and advanced optoelectronic properties. For example, the reported 2D-Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly-freeelectron Fermi surface that closely approaches the Dirac points of the graphene overlayer.Major advances in fundamental science have followed from the exfoliation, stacking, and encapsulation of atomically thin 2D layers 1 . The next step towards technological impact of 2D layers and heterostructures is to transition sophisticated "pick and place" devices to a wafer-scale platform. However, the sensitivity of 2D systems to interfacial reactions and environmental influences -especially for two-dimensional metals or small-gap semiconductors -poses challenges for large-scale integration. Very few metals resist degradation of their top few atomic layers upon environmental exposure, and for a 2D metal, these layers constitute the entire system. A general platform for producing environmentally stable and wafer-scale 2D metals that are not prone to interfacial interactions would represent a significant advance. Inspired by the success of wide-bandgap 2D gallium nitride 2 , we turn focus onto the metal alone and demonstrate a platform dubbed confinement heteroepitaxy (CHet), where the interface between epitaxial graphene (EG) and silicon carbide (SiC) stabilizes crystalline 2D forms of Group-III (Ga, In) and group-IV (Sn) elements. Defect engineering of the graphene overlayer enables uniform, large-area intercalation at the high-energy SiC/EG interface; this interface then templates intercalant crystallization at a thermodynamically defined number of atomic layers. The unreactive nature of as-grown EG on SiC (graphene plus buffer layer) performs multiple services: (1) it only partially passivates the SiC surface underneath, thereby sustaining the high-energy interface that drives intercalation; (2) it lowers the energy of the (otherwise exposed) upper surface of the metal, thus facilitating 2D morphologies; (3) it protects the newly formed 2D metal from environmental degradation after intercalation through in situ healing of the graphene defects. Stability of these 2D metals in air over months gr...

Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe. Renormalization-group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge-density wave-transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal-insulator transition at 135 K that is driven by lattice distortions. The heterointerface plays an important role in driving this novel metal-insulator transition in the family of monolayer transition-metal dichalcogenides.

In high-temperature superconductivity, the process that leads to the formation of Cooper pairs, the fundamental charge carriers in any superconductor, remains mysterious. We used a femtosecond laser pump pulse to perturb superconducting Bi(2)Sr(2)CaCu(2)O(8+δ) and studied subsequent dynamics using time- and angle-resolved photoemission and infrared reflectivity probes. Gap and quasiparticle population dynamics revealed marked dependencies on both excitation density and crystal momentum. Close to the d-wave nodes, the superconducting gap was sensitive to the pump intensity, and Cooper pairs recombined slowly. Far from the nodes, pumping affected the gap only weakly, and recombination processes were faster. These results demonstrate a new window into the dynamical processes that govern quasiparticle recombination and gap formation in cuprates.

A universal high energy anomaly in the single particle spectral function is reported in three different families of high temperature superconductors by using angle-resolved photoemission spectroscopy. As we follow the dispersing peak of the spectral function from the Fermi energy to the valence band complex, we find dispersion anomalies marked by two distinctive high energy scales, E1 ≈ 0.38 eV and E2 ≈ 0.8 eV. E1 marks the energy above which the dispersion splits into two branches. One is a continuation of the near parabolic dispersion, albeit with reduced spectral weight, and reaches the bottom of the band at the Γ point at ≈ 0.5 eV. The other is given by a peak in the momentum space, nearly independent of energy between E1 and E2. Above E2, a band-like dispersion re-emerges. We conjecture that these two energies mark the disintegration of the low energy quasiparticles into a spinon and holon branch in the high Tc cuprates.Understanding how doped oxygen holes are transported in the environment of antiferromagnetically coupled copper spin is one of the most fundamental problems in the field of high temperature superconductivity. In 1988 Zhang and Rice [1] proposed that the doped holes in the oxygen 2pσ orbitals form singlets with the spins of the neighboring coppers. The resulting charge-e and spin-0 object is called the Zhang-Rice singlet (ZRS). As the ZRS moves through the CuO 2 plane, the copper spins get rearranged. As a result, the ZRS couples very strongly to the antiferromagnetic environment. Remarkably as a consequence of such strong coupling, quasiparticles emerge at low energies. This is evidenced by the sharp nodal quasiparticle peaks seen in angle-resolved photoemission (ARPES) of almost all cuprate compounds [2,3]. In simple physical terms a quasiparticle is a composite object made of a ZRS and a S=1/2 copper spins. It is widely believed that, at sufficiently low temperatures, superconducting pairing occurs between these quasiparticles giving rise to the high temperature superconducting state. Thus a microscopic understanding of the pairing mechanism of high Tc superconductors requires an in-depth understanding of how a ZRS is dressed into a quasiparticle.Here we present the first systematic study of the evolution of the ARPES spectral function from the Fermi level (E F ≡ 0) to the valence band complex (at energy ≈ 1 eV [4]) for three different families of high temperature superconductors. Our results provide a surprising new experimental understanding on the important quasiparticle formation process discussed above. Specifically, by covering a much broader energy range than typically studied [2], we have identified anomalies in the ARPES spectra occurring at two universal high energy scales, E 1 ≈ 0.38 eV and E 2 ≈ 0.8 eV from E F . We conjecture that these two energies mark the threshold for the disintegration of the low-energy quasiparticles at two different binding levels.ARPES data have been collected at the Advanced Light Source, beamlines 7.0.1, 10.0.1 and 12.0.1. for three different familie...

Electronic flat bands in momentum space, arising from strong localization of electrons in real space, are an ideal stage to realize strongly-correlated phenomena. Theoretically, the flat bands can naturally arise in certain geometrically frustrated lattices, often with nontrivial topology if combined with spin-orbit coupling. Here, we report the observation of topological flat bands in frustrated kagome metal CoSn, using angle-resolved 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 frustration-driven nature of the flat band is directly confirmed by the chiral d-orbital texture of the corresponding real-space Wannier functions. Spin-orbit 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 kagome-derived flat bands are a promising platform for novel emergent phases of matter at the confluence of strong correlation and topology.

Recently discovered materials called threedimensional topological insulators 1-5 constitute examples of symmetry protected topological states in the absence of applied magnetic fields and cryogenic temperatures. A hallmark characteristic of these non-magnetic bulk insulators is the protected metallic electronic states confined to the material's surfaces. Electrons in these surface states are spin polarized with their spins governed by their direction of travel (linear momentum), resulting in a helical spin texture in momentum space.6 Spin-and angle-resolved photoemission spectroscopy (spin-ARPES) has been the only tool capable of directly observing this central feature with simultaneous energy, momentum, and spin sensitivity.6-12 By using an innovative photoelectron spectrometer 13 with a highflux laser-based light source, we discovered another surprising property of these surface electrons which behave like Dirac fermions. We found that the spin polarization of the resulting photoelectrons can be fully manipulated in all three dimensions through selection of the light polarization. These surprising effects are due to the spin-dependent interaction of the helical Dirac fermions with light, which originates from the strong spin-orbit coupling in the material. Our results illustrate unusual scenarios in which the spin polarization of photoelectrons is completely different from the spin state of electrons in the originating initial states. The results also provide the basis for a novel source of highly spinpolarized electrons with tunable polarization in three dimensions.The topological electronic bandstructure of a bulk topological insulator ensures the presence of gapless surface electronic states with Dirac-like dispersions similar to graphene. Unlike graphene, the topological surface states are spin polarized, with their spins locked perpendicular to their momentum, forming helical spinmomentum textures 6 (see Fig. 1(a)). The presence of such 'helical Dirac fermions' forms an operational definition of a 3D topological insulator, and much of the excitement surrounding topological insulators involves the predicted exotic phenomena and potential applications of these metallic states.4,5 These include novel magnetoelectric effects, 14 exotic quasiparticles (in a proximity induced superconducting state) called Majorana fermions which are their own antiparticles, 15 and applications ranging from spintronics to quantum computing. 16Establishing methods that are sensitive to these states and their predicted behaviors have therefore generated much interest. 6,17-20Angle-resolved photoemission spectroscopy (ARPES) directly maps the dispersions and Fermi surfaces of such electronic states in energy-momentum space. Spinresolved ARPES also measures the spin polarization of the corresponding photoelectrons. Following a common assumption that electron spin is conserved in the photoemission process, the technique has been used to identify the presence of the predicted helical spin textures of topological surface states.6-12 Ut...