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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. The experimental realization of lattice-born flat bands with nontrivial topology has been elusive. Here, the authors observe topological flat bands near the Fermi level in a kagome metal CoSn, with flat bands as well as Dirac bands originating from 3d orbitals in a frustrated kagome geometry.

A hallmark of strongly correlated quantum materials is the rich phase diagram resulting from competing and intertwined phases with nearly degenerate ground-state energies 1 , 2 . A well-known example is the copper oxides, in which a charge density wave (CDW) is ordered well above and strongly coupled to the magnetic order to form spin-charge-separated stripes that compete with superconductivity 1 , 2 . Recently, such rich phase diagrams have also been shown in correlated topological materials. In 2D kagome lattice metals consisting of corner-sharing triangles, the geometry of the lattice can produce flat bands with localized electrons 3 , 4 , non-trivial topology 5 – 7 , chiral magnetic order 8 , 9 , superconductivity and CDW order 10 – 15 . Although CDW has been found in weakly electron-correlated non-magnetic A V 3 Sb 5 ( A  = K, Rb, Cs) 10 – 15 , it has not yet been observed in correlated magnetic-ordered kagome lattice metals 4 , 16 – 21 . Here we report the discovery of CDW in the antiferromagnetic (AFM) ordered phase of kagome lattice FeGe (refs.  16 – 19 ). The CDW in FeGe occurs at wavevectors identical to that of A V 3 Sb 5 (refs.  10 – 15 ), enhances the AFM ordered moment and induces an emergent anomalous Hall effect 22 , 23 . Our findings suggest that CDW in FeGe arises from the combination of electron-correlations-driven AFM order and van Hove singularities (vHSs)-driven instability possibly associated with a chiral flux phase 24 – 28 , in stark contrast to strongly correlated copper oxides 1 , 2 and nickelates 29 – 31 , in which the CDW precedes or accompanies the magnetic order. Analysis of the antiferromagnetic ordered phase of kagome lattice FeGe suggests that charge density wave is the result of a combination of electronic-correlations-driven antiferromagnetic order and instability driven by van Hove singularities.

A kagome lattice of 3 d 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-3 d 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.

We describe the synthesis of bilayer graphene thin films deposited on insulating silicon carbide and report the characterization of their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic-scale electronic devices.

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, single-crystal 2D gallium, indium and tin that are stabilized at the interface of epitaxial graphene and silicon carbide. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to the graphene overlayer; that is, they are ‘half van der Waals’ metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals offer 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 free-electron Fermi surface that closely approaches the Dirac points of the graphene overlayer. Single-crystal 2D metals are stabilized at the interface between epitaxial graphene and silicon carbide, with strong internal gradients in bonding character. The confined 2D metals demonstrate compelling superconducting properties.

A hallmark of graphene is its unusual conical band structure that leads to a zero-energy band gap at a single Dirac crossing point. By measuring the spectral function of charge carriers in quasi-freestanding graphene with angle-resolved photoemission spectroscopy, we showed that at finite doping, this well-known linear Dirac spectrum does not provide a full description of the charge-carrying excitations. We observed composite \"plasmaron\" particles, which are bound states of charge carriers with plasmons, the density oscillations of the graphene electron gas. The Dirac crossing point is resolved into three crossings: the first between pure charge bands, the second between pure plasmaron bands, and the third a ring-shaped crossing between charge and plasmaron bands.

Excitons, bound electron-hole pairs, influence the optical properties in strongly interacting solid-state systems and are typically most stable and pronounced in monolayer materials. Bulk systems with large exciton binding energies, on the other hand, are rare and the mechanisms driving their stability are still relatively unexplored. Here, we report an exceptionally large exciton binding energy in single crystals of the bulk van der Waals antiferromagnet CrSBr. Utilizing state-of-the-art angle-resolved photoemission spectroscopy and self-consistent ab-initio GW calculations, we present direct spectroscopic evidence supporting electronic localization and weak dielectric screening as mechanisms contributing to the amplified exciton binding energy. Furthermore, we report that surface doping enables broad tunability of the band gap offering promise for engineering of the optical and electronic properties. Our results indicate that CrSBr is a promising material for the study of the role of anisotropy in strongly interacting bulk systems and for the development of exciton-based optoelectronics. Chromium sulfur bromide is an air-stable van der Waals magnet, which has a large optical response. Here, Smolenski et al find a large exciton binding energy CrSBr, which they attribute to localization due to the quasi one-dimensional electronic bands.

In two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), new electronic phenomena such as tunable bandgaps1–3 and strongly bound excitons and trions emerge from strong many-body effects4–6, beyond the spin and valley degrees of freedom induced by spin–orbit coupling and by lattice symmetry7. Combining single-layer TMDs with other 2D materials in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these many-body effects, by means of engineered interlayer interactions8–10. Here, we use micro-focused angle-resolved photoemission spectroscopy (microARPES) and in situ surface doping to manipulate the electronic structure of single-layer WS2 on hexagonal boron nitride (WS2/h-BN). Upon electron doping, we observe an unexpected giant renormalization of the spin–orbit splitting of the single-layer WS2 valence band, from 430 meV to 660 meV, together with a bandgap reduction of at least 325 meV, attributed to the formation of trionic quasiparticles. These findings suggest that the electronic, spintronic and excitonic properties are widely tunable in 2D TMD/h-BN heterostructures, as these are intimately linked to the quasiparticle dynamics of the materials11–13.

Thermal annealing of SiC produces graphene layers on an insulating substrate, but the material is highly inhomogeneous. It is now shown that an argon atmosphere during annealing improves uniformity of the graphene layers dramatically and yields better transport characteristics. This is a very important result for the development of graphene-based electronic devices. Graphene, a single monolayer of graphite, has recently attracted considerable interest owing to its novel magneto-transport properties 1 , 2 , 3 , high carrier mobility and ballistic transport up to room temperature 4 . It has the potential for technological applications as a successor of silicon in the post Moore’s law era 5 , 6 , 7 , as a single-molecule gas sensor 8 , in spintronics 9 , 10 , 11 , in quantum computing 12 or as a terahertz oscillator 13 . For such applications, uniform ordered growth of graphene on an insulating substrate is necessary. The growth of graphene on insulating silicon carbide (SiC) surfaces by high-temperature annealing in vacuum was previously proposed to open a route for large-scale production of graphene-based devices 5 , 6 . However, vacuum decomposition of SiC yields graphene layers with small grains (30–200 nm; refs  14–16 ). Here, we show that the ex situ graphitization of Si-terminated SiC(0001) in an argon atmosphere of about 1 bar produces monolayer graphene films with much larger domain sizes than previously attainable. Raman spectroscopy and Hall measurements confirm the improved quality of the films thus obtained. High electronic mobilities were found, which reach μ =2,000 cm  2  V −1  s −1 at T =27 K. The new growth process introduced here establishes a method for the synthesis of graphene films on a technologically viable basis.

The physics of doped Mott insulators remains controversial after decades of active research, hindered by the interplay among competing orders and fluctuations. It is thus highly desired to distinguish the intrinsic characters of the Mott-metal crossover from those of other origins. Here we investigate the evolution of electronic structure and dynamics of the hole-doped pseudospin-1/2 Mott insulator Sr 2 IrO 4 . The effective hole doping is achieved by replacing Ir with Rh atoms, with the chemical potential immediately jumping to or near the top of the lower Hubbard band. The doped iridates exhibit multiple iconic low-energy features previously observed in doped cuprates—pseudogaps, Fermi arcs and marginal-Fermi-liquid-like electronic scattering rates. We suggest these signatures are most likely an integral part of the material’s proximity to the Mott state, rather than from many of the most claimed mechanisms, including preformed electron pairing, quantum criticality or density-wave formation. The physics of Mott insulators is obscured by the interplay between competing orders and fluctuations. Here, the authors track the evolution of the electronic structure of Mott insulator strontium iridate as the iridium atoms are replaced by rhodium, providing insight into this exotic state of matter.