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
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.
Controlling the Electronic Structure of Bilayer Graphene Ohta, Taisuke; Bostwick, Aaron; Seyller, Thomas ...
Science (American Association for the Advancement of Science),
08/2006, Letnik:
313, Številka:
5789
Journal Article
Recenzirano
Odprti dostop
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.
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.
The scientific opportunities for microARPES and nanoARPES techniques are discussed, and the benefits to these techniques at diffraction‐limited light sources are presented, in particular the impact ...on spectromicroscopic ARPES (angle‐resolved photoemission spectroscopy) of upgrading the Advanced Light Source to diffraction‐limited performance. The most important consideration is whether the space‐charge broadening, impacting the energy and momentum resolution, will limit the possible benefits for ARPES. Calculations of energy broadening due to space‐charge effects will be presented over a wide range of parameters, and optimum conditions for ARPES will be discussed. The conclusion is that spectromicroscopic ARPES will greatly benefit from the advent of diffraction‐limited light sources; space‐charge broadening effects will not be a limiting factor.
van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is ...to modify the band gap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable band gap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the band gap of TMDs at the zone corners is modulated in the range of 0.8–2.0 eV, which covers a wide spectral range from visible to near-infrared, with a tendency from indirect to direct band gap. A key clue to understanding the mechanism of this band-gap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin-splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.
Bilayer graphene has been predicted to host a moiré miniband with flat dispersion if the layers are stacked at specific twist angles known as the ’magic angles’1,2. Recently, twisted bilayer graphene ...(tBLG) with a magic angle twist was reported to exhibit a correlated insulating state and superconductivity3,4, where the presence of the flat miniband in the system is thought to be essential for the emergence of these ordered phases in the transport measurements. Although tunnelling spectroscopy5–9 and electronic compressibility measurements10 in tBLG have found a van Hove singularity that is consistent with the presence of the flat miniband, a direct observation of the flat dispersion in the momentum space of such a moiré miniband in tBLG is still lacking. Here, we report the visualization of this flat moiré miniband by using angle-resolved photoemission spectroscopy with nanoscale resolution. The high spatial resolution of this technique enabled the measurement of the local electronic structure of the tBLG. The measurements demonstrate the existence of the flat moiré band near the charge neutrality for tBLG close to the magic angle at room temperature.The flat electronic bands that are associated with ordered phases in twisted bilayer graphene at a magic twist angle have been imaged using angle-resolved photoemission spectroscopy.
We investigate the effects of external dielectric screening on the electronic dispersion and the band gap in the atomically thin, quasi-two-dimensional (2D) semiconductor WS_{2} using angle-resolved ...photoemission and optical spectroscopies, along with first-principles calculations. We find the main effect of increased external dielectric screening to be a reduction of the quasiparticle band gap, with rigid shifts to the bands themselves. Specifically, the band gap of monolayer WS_{2} is decreased by about 140 meV on a graphite substrate as compared to a hexagonal boron nitride substrate, while the electronic dispersion of WS_{2} remains unchanged within our experimental precision of 17 meV. These essentially rigid shifts of the valence and conduction bands result from the special spatial structure of the changes in the Coulomb potential induced by the dielectric environment of the monolayer.
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 VSe2 on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe2. 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 VSe2. 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.
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.
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