We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy and ab initio calculations. We observe two noninteracting cones near the ...Dirac crossing energy and the emergence of van Hove singularities where the cones overlap for large twist angles (>5°). Besides the expected interaction between the Dirac cones, minigaps appeared at the Brillouin zone boundaries of the moiré superlattice formed by the misorientation of the two graphene layers. We attribute the emergence of these minigaps to a periodic potential induced by the moiré. These anticrossing features point to coupling between the two graphene sheets, mediated by moiré periodic potentials.
Vanadium disulfide (VS2) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not ...been well understood yet. Here we report synthesis of epitaxial 1T VS2 monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transition above room temperature. Our observations provide important evidence to understand the strongly correlated electron physics and the related surface catalytic properties in two-dimensional transition-metal dichalcogenides (TMDCs).
Epitaxial growth and Fermi surface mapping of monolayer VS2 on graphene. Display omitted
•We report an epitaxial growth of monolayer VS2 film on bilayer graphene by molecular beam epitaxy.•We demonstrate electronic band structure of monolayer VS2 with 1T phase by angle-resolved photoemission spectroscopy.•We observe temperature-dependent evolution of charge density wave gap with transition temperature at ~400 K.•The VS2 is an important building block for establishing next-generation electronics and renewable energy applications.
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.
Two-dimensional materials with engineered composition and structure will provide designer materials beyond conventional semiconductors. However, the potentials of defect engineering remain largely ...untapped, because it hinges on a precise understanding of electronic structure and excitonic properties, which are not yet predictable by theory alone. Here, we utilize correlative, nanoscale photoemission spectroscopy to visualize how local introduction of defects modifies electronic and excitonic properties of two-dimensional materials at the nanoscale. As a model system, we study chemical vapor deposition grown monolayer WS2, a prototypical, direct gap, two-dimensional semiconductor. By cross-correlating nanoscale angle-resolved photoemission spectroscopy, core level spectroscopy, and photoluminescence, we unravel how local variations in defect density influence electronic structure, lateral band alignment, and excitonic phenomena in synthetic WS2 monolayers.
Graphene, a single monolayer of graphite, has recently attracted considerable interest owing to its novel magneto-transport properties, high carrier mobility and ballistic transport up to room ...temperature. It has the potential for technological applications as a successor of silicon in the post Moore's law era, as a single-molecule gas sensor, in spintronics, in quantum computing or as a terahertz oscillator. 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. 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.
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.
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 Sr2IrO4. 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.
In nodal-line semimetals, linearly dispersing states form Dirac loops in the reciprocal space with a high degree of electron-hole symmetry and a reduced density of states near the Fermi level. The ...result is reduced electronic screening and enhanced correlations between Dirac quasiparticles. Here we investigate the electronic structure of ZrSiSe, by combining time- and angle-resolved photoelectron spectroscopy with ab initio density functional theory (DFT) complemented by an extended Hubbard model (DFT + U + V) and by time-dependent DFT + U + V. We show that electronic correlations are reduced on an ultrashort timescale by optical excitation of high-energy electrons-hole pairs, which transiently screen the Coulomb interaction. Our findings demonstrate an all-optical method for engineering the band structure of a quantum material.
The iron arsenic high-temperature superconductors exhibit particularly rich phase diagrams. In the AE(Fe1−xTx)2As2 family (known as '122', with AE being Ca, Sr or Ba and T being a transition metal), ...the simultaneous structural/magnetic phase transition that occurs at elevated temperature in the undoped material splits and is suppressed by carrier doping. A superconducting region appears as likely in the orthorhombic/antiferromagnetic (AFM) state as in the tetragonal/paramagnetic state. An important question then is what determines the critical doping at which superconductivity emerges, as the AFM order is fully suppressed only close to optimal doping. Here we report evidence from angle-resolved photoemission spectroscopy that marked changes in the Fermi surface coincide with the onset of superconductivity in electron-doped Ba(Fe1−xCox)2As2. The presence of the AFM order leads to a reconstruction of the electronic structure, most significantly the appearance of the petal-like hole pockets at the Fermi level. These hole pockets vanish-that is, undergo a Lifshitz transition-as the cobalt concentration is increased sufficiently to support superconductivity. Superconductivity and magnetism are competing states in this system: when petal-like hole pockets are present, superconductivity is fully suppressed, whereas in their absence the two states can coexist.
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