Generating electron coherence in quantum materials is essential in optimal control of many-body interactions and correlations. In a multidomain system this signifies nonlocal coherence and emergence of collective phenomena, particularly in layered 2D quantum materials possessing novel electronic structures and high carrier mobilities. Here we report nonlocal ac electron coherence induced in dispersed MoS 2 flake domains, using coherent spatial self-phase modulation (SSPM). The gap-dependent nonlinear dielectric susceptibility χ (3) measured is surprisingly large, where direct interband transition and two-photon SSPM are responsible for excitations above and below the bandgap, respectively. A windchime model is proposed to account for the emergence of the ac electron coherence. Furthermore, all-optical switching is achieved based on SSPM, especially with two-color intraband coherence, demonstrating that electron coherence generation is a ubiquitous property of layered quantum materials.electron coherence | transition metal dichalcogenide | self-phase modulation | optical switching | emergent phenomena R ecently 2D layered quantum materials have attracted tremendous interest since the discovery of graphene a decade ago (1). Various layered materials, ranging from boron nitride sheets to transition metal dichalcogenides and from topological insulators to high-temperature superconductors, have been intensively investigated (2-11). Strict 2D atomic crystals can now be produced at a macroscopic scale, using a variety of methods (11,12). Among them molybdenum disulfide (MoS 2 ) and related layered quantum materials are particularly interesting due to their novel optical properties and potential valleytronics applications (4,5,8,9) at a thickness of monolayer and few layers (2, 10, 13). Layered materials share common physical properties rooted in their ubiquitous 2D quantum nature, for which achieving pure coherence among electrons (lattices) is of particular interest (14-19). The presence of multiple domains is ubiquitous in many known 2D quantum materials, ranging from stripe-order cuprate superconductors to polycrystalline strongly correlated systems. For example, phase locking between different layers of stripe orders is crucial for enhancing the superconducting phase in layered hightemperature superconductors (20,21).In this work we demonstrate unambiguously that nonlocal and intraband ac electron coherence, of which the electronic wave function oscillates at an optical frequency of 10 14 Hz, can be generated in separate MoS 2 flakes, using spatial self-phase modulation (SSPM). The SSPM is a coherent third-order nonlinear optical process systematically investigated decades ago (22), where the nonlinear optical susceptibility χ (3) is uniquely determined by the laser-intensity-dependent refractive index n = n 0 + n 2 I, where n 0 and n 2 are linear and nonlinear refractive indexes, respectively. If this effect is strong enough in a material, the phenomenon of selffocusing can be directly observed. The SSPM is also f...
Group-V elemental monolayers were recently predicted to exhibit exotic physical properties such as nontrivial topological properties, or a quantum anomalous Hall effect, which would make them very suitable for applications in next-generation electronic devices. The free-standing group-V monolayer materials usually have a buckled honeycomb form, in contrast with the flat graphene monolayer. Here, we report epitaxial growth of atomically thin flat honeycomb monolayer of group-V element antimony on a Ag(111) substrate. Combined study of experiments and theoretical calculations verify the formation of a uniform and single-crystalline antimonene monolayer without atomic wrinkles, as a new honeycomb analogue of graphene monolayer. Directional bonding between adjacent Sb atoms and weak antimonene-substrate interaction are confirmed. The realization and investigation of flat antimonene honeycombs extends the scope of two-dimensional atomically-thick structures and provides a promising way to tune topological properties for future technological applications.
Planar borophene, the truly 2D monolayer boron, has been independently successfully grown on Ag(1 1 1) by two groups (2016 Nat. Chem. 8 563 and 2015 Science 350 1513), which has received widespreading attentions. The superconducting property has not been unambiguously observed, which is unexpected because light element boron should have strong electron-phonon coupling. To resolve this puzzle, we show that the superconducting transition temperature T c of β 12 borophene is effectively suppressed by the substrate-induced tensile strain and electron doping via first principles calculations. The biaxial tensile strain of 2% induced by Ag(1 1 1) significantly reduces T c from 14 K to 2.95 K; electron doping of 0.1 eper boron atom further shrinks T c to 0.09 K. We also predict that the superconducting transition temperature in β 12 can be enhanced to 22.82 K with proper compressive strain (−1%) and 18.97 K with hole doping (0.1 h + per boron). Further studies indicate that the variation of T c is closely related to the density of states of σ bands near the Fermi surface. Our results help to explain the challenges to experimentally probe superconductivity in substratesupported borophene.
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