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The topology of Weyl semimetals requires the existence of unique surface states. Surface states have been visualized in spectroscopy measurements, but their connection to the topological character of the material remains largely unexplored. 1T'-MoTe2, presents a unique opportunity to study this connection. This material undergoes a phase transition at 240 K that changes the structure from orthorhombic (putative Weyl semimetal) to monoclinic (trivial metal), while largely maintaining its bulk electronic structure. Here, we show from temperature-dependent quasiparticle interference measurements that this structural transition also acts as a topological switch for surface states in 1T'-MoTe2. At low temperature, we observe strong quasiparticle scattering, consistent with theoretical predictions and photoemission measurements for the surface states in this material. In contrast, measurements performed at room temperature show the complete absence of the scattering wavevectors associated with the trivial surface states. These distinct quasiparticle scattering behaviors show that 1T'-MoTe2 is ideal for separating topological and trivial electronic phenomena via temperature-dependent measurements. Weyl semimetals: switching off topology A temperature-induced structural phase transition can provide insight into the effect of Fermi arcs on the properties of Weyl semimetals. Weyl semimetals are topological materials at the surface of which Weyl points—the points at which electron and hole pockets touch—are connected in pairs by Fermi arc surface states. Probing the topological nature of the surface states has so far proven difficult, and a connection with the band structure of the material is complex to establish. An international team of researchers led by Abhay Pasupathy at Columbia University demonstrated, by combining experiments and theory, that the Weyl semimetal MoTe2 undergoes a temperature-driven structural phase transition that switches off the Weyl behavior. This makes it possible to systematically compare the electronic properties of the trivial and topological states. [ABSTRACT FROM AUTHOR]

Using angle-resolved photoemission on micrometer-scale sample areas, we directly measure the interlayer twist angle-dependent electronic band structure of bilayer molybdenum-disulfide (MoS2). Our measurements, performed on arbitrarily stacked bilayer MoS2 flakes prepared by chemical vapor deposition, provide direct evidence for a downshift of the quasiparticle energy of the valence band at the Brillouin zone center (Γ point) with the interlayer twist angle, up to a maximum of 120 meV at a twist angle of ~40°. Our direct measurements of the valence band structure enable the extraction of the hole effective mass as a function of the interlayer twist angle. While our results at Γ agree with recently published photoluminescence data, our measurements of the quasiparticle spectrum over the full 2D Brillouin zone reveal a richer and more complicated change in the electronic structure than previously theoretically predicted. The electronic structure measurements reported here, including the evolution of the effective mass with twist-angle, provide new insight into the physics of twisted transition-metal dichalcogenide bilayers and serve as a guide for the practical design of MoS2 optoelectronic and spin-/valley-tronic devices. [ABSTRACT FROM AUTHOR]

We report angle-resolved photoemission spectroscopic measurements of the evolution of the thicknessdependent electronic band structure of the atomically heavy two-dimensional layered dichalcogenide, tungsten diselenide (WSe2). Our data, taken on mechanically exfoliated WSe2 single crystals, provide direct evidence for shifting of the valence-band maximum from Γ (multilayer WSe2) to K (single-layer WSe2). Further, our measurements also set a lower bound on the energy of the direct band gap and provide direct measurement of the hole effective mass. [ABSTRACT FROM AUTHOR]