Electronic properties of candidate type-II Weyl semimetal WTe ₂ . A review perspective

Abstract

Currently, there is a flurry of research interest on materials with an unconventional electronic structure, and we have already seen significant progress in their understanding and engineering towards real-life applications. The interest erupted with the discovery of graphene and topological insulators in the previous decade. The electrons in graphene simulate massless Dirac Fermions with a linearly dispersing Dirac cone in their band structure, while in topological insulators, the electronic bands wind non-trivially in momentum space giving rise to gapless surface states and bulk bandgap. Weyl semimetals in condensed matter systems are the latest addition to this growing family of topological materials. Weyl Fermions are known in the context of high energy physics since almost the beginning of quantum mechanics. They apparently violate charge conservation rules, displaying the 'chiral anomaly', with such remarkable properties recently theoretically predicted and experimentally verified to exist as low energy quasiparticle states in certain condensed matter systems. Not only are these new materials extremely important for our fundamental understanding of quantum phenomena, but also they exhibit completely different transport phenomena. For example, massless Fermions are susceptible to scattering from non-magnetic impurities. Dirac semimetals exhibit non-saturating extremely large magnetoresistance as a consequence of their robust electronic bands being protected by time reversal symmetry. These open up whole new possibilities for materials engineering and applications including quantum computing. In this review, we recapitulate some of the outstanding properties of WTe 2 , namely, its non-saturating titanic magnetoresistance due to perfect electron and hole carrier balance up to a very high magnetic field observed for the very first time. It also indicative of hosting Lorentz violating type-II Weyl Fermions in its bandstructure, again first predicted candidate material to host such a remarkable phase. We primarily focus on the findings of our ARPES, spin-ARPES, and time-resolved ARPES studies complemented by first-principles calculations.

Reversible Iodine Intercalation into Tungsten Ditelluride

“…In WTe2, an avoided crossing of two bands due to spin-orbit coupling near the Fermi level along the Γ-X direction gives rise to Weyl semimetal behavior. [12,20] The addition of iodine to form These bands have very high dispersion, indicating small effective electron masses and large charge carrier mobility.…”

Positive Magnetoresistance and Chiral Anomaly in Exfoliated Type-II Weyl Semimetal Td-WTe2

“…The presence of tilted Weyl cones and of low energy excitations violating the Lorentz invariance in the vicinity of the Weyl points were predicted for the electronic band structure of WTe 2 , a transition metal dichalcogenide (TMDC) semimetal with layered van der Waals structure [21,22]. In addition to the presence of the Fermi arc at the Fermi surface [23][24][25], anisotropic transport properties including extreme transverse magnetoresistance and the anisotropic ABJ anomaly, are reported for both bulk and few layers WTe 2 flakes [15,[26][27][28][29][30][31][32][33][34][35][36]. The tilted Weyl cone was also observed in the TMDC semimetal MoTe 2 [37,38] and in LaAlGe [39].…”

Progress in Epitaxial Thin‐Film Na3Bi as a Topological Electronic Material

“…TRS‐breaking Weyl semimetals are uncommon: their existence was predicted only in few magnetically ordered materials—like pyrochlore iridates [ 7 ] (Y 2 Ir 2 O 7 ) and Co‐based Heusler materials, [ 8,9 ] and verified experimentally only very recently. [ 10–12 ] The family of IS‐breaking materials is much broader, with theoretical predictions confirmed experimentally, among others (like TaAs [ 13–15 ] ), for the transition metal dichalcogenides WTe 2 [ 16–18 ] and MoTe 2 . [ 19,20 ]…”