Ubiquitous formation of bulk Dirac cones and topological surface states from a single orbital manifold in transition-metal dichalcogenides

Abstract

Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied bulk properties, while their single-layer variants have become one of the most prominent examples of two-dimensional materials beyond graphene. Their disparate ground states largely depend on transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin-and angle-resolved photoemission, we find that these generically host a coexistence of type-I and type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics.The classification of electronic structures based on their topological properties has opened powerful routes for understanding solid state materials. 1 The nowfamiliar Z 2 topological insulators are most renowned for their spin-polarised Dirac surface states residing in inverted bulk band gaps. 1 In systems with rotational invariance, a band inversion on the rotation axis can generate protected Dirac cones with a point-like Fermi surface of the bulk electronic structure. 2-8 If either inversion or time-reversal symmetry is broken, a bulk Dirac point can split into a pair of spin-polarised Weyl points. 9-15 Unlike for elementary particles, Lorentz-violating Weyl fermions can also exist in the solid state, manifested as a tilting of the Weyl cone. If this tilt is sufficiently large, so-called type-II Weyl points can occur, now formed at the touching of open electron and hole pockets. [15][16][17][18][19][20][21][22] Realising such phases in solid-state materials not only offers unique environments and opportunities for studying the fundamental properties of fermions, but also holds potential for applications exploiting their exotic surface excitations and bulk electric and thermal transport properties. [23][24][25][26][27] Consequently, there is an intense current effort focused on identifying compounds which host the requisite band inversions. In many cases, however, this arXiv:1702.08177v2 [cond-mat.mtrl-sci]

Observation of bulk states and spin-polarized topological surface states in transition metal dichalcogenide Dirac semimetal candidate NiTe2

“…The electronic configuration of Ni is 3s 2 3d 8 and that of Te is 4d 10 5p 4 . We find that similar to other group-X TMDs [32,46], the Te 5p orbital manifold in NiTe 2 , aided by the interplay between intralayer hopping, crystal field splitting, and SOC strength, gives rise to most of the bulk Dirac nodes and multiple inverted band-gaps. To highlight this, we show the evolution of the p-orbital manifold of the Te atoms in Fig.…”

“…In contrast to their high energy counter-parts, these emergent quasi-particles are not protected by Lorentz symmetry, and can also occur in a tilted form, giving rise to type-I and type-II Dirac fermions. Specifically, Na 3 Bi [26,29] and Cd 3 As 2 [25,30] are type-I Dirac semimetal (DSM), while the transitionmetal dichalcogenides (TMDs) PtTe 2 [31][32][33], PtSe 2 [34], and PdTe 2 [32,35,36] are type-II DSM.…”

“…In group X Pd-and Pt-based dichalcogenides, the bulk Dirac node lies deep below the Fermi level (∼0.6, ∼0.8 and ∼1.2 eV in PdTe 2 , PtTe 2 , and PtSe 2 , respectively) [31][32][33][34][35][36], hindering their successful exploitation in technology. In contrast, NiTe 2 has been predicted to host type-II Dirac fermions in vicinity of the Fermi energy [37].…”

“…Extrapolating the fitted DFT band structure, we find that the Dirac cone is located just above (∼20 meV) the Fermi energy. in contrast to other TMDbased type-II DSMs like PtTe 2 [31][32][33], PtSe 2 [34], and PdTe 2 [32,35,36]. Our attempt to electronically dope the sample via alkali metal (potassium) deposition (see Fig.…”

The study on quantum material WTe2

“…TaIrTe 4 [55] and LaAlGe [56] were also demonstrated to hold type-II Weyl fermions. Recently, the band structure with highly tilted cone has also been found in type-II Dirac semi-metals [57][58][59][60][61].…”