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]

Several transition-metal dichalcogenides exhibit a striking crossover from indirect to direct band gap semiconductors as they are thinned down to a single monolayer. Here, we demonstrate how an electronic structure characteristic of the isolated monolayer can be created at the surface of a bulk MoS2 crystal. This is achieved by intercalating potassium in the interlayer van der Waals gap, expanding its size while simultaneously doping electrons into the conduction band. Our angle-resolved photoemission measurements reveal resulting electron pockets centered at the K̅ and K' points of the Brillouin zone, providing the first momentum-resolved measurements of how the conduction band dispersions evolve to yield an approximately direct band gap of ∼1.8 eV in quasi-freestanding monolayer MoS2. As well as validating previous theoretical proposals, this establishes a novel methodology for manipulating electronic structure in transition-metal dichalcogenides, opening a new route for the generation of large-area quasi-freestanding monolayers for future fundamental study and use in practical applications.

Semiconductors are typically considered weakly interacting systems, well described by conventional band theory. The exchange and correlation energies arising from electronelectron interactions can, however, dominate the kinetic energy in the dilute doping limit.This stabilises a small regime of negative electronic compressibility (NEC), κ = weak anti-localisation 17 and a density-tuned dome of superconductivity. 18 A detailed understanding of the underlying gate-induced electronic structure evolution driving such emergent properties has, however, remained elusive.Here, we mimic the effects of field-effect doping in the TMD WSe 2 by the sub-monolayer 2 deposition of alkali metals at the vacuum-cleaved surface. Such "chemical gating" leaves the surface accessible for detailed spectroscopic measurements. From angle-resolved photoemission (ARPES), we uncover how the resulting charge accumulation drives a pronounced reconstruction of the bulk electronic structure, not only mediating the formation of a multivalley 2DEG and a giant tuneable valence band spin splitting, but also inducing a pronounced decrease of the surface chemical potential with increasing electron doping. This direct spectroscopic observation of NEC, which we find persists to remarkably high electron densities, reveals a dominant role of many-body interactions shaping the underlying electronic landscape of electrostatically-tuned TMDs.In Figure 1, we show the occupied electronic structure of bulk and chemically-gated WSe 2 as measured by ARPES. No electronic states cross the Fermi level for the pristine cleaved material ( Fig. 1(b)), consistent with its semiconducting bulk. While the uppermost valence bands near the zone centre are strongly three-dimensional, those at the zone-corner K point have negligible dispersion along k z , with electronic wavefunctions localised to single Se-W-Se monlayers (half of the unit cell). 8,19 These two-dimensional states, which form the lowest energy band extrema in monolayer TMDs, are strongly spin-polarised even in the bulk. 6,8The spin is coupled to the valley degree of freedom, alternating sign at neighbouring corners of the Brillouin zone just as for monolayer MoS 2 and WSe 2 . 5,7 For the 2H structure, spin also becomes locked to the layer pseudospin, reversing sign for neighbouring Se-W-Se layers. 6-8An energetic degeneracy of the states in neighbouring layers thus enforces the total electronic structure to be spin degenerate, as required by the structural inversion symmetry of bulk WSe 2 ( Fig. 1(a)).We show that breaking such inversion symmetry, achieved here by our surface doping approach, drives a number of striking changes of the electronic structure ( Fig. 1(c)). Deposition of minute quantities of alkali metals, electron doping the surface, causes the conduction band states to become populated at the K point (only weakly visible) and approximately mid-way along the Γ − K direction (denoted here as T ). The latter have the larger occupied bandwidth, maintaining an indirect band gap as for bulk WSe 2 . Unl...

Extraordinary (X) waves are perpendicularly injected for electron Bernstein (B) wave heating into an Ohmically heated plasma from the inboard side in the WT-3 tokamak. Measurements show that absorption does not take place at the electron cyclotron resonance layer nor the upper hybrid resonance layer, but does happen midway between them. This is consistent with the ray tracing prediction, i.e., the poloidal field and poloidal inhomogeneity of toroidal field lead the B waves to have a large parallel refractive index N( parallel) (>1), and the B waves are damped away via the Doppler-shifted cyclotron resonance.

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