How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below T = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diffraction. These observations point to a charge-density wave instability in the monolayer that is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of X-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density functional theory. Our study thus points to a delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides.

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]

T.O.), Andy.Mackenzie@cpfs.mpg.de (A.P.M.), philip.king@st-andrews.ac.uk (P.D.C.K.) † These authors contributed equally to this work.Abstract: A nearly free electron metal and a Mott insulating state can be thought of as opposite ends of possibilities for the motion of electrons in a solid. In the magnetic oxide metal PdCrO 2 , these two coexist as alternating layers. Using angle resolved photoemission, we surprisingly find sharp band-like features in the one-electron removal spectral function of the correlated subsystem. We show that these arise because a hole created in the Mott layer moves to and propagates in the metallic layer while retaining memory of the Mott layer's magnetism. This picture is quantitatively supported by a strong coupling analysis capturing the physics of PdCrO 2 in terms of a Kondo lattice Hamiltonian. Our findings open new routes to use the non-magnetic probe of photoemission to gain insights into the spin-susceptibility of correlated electron systems.One Sentence Summary: An intrinsically non-magnetic spectroscopy is shown to have strong magnetic sensitivity in Kondo-coupled PdCrO 2 .Main Text: PdCrO 2 is a member of the broad class of layered triangular lattice materials whose layer stacking sequence (see Fig. 1A) is that of the delafossite structural family ABO 2 (1). In a simple ionic picture of the delafossites, triangular co-ordinated layers of A + ions are stacked between BO 2 octahedra in which the B ions also have triangular co-ordination (2, 3). Most delafossites are insulating or semiconducting. PdCoO 2 and PtCoO 2 , however, are extremely high

We have used angle-resolved photoemission spectroscopy to investigate the band structure of ReS 2 , a transitionmetal dichalcogenide semiconductor with a distorted 1T crystal structure. We find a large number of narrow valence bands, which we attribute to the combined influence of structural distortion and spin-orbit coupling. We further show how this leads to a strong in-plane anisotropy of the electronic structure, with quasi-one-dimensional bands reflecting predominant hopping along zigzag Re chains. We find that this does not persist up to the top of the valence band, where a more three-dimensional character is recovered with the fundamental band gap located away from the Brillouin zone center along k z . These experiments are in good agreement with our density-functional theory calculations, shedding light on the bulk electronic structure of ReS 2 , and how it can be expected to evolve when thinned to a single layer.

The semiconducting single-layer transition metal dichalcogenides have been identified as ideal materials for accessing and manipulating spin-and valley-quantum numbers due to a set of favorable optical selection rules in these materials. Here, we apply time-and angle-resolved photoemission spectroscopy to directly probe optically excited free carriers in the electronic band structure of a high quality single layer of WS2. We observe that the optically generated free hole density in a single valley can be increased by a factor of 2 using a circularly polarized optical excitation. Moreover, we find that by varying the photon energy of the excitation we can tune the free carrier density in a given spin-split state around the valence band maximum of the material. The control of the photon energy and polarization of the excitation thus permits us to selectively excite free electron-hole pairs with a given spin and within a single valley.In two-dimensional (2D) materials, control of the spinand valley-degrees of freedom has been suggested as a new type of tuning knob for carrier dynamics [1]. Singlelayer (SL) transition metal dichalcogenides (TMDCs) such as SL MoS 2 and WS 2 are particularly promising candidates for new spin-and valley-tronic device paradigms, owing to the broken inversion symmetry of their crystal lattices, a strong spin-orbit coupling and a direct band gap at theK andK ′ valleys in the electronic structure of the materials [2-4]. These properties have been indirectly verified in SL TMDCs through selective excitation of bound electron-hole pairs with circularly polarized light, leading to the observation of valley polarized excitons [5][6][7][8], which could be coherently manipulated [9]. Device measurements have shown indications of spin-and valley-coupled photocurrents [10] and Hall effects [11,12]. Direct measurements of the electronic structure and associated quasiparticles have been carried out for the bulk TMDC material WSe 2 using photoemission spectroscopies with spin [13] and time resolution [14]. Such measurements can, to some degree, even give information about the situation in a single layer, due to the surface sensitivity of photoemission spectroscopy. However, there is a need for experimental evidence and quantification of these properties for free carriers in the electronic structure of actual SL TMDCs, which are truly non-inversion symmetric materials.Time-and angle-resolved photoemission spectroscopy (TR-ARPES) is a powerful experimental approach for detecting optically excited free carriers with time-, energyand momentum-resolution with extremely high sensitivity towards 2D materials [15,16]. In this technique, a pump pulse with tuneable photon energy and polarization optically excites the material in question. The excited state is then probed by ARPES with an ultraviolet pulse, produced via high harmonic generation (HHG), which can provide sufficiently high photon energies to reach theK valleys at the corner of the SL TMDC's Brillouin zone (BZ). Since the two pulses are time-delaye...