The transition-metal dichalcogenide 1T-TiSe2 is a quasi-two-dimensional layered material with a charge density wave (CDW) transition temperature of T(CDW) ≈ 200 K. Self-doping effects for crystals grown at different temperatures introduce structural defects, modify the temperature-dependent resistivity, and strongly perturbate the CDW phase. Here, we study the structural and doping nature of such native defects combining scanning tunneling microscopy or spectroscopy and ab initio calculations. The dominant native single atom dopants we identify in our single crystals are intercalated Ti atoms, Se vacancies, and Se substitutions by residual iodine and oxygen.

Spin-and angle-resolved photoemission spectroscopy is used to reveal that a large spin polarization is observable in the bulk centrosymmetric transition metal dichalcogenide MoS 2 . It is found that the measured spin polarization can be reversed by changing the handedness of incident circularly polarized light. Calculations based on a three-step model of photoemission show that the valley and layer-locked spinpolarized electronic states can be selectively addressed by circularly polarized light, therefore providing a novel route to probe these hidden spin-polarized states in inversion-symmetric systems as predicted by Zhang et al. [Nat. Phys. 10, 387 (2014).]. DOI: 10.1103/PhysRevLett.118.086402 Transition metal dichalcogenide (TMDC) monolayers have been heavily investigated due to the locking of the spin with valley pseudospins and the presence of a direct gap, which makes them ideal candidates for valleytronic devices [1]. Thanks to the lack of inversion symmetry and the non-negligible spin-orbit coupling, TMDC monolayers also feature well-defined spin-polarized ground states [2], which can, in principle, be investigated by spin-and angleresolved photoemission spectroscopy (spin-ARPES). However, while few ARPES studies clearly observed the indirect to direct band gap transition going from the bulk crystal to the monolayer [3-6], spin-ARPES investigation of TMDC monolayers is more challenging, given the low cross section of photoemission from single layers.ARPES measurements on the bulk system are instead less demanding, but early studies detected spin-resolved signals only from TMDCs with broken inversion symmetry [7]. Interestingly, a recent theoretical study [8] suggested that the spin texture of the TMDC could be probed by photoemission, even in the inversion symmetric bulk TMDC crystals, as a result of the localization of two spin-degenerated valence band maxima on different layers of the unit cell and of the finite penetration depth of the photoemission process probing preferentially the uppermost layer. Experimentally this effect has been observed for WSe 2 , where the spin orbit (SO) coupling is the strongest one among the TMDCs [9].Here we show that a large out-of-plane spin-polarization is observable in the bulk dichalcogenide MoS 2 , and more importantly, that its sign depends on the handedness of the incident circularly polarized light. Our calculations, based on a three step model of the photoemission process demonstrate that the observed spin reversal is an initial state effect. Using left-(C L ) and right-handed (C R ) circularly polarized light results in selecting different initial states that present a positive or negative out-of-plane spin polarization depending on their localization on the S-Mo-S layers of the 2H-stacked MoS 2 unit cell. Our findings not only highlight the locking of the spin with layer and valley pseudospins in MoS 2 but also provide a novel and improved route, other than taking profit of the inelastic mean free path (IMFP) [8], for selectively probing hidden spin-polari...

Several experiments have been performed on 1T-TiSe_{2} in order to identify whether the electronic structure is semimetallic or semiconducting without reaching a consensus. In this Letter, we theoretically study the impact of electron-hole and electron-phonon correlations on the bare semimetallic and semiconducting electronic structure. The resulting electron spectral functions provide a direct comparison of both cases and demonstrate that 1T-TiSe_{2} is of predominant semiconducting character with some spectral weight crossing the Fermi level.

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We present a detailed low temperature scanning tunneling microscopy study of the commensurate charge density wave (CDW) in 1T -TiSe2 in the presence of single atom defects. We find no significant modification of the CDW lattice in single crystals with native defects concentrations where some bulk probes already measure substantial reductions in the CDW phase transition signature. Systematic analysis of STM micrographs combined with density functional theory modelling of atomic defect patterns indicate that the observed CDW modulation lies in the Se surface layer. The defect patterns clearly show there are no 2H-polytype inclusions in the CDW phase, as previously found at room temperature [Titov A.N. et al, Phys. Sol. State 53, 1073. They further provide an alternative explanation for the chiral Friedel oscillations recently reported in this compound [J. Ishioka et al., Phys. Rev. B 84, 245125, (2011)].PACS numbers: 68.37. Ef, 71.15.Mb, 74.70.Xa, 73.20.Hb The transition metal dichalcogenide (TMD) 1T -TiSe 2 has kept the scientific community wondering about a number of its striking physical properties for more than four decades [1][2][3][4][5][6][7]. 1T -TiSe 2 is a layered compound consisting of a hexagonal layer of Ti sandwiched between two hexagonal layers of Se to form Se-Ti-Se sandwiches that stack via weak Van-der-Waals (VdW) forces to form a single crystal. The band structure of 1T -TiSe 2 , as determined by angle-resolved photoemission spectroscopy, consists primarily of a Se 4p-valence band at the Γ point and a Ti 3d-conduction band at the L point of the Brillouin zone. But it is still debated whether it is a semiconductor or a semimetal with evidences claimed for both alternatives [6,[8][9][10].Below T CDW ≈ 202K, 1T -TiSe 2 undergoes a second order phase transition into a commensurate charge density wave (CDW). A comprehensive theory of this CDW formation is yet to be developed. Two main mechanisms are currently considered, driven either by a JahnTeller distortion [4,11] or an excitonic ground state [2,9,12,13]. The CDW phase has been found to melt upon copper intercalation [5] or when applying pressure [7]. In both instances, superconductivity develops in a dome shaped region around some optimal doping or optimal pressure, with a maximum critical temperature of 4.1K and 1.8K, respectively. More recently, chiral properties have been reported for the CDW in pristine and copper intercalated 1T -TiSe 2 based on polarized optical reflectometry and scanning tunneling microscopy (STM) [14][15][16].Here we focus on the CDW instability in 1T -TiSe 2 in the presence of native atomic scale defects. Past studies performed using macroscopic probes including resistivity, magnetic susceptibility and optical reflectivity have found atomic intercalation and substitution to be detrimental to the CDW [1,17]. This compound is usually non-stoichiometric with a strong correlation between increasing crystal growth temperature and Ti self-doping leading to the collapse of the CDW phase transition signature in temperature ...