Selective Probing of Hidden Spin-Polarized States in Inversion-Symmetric Bulk MoS2

Abstract

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...

Hidden spin-polarized bands in semiconducting 2H-MoTe2

“…The electronic band structure of MoX 2 has been intensively studied both experimentally and theoretically, and special attention has been paid to the region of the K point in the Brillouin zone (BZ), where the first direct electronic transitions take place (A and B). Spin-and angle-resolved photoemission spectroscopy [7][8][9] evidenced the presence of spin-polarized states in bulk TMDCs which was previously believed to take place only for non-centrosymmetric materials such as monolayers (MLs). However, since the photoemission spectroscopy mostly probes the outermost layer, it is highly desirable to use alternative optical techniques to confirm the presence of spin-polarized states in bulk TMDCs.…”

NMR in an electric field: A bulk probe of the hidden spin and orbital polarizations

“…4 Third, in certain materials such as MoS 2 , spin-dependent dipole selection rules allow to probe the hidden spin polarization under irradiation by circularly polarized light. 5 In this work, we will be interested in another way of detecting the hidden spin and orbital polarizations. We begin by recognizing that the sum over occupied Bloch states in Eq.…”

Spin Accumulation and Dynamics in Inversion-Symmetric van der Waals Crystals

“…Recently it has been proposed that bulk inversion symmetric materials can show a spatially dependent spin texture resulting from a local inversion asymmetry [2], which in the special case of layered transition metal dichalcogenides (TMDs) results in coupled spin, layer, and valley degrees of freedom [3]. Studies using photoemission spectroscopy showed spin-polarized bands [4] and ultrafast decay (less than 1 ps) of the spin accumulation at the surface of inversion symmetric TMD crystals [5][6][7][8]. The high-energy beams used in these studies have a low penetration depth therefore allowing to isolate the surface signals from the bulk.…”