title: Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides.
abstract: We report systematic optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at multilayers to a direct-gap one at monolayers. A hot luminescence peak (B) is observed at ~0.4 eV above the prominent band edge peak (A) in all samples. The magnitude of A-B splitting is independent of the number of layers and coincides with the spin-valley coupling strength in monolayers. Ab initio calculations show that this thickness independent splitting pattern is a direct consequence of the giant spin-valley coupling which fully suppresses interlayer hopping at valence band edge near K points because of the sign change of the spin-valley coupling from layer to layer in the 2H stacking order.
authors.affiliation: Department of Physics, The University of Hong Kong, Pokfulam road, Hong Kong, China.
authors.forename: Hualing
authors.initials: H
authors.lastname: Zeng
authors.forename: Gui-Bin
authors.initials: GB
authors.lastname: Liu
authors.forename: Junfeng
authors.initials: J
authors.lastname: Dai
authors.forename: Yajun
authors.initials: Y
authors.lastname: Yan
authors.forename: Bairen
authors.initials: B
authors.lastname: Zhu
authors.forename: Ruicong
authors.initials: R
authors.lastname: He
authors.forename: Lu
authors.initials: L
authors.lastname: Xie
authors.forename: Shijie
authors.initials: S
authors.lastname: Xu
authors.forename: Xianhui
authors.initials: X
authors.lastname: Chen
authors.forename: Wang
authors.initials: W
authors.lastname: Yao
authors.forename: Xiaodong
authors.initials: X
authors.lastname: Cui
mesh_terms: D018011:Chalcogens / Q000737:chemistry*; D003198:Computer Simulation; D008027:Light; D008163:Luminescent Measurements / Q000379:methods*; D008422:Materials Testing; D008956:Models, Chemical; D012542:Scattering, Radiation; D013113:Spin Labels; D017973:Tungsten Compounds / Q000737:chemistry*
chemical_list: D018011:Chalcogens; D013113:Spin Labels; D017973:Tungsten Compounds