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Motivated by the triumph and limitation of graphene for electronicapplications, atomically thin layers of group VI transition metaldichalcogenides are attracting extensive interest as a class of graphene-likesemiconductors with a desired band-gap in the visible frequency range. Themonolayers feature a valence band spin splitting with opposite sign in the twovalleys located at corners of 1st Brillouin zone. This spin-valley coupling,particularly pronounced in tungsten dichalcogenides, can benefit potentialspintronics and valleytronics with the important consequences of spin-valleyinterplay and the suppression of spin and valley relaxations. Here we reportthe first optical studies of WS2 and WSe2 monolayers and multilayers. Theefficiency of second harmonic generation shows a dramatic even-odd oscillationwith the number of layers, consistent with the presence (absence) of inversionsymmetry in even-layer (odd-layer). Photoluminescence (PL) measurements showthe crossover from an indirect band gap semiconductor at mutilayers to adirect-gap one at monolayers. The PL spectra and first-principle calculationsconsistently reveal a spin-valley coupling of 0.4 eV which suppressesinterlayer hopping and manifests as a thickness independent splitting patternat valence band edge near K points. This giant spin-valley coupling, togetherwith the valley dependent physical properties, may lead to rich possibilitiesfor manipulating spin and valley degrees of freedom in these atomically thin 2Dmaterials.

Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides