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Excitonic properties and valley dynamics in monolayer transitioin metal dichalcogenides

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Transition metal dichalcogenides (TMDCs) such as WSe2 and MoS2 are layered semiconductors, with strong in-plane bonds and Van-der-Waals bonding between the individual layers. Although indirect semiconductors in bulk form, when thinned down to one monolayer (ML) they become direct semiconductors in the visible/IR region of the optical spectrum [1]. These atomically flat 2D materials have unique physical properties for manipulating electrons in non-equivalent valleys in momentum space and can potentially serve as building blocks in Van-der-Waals heterostructures for optoelectronics applications. In this talk we will review the optical properties of ML TMD Cs governed by robust excitons, Coulomb bound electron-hole pairs. We discuss valley state manipulation with polarized lasers and the numerous open questions and challenges in this fast growing field.

ML TMD Cs share common properties: The ML crystals have no inversion centre, contrary to graphene or bilayer TMD Cs. In addition, the spin-orbit (SO) coupling is strong due to the heavy transition metal atoms. This combination leads to a unique coupling of electron spin and k-space valley physics, initially described in a single particle picture [2]. When electrons and holes are simultaneously present, they will form excitons as the Coulomb interaction is enhanced by the strong quantum confinement, the large effective masses, and the reduced dielectric screening in these ideal 2D systems. Excitons with typical binding energies of 500 meV dominate the optical properties of ML TMD Cs as the 1s, 2s, 2p…. exciton states are resonantly excited, also Raman scattering amplitudes are strongly affected by exciton resonances [3]. Due to the high oscillator strength of excitonic transitions, the intrinsic radiative lifetimes are expected to be short, and in time resolved photoluminescence experiments on ML MoS2, WSe2 and MoSe2 we find indeed emission times in the ps range [4].

In addition to these common properties for ML TMD Cs we also discuss important differences. We show how valley polarization and coherence can be optically initialized in ML W Se2 but not ML MoSe2. We also discuss the competition between bright and dark excitons that determines the light emission yield in these binary materials and their ternary alloys MoWSe2 [5].

References [1] Mak et al, PRL 105 , 136805 (2010); Lezema et al NanoLett 15, 2336 (2015) [2] Di Xiao et al, PRL 108 , 196802 (2012) [3] Wang et al, PRL 114 , 097403 (2015) & Wang et al PRL 115 , 117401 (2015)
 [4] Lagarde et al, PRL 112 , 047401 (2014) & Wang et al PRB 90 , 075413 (2014)
 [5] Wang et al, arXiv 1506.08114

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