Novel truly two-dimensional materials, such as monolayers of transition metal dichalcogenides (TMDCs) combine unusual optical and transport properties and attract a lot of interest nowadays [1,2]. TMDCs such as MoS₂ or WSe₂ have a hexagonal crystalline lattice and direct band gaps on the order of 2 eV realized at the edges of the Brillouin zone denoted as the K₊ and K_- valleys. An extremely strong spin-orbit coupling in TMDCs yields the spin-valley locking: Certain spin components of individual charge carriers correspond to the specific valley of the energy spectrum [3]. Interestingly, both the electron and hole states are already spin-split at the K₊ and K_- points. Moreover, spin-valley locking results also in the chiral selection rules for interband optical transitions. Namely, the absorption of right- or left-handed circularly polarized light excites transitions in the the K₊ and K_- valley, respectively [4].

 

Optical properties of TMDCs are governed by robust excitons, Coulomb-bound electron-hole pairs. After brief introduction to the physics of excitons and trions in these materials and a review of experimental data on exciton/trion binding energies as well as corresponding theoretical models, I will present a theory of fine structure of exciton energy spectrum and of exciton spin dynamics [5,6]. It will be demonstrated that the long-range exchange interaction between an electron and a hole results in simultaneous intervalley transfer of charge carriers forming the exciton. This contribution will be calculated both by means of macroscopic (electrodynamical) and microscopic (quantum mechanical) methods. The long-range exchange interaction results in the efficient depolarization of excitons as confirmed by polarized photoluminescence and pump-probe Kerr rotation experiments. The possibility to preserve exciton circular polarization (also termed as valley polarization) and alignment (termed as valley coherence) in double resonant Raman process involving excited excitonic states is demonstrated [7]. 

 

A brief review of the exciton fine structure, spin and valley dynamics is given in Ref. [8].

 

[1] A. K. Geim and K. S. Novoselov, Nat. Mater. 6, 183 (2007).

[2] L. Britnell et al., Science 340, 1311 (2013).

[3] D. Xiao, G.-B. Liu, W. Feng, X. Xu, and W. Yao, Phys. Rev. Lett. 108, 196802 (2012).

[4] K. F. Mak, K. He, J. Shan, and T. F. Heinz, Nat. Nano. 7, 494 (2012).

[5] M. M. Glazov, et al., Phys. Rev. B 89, 201302(R) (2014).

[6] C. R. Zhu, K. Zhang, M. Glazov, et al., Phys. Rev. B 90, 161302(R) (2014).

[7] G. Wang, M. M. Glazov, et. al., Phys. Rev. Lett. 115, 117401 (2015).

 

[8] M. M. Glazov, et al., Phys. Status Solidi B. doi: 10.1002/pssb.2015522112 (2015).