Magnetoluminescence and valley polarized state of a two-dimensional electron gas in WS2 monolayers

Abstract

Materials often exhibit fundamentally new phenomena in reduced dimensions that potentially lead to novel applications. This is true for single-layer, two-dimensional semiconductor crystals of transition-metal dichalcogenides, MX2 (M = Mo, W and X = S, Se). They exhibit direct bandgaps with energies in the visible region at the two non-equivalent valleys in the Brillouin zone. This makes them suitable for optoelectronic applications that range from light-emitting diodes to light harvesting and light sensors, and to valleytronics. Here, we report the results of a magnetoluminescence study of WS2 single-layer crystals in which the strong spin-orbit interaction additionally locks the valley and spin degrees of freedom. The recombination of the negatively charged exciton in the presence of a two-dimensional electron gas (2DEG) is found to be circularly polarized at zero magnetic field despite being excited with unpolarized light, which indicates that the existence of a valley polarized 2DEG is caused by valley and spin locking and strong electron-electron interactions.

Strong Photoluminescence Fluctuations in Laser-Thinned Few-Layer WS_{2}

“…[14]. Note that in earlier papers only X and X − emission lines were attributed [15][16][17]. In Fig.…”

Probing the Interlayer Exciton Physics in a MoS2/MoSe2/MoS2 van der Waals Heterostructure

“…This is in strong contrast with the co-polarized emission of the A-exciton in MoSe 2 . 33 In addition, under linearly polarized excitation, which simultaneously excites both valleys, I X does not exhibit any detectable degree of circular polarization, which rules out a "built in" imbalance of the valley population 41 or carrier lifetimes as the origin of the observed polarization behavior. The opposite helicity compared to the excitation points to a more complex valley and spin transfer mechanism than that previously invoked to explain the non-trivial dependence of the polarization of inter-layer excitons in gated heterostructures.…”

Dark excitons and the elusive valley polarization in transition metal dichalcogenides

“…Experimentally, a significant degree of circular polarization of photoluminescence (PL) after circularly polarized excitation has been observed in MoS 2 , WS 2 and WSe 2 [25][26][27][28][29][30][31]. In addition, coherent emission from K K and ′ K K valleys was observed in WSe 2 [32].…”