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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 sensors1,2,3,4,5,6,7, and to valleytronics8,9,10,11,12,13,14,15,16,17. 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.

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Figure 1: PL spectra and optical image of monolayer WS2.
Figure 2: Valley-spin polarization from a WS2 monolayer at zero magnetic field and a temperature of 5 K.
Figure 3: Circular polarization as a function of magnetic field at a temperature of 5 K.
Figure 4: Band structure and quantum numbers.
Figure 5: Modelling of the dependence of the circular polarization on the magnetic field.

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Acknowledgements

The authors thank S. Banerjee and his group at SUNY Buffalo for technical support. Work at SUNY Buffalo has been supported by the Office of Naval Research. I.O., M.K. and P.H. acknowledge support of National Research Council Quantum Photonic Sensing and Security program and of the Natural Sciences and Engineering Research Council. G.K. acknowledges support by the Greek General Secretariat for Research and Technology project ERC02-EXEL (contract No. 6260).

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T.S., Y.T., B.B., L.S., A.P. and G.K. designed and performed the experimental measurements, and I.O., M.K. and P.H. provided the theoretical analysis. A.P., G.K., M.K. and P.H. wrote the manuscript.

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Correspondence to A. Petrou or P. Hawrylak.

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The authors declare no competing financial interests.

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Scrace, T., Tsai, Y., Barman, B. et al. Magnetoluminescence and valley polarized state of a two-dimensional electron gas in WS2 monolayers. Nature Nanotech 10, 603–607 (2015). https://doi.org/10.1038/nnano.2015.78

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