Abstract
Crystal symmetry governs the nature of electronic Bloch states. For example, in the presence of time-reversal symmetry, the orbital magnetic moment and Berry curvature of the Bloch states must vanish unless inversion symmetry is broken1. In certain two-dimensional electron systems such as bilayer graphene, the intrinsic inversion symmetry can be broken simply by applying a perpendicular electric field2,3. In principle, this offers the possibility of switching on/off and continuously tuning the magnetic moment and Berry curvature near the Dirac valleys by reversible electrical control4,5. Here we investigate this possibility using polarization-resolved photoluminescence of bilayer MoS2, which has the same symmetry as bilayer graphene but has a bandgap in the visible spectrum6,7 allowing direct optical probing5,8,9,10,11,12. We find that in bilayer MoS2 the circularly polarized photoluminescence can be continuously tuned from −15% to 15% as a function of gate voltage, whereas in structurally non-centrosymmetric monolayer MoS2 the photoluminescence polarization is gate independent. The observations are well explained as resulting from the continuous variation of orbital magnetic moments between positive and negative values through symmetry control.
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Acknowledgements
The authors thank B. Spivak for helpful discussions. This work is mainly supported by the US DoE, BES, Division of Materials Sciences and Engineering (DE-SC0008145), and device fabrication is partially supported by the NSF (DMR-1150719). A.J. was supported by the NSF Graduate Research Fellowship (DGE-0718124). G-B.L. and W.Y. were supported by the Research Grant Council of Hong Kong (HKU 706412P). W.Z. and D.X. were supported by US DoE, BES, Division of Materials Sciences and Engineering. D.C. and Z.F. were supported by the DoE BES (DE-SC0002197). The DFT calculations were performed at the National Energy Research Scientific Computing Center supported by the DoE Office of Science. Device fabrication was performed at the University of Washington Micro Fabrication Facility and NSF-funded Nanotech User Facility.
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Wu, S., Ross, J., Liu, GB. et al. Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS2. Nature Phys 9, 149–153 (2013). https://doi.org/10.1038/nphys2524
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DOI: https://doi.org/10.1038/nphys2524
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