Breaking space–time symmetries in two-dimensional crystals can markedly influence their macroscopic electronic properties. Monolayer transition metal dichalcogenides (TMDs) are prime examples where the intrinsically broken crystal inversion symmetry permits the generation of valley-selective electron populations1,2,3,4, even though the two valleys are energetically degenerate, locked by time-reversal symmetry. Lifting the valley degeneracy in these materials is of great interest because it would allow for valley-specific band engineering and offer additional control in valleytronic applications. Although applying a magnetic field should, in principle, accomplish this task, experiments so far have not shown valley-selective energy level shifts in fields accessible in the laboratory. Here, we show the first direct evidence of lifted valley degeneracy in the monolayer TMD WS2. By applying intense circularly polarized light, which breaks time-reversal symmetry, we demonstrate that the exciton level in each valley can be selectively tuned by as much as 18 meV through the optical Stark effect. These results offer a new way to control the valley degree of freedom, and may provide a means to realize new Floquet topological phases5,6,7 in two-dimensional TMDs.
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The authors acknowledge technical assistance from Q. Ma and Y. Bie during the measurement of the equilibrium absorption of monolayer WS2, and helpful discussions with Z. Alpichshev, I. M. Vishik and Y. H. Wang. This work is supported by US Department of Energy (DOE) award numbers DE-FG02-08ER46521 and DE-SC0006423 (data acquisition and analysis). Y-H.L. and J.K. acknowledge support from NSF DMR 0845358 (material growth and characterization). Y-H.L. also acknowledges partial support from the Ministry of Science and Technology of the Republic of China (103-2112-M-007-001-MY3). L.F. acknowledges support from the STC Center for Integrated Quantum Materials (CIQM), NSF Grant No. DMR-1231319 (theory).
The authors declare no competing financial interests.
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Sie, E., McIver, J., Lee, YH. et al. Valley-selective optical Stark effect in monolayer WS2. Nature Mater 14, 290–294 (2015). https://doi.org/10.1038/nmat4156
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