Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light–matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX2 (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe2 and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitons are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. These studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.
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Support at SLAC/Stanford was provided by the AMOS program, Chemical Sciences, Geosciences, and Biosciences Division, Basic Energy Sciences, US Department of Energy under contract no. DE-AC02-76-SFO0515 and the Betty and Gordon Moore Foundation's EPiQS Initiative through grant no. GBMF4545 (T.F.H.). for data analysis, and by Air Force Office of Scientific Research through the MURI Center for dynamic magneto-optics under grant. no. FA9550-14-1-0040 for optical measurements. The theoretical studies were supported by the Theory of Materials Program funded by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, US Department of Energy under contract no. DE-AC02-05CH11231, which supported the GW and GW-BSE calculations, and by the National Science Foundation (NSF) grant no. DMR-1508412, which supported the magnetic-field-induced exciton mixing analyses. Computational resources were provided by the DOE at Lawrence Berkeley National Laboratory's NERSC facility and by the NSF through XSEDE resources at NICS. Y.-C.L. and J.A.R. acknowledge the support from the Center for Low Energy Systems Technology (LEAST), one of six centres supported by the STARnet phase of the Focus Center Research Program (FCRP), a Semiconductor Research Corporation program sponsored by MARCO and DARPA. Sample preparation at Columbia University was supported by the NSF MRSEC for Precision Assembly of Superstratic and Superatomic Solids through grant no. DMR-1420634. Z.L. and D.S. acknowledge the support from the US Department of Energy (grant no. DE-FG02-07ER46451) for high-field CW PL measurements that were performed at the National High Magnetic Field Laboratory, which is supported by the NSF Cooperative Agreement no. DMR-1157490 and the State of Florida.
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Physical Review B (2019)