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Nat Nanotechnol. 2017 Sep;12(9):883-888. doi: 10.1038/nnano.2017.105. Epub 2017 Jun 26.

Magnetic brightening and control of dark excitons in monolayer WSe2.

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Department of Physics, Columbia University, New York, New York 10027, USA.
Department of Applied Physics, Stanford University, Stanford, California 94305, USA.
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
Department of Physics, University of California, Berkeley, California 94720, USA.
Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA.
National High Magnetic Field Laboratory, Tallahassee, Florida 32312, USA.
Department of Physics, Florida State University, Tallahassee, Florida 32310, USA.
Department of Materials Science and Engineering and Center for 2-Dimensional and Layered materials, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA.


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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