title: Experimental Evidence for Dark Excitons in Monolayer WSe_{2}.
abstract: Transition metal dichalcogenides in the class MX_{2} (M=Mo, W; X=S, Se) have been identified as direct-gap semiconductors in the monolayer limit. Here, we examine light emission of monolayer WSe_{2} using temperature-dependent photoluminescence and time-resolved photoluminescence spectroscopy. We present experimental evidence for the existence of an optically forbidden dark state of the band-gap exciton that lies tens of meV below the optically bright state. The presence of the dark state is manifest in the strong quenching of light emission observed at reduced temperatures. The experimental findings are consistent with theoretical predictions of spin-polarized conduction and valence bands at the K point of the Brillouin zone, with the minimum gap occurring between bands of opposite electron spin.
authors.affiliation: Departments of Physics and Electrical Engineering, Columbia University, 538 West 120th Street, New York, New York 10027, USA.
authors.forename: Xiao-Xiao
authors.initials: XX
authors.lastname: Zhang
authors.affiliation: Departments of Physics and Electrical Engineering, Columbia University, 538 West 120th Street, New York, New York 10027, USA.|Ordered Matter Science Research Center, Southeast University, Nanjing 211189, China.
authors.forename: Yumeng
authors.initials: Y
authors.lastname: You
authors.affiliation: Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: Shu Yang Frank
authors.initials: SY
authors.lastname: Zhao
authors.affiliation: Department of Applied Physics, Stanford University, Stanford, California 94305, USA.|SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
authors.forename: Tony F
authors.initials: TF
authors.lastname: Heinz