Identifying optical signatures of momentum-dark excitons in transition metal dichalcogenide monolayers

Jessica Lindlau, Cedric Robert, Victor Funk, Jonathan Förste, Michael Förg, Léo Colombier, Andre Neumann, Emmanuel Courtade, Shivangi Shree, Takashi Taniguchi, Kenji Watanabe, Mikhail M. Glazov, Xavier Marie, Bernhard Urbaszek, Alexander Högele

Published 2017-10-03Version 1

Transition metal dichalcogenide (TMD) monolayers (MLs) exhibit rich photoluminescence spectra associated with interband optical transitions of direct-gap semiconductors. Dipolar selection rules of Coulomb-correlated electrons and holes in the conduction and valence band with identical valley index result in spin-allowed and spin-forbidden optical transitions of excitons with zero center-of-mass momentum. The corresponding spectral signatures, however, are insufficient to explain the realm of characteristic peaks observed in the photoluminescence spectra of ML TMDs on the basis of momentum-direct excitons alone. Here, we show that the notion of these momentum-indirect excitons is the key to the understanding of the versatile photoluminescence features. Taking into account phonon-assisted radiative recombination pathways for electrons and holes from dissimilar valleys, we interpret unidentified peaks in the emission spectra as acoustic and optical phonon sidebands of momentum-dark excitons. Our approach represents a crucial step towards a unified understanding of TMD photophysics and will facilitate the interpretation of optical, valley and spin phenomena in TMDs arising from bright and dark exciton manifolds.