Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe2

Kai-Qiang Lin (), Chin Shen Ong, Sebastian Bange, Paulo E. Faria Junior, Bo Peng, Jonas D. Ziegler, Jonas Zipfel, Christian Bäuml, Nicola Paradiso, Kenji Watanabe, Takashi Taniguchi, Christoph Strunk, Bartomeu Monserrat, Jaroslav Fabian, Alexey Chernikov, Diana Y. Qiu, Steven G. Louie and John M. Lupton ()
Additional contact information
Kai-Qiang Lin: University of Regensburg
Chin Shen Ong: University of California at Berkeley
Sebastian Bange: University of Regensburg
Paulo E. Faria Junior: University of Regensburg
Bo Peng: University of Cambridge
Jonas D. Ziegler: University of Regensburg
Jonas Zipfel: University of Regensburg
Christian Bäuml: University of Regensburg
Nicola Paradiso: University of Regensburg
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Christoph Strunk: University of Regensburg
Bartomeu Monserrat: University of Cambridge
Jaroslav Fabian: University of Regensburg
Alexey Chernikov: University of Regensburg
Diana Y. Qiu: University of California at Berkeley
Steven G. Louie: University of California at Berkeley
John M. Lupton: University of Regensburg

Nature Communications, 2021, vol. 12, issue 1, 1-8

Abstract: Abstract Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe2 with an energy of ~3.4 eV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8 meV. The HX is populated through momentum-selective optical excitation in the K-valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW-BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons.

Date: 2021
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DOI: 10.1038/s41467-021-25499-2

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