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; John M Lupton

doi:10.1038/s41467-021-25499-2

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

Nat Commun. 2021 Sep 17;12(1):5500. doi: 10.1038/s41467-021-25499-2.

Authors

Kai-Qiang Lin¹, Chin Shen Ong^{2

3}, Sebastian Bange⁴, Paulo E Faria Junior⁴, Bo Peng⁵, Jonas D Ziegler⁴, Jonas Zipfel⁴, Christian Bäuml⁴, Nicola Paradiso⁴, Kenji Watanabe⁶, Takashi Taniguchi⁷, Christoph Strunk⁴, Bartomeu Monserrat^{5

8}, Jaroslav Fabian⁴, Alexey Chernikov^{4

9}, Diana Y Qiu^{2

3

10}, Steven G Louie^{2

3}, John M Lupton¹¹

Affiliations

¹ Department of Physics, University of Regensburg, Regensburg, Germany. [email protected].
² Department of Physics, University of California at Berkeley, Berkeley, CA, USA.
³ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
⁴ Department of Physics, University of Regensburg, Regensburg, Germany.
⁵ Cavendish Laboratory, University of Cambridge, Cambridge, UK.
⁶ Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Japan.
⁷ International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan.
⁸ Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK.
⁹ Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Würzburg-Dresden Cluster of Excellence ct.qmat, Technische Universität Dresden, Dresden, Germany.
¹⁰ Department of Mechanical Engineering and Materials Science, Yale University, Yale, CT, USA.
¹¹ Department of Physics, University of Regensburg, Regensburg, Germany. [email protected].

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.

Grants and funding

314695032/Deutsche Forschungsgemeinschaft (German Research Foundation)