• Open Access

Marrying Excitons and Plasmons in Monolayer Transition-Metal Dichalcogenides

Dinh Van Tuan, Benedikt Scharf, Igor Žutić, and Hanan Dery
Phys. Rev. X 7, 041040 – Published 17 November 2017

Abstract

Just as photons are the quanta of light, plasmons are the quanta of orchestrated charge-density oscillations in conducting media. Plasmon phenomena in normal metals, superconductors, and doped semiconductors are often driven by long-wavelength Coulomb interactions. However, in crystals whose Fermi surface is comprised of disconnected pockets in the Brillouin zone, collective electron excitations can also attain a shortwave component when electrons transition between these pockets. In this work, we show that the band structure of monolayer transition-metal dichalcogenides gives rise to an intriguing mechanism through which shortwave plasmons are paired up with excitons. The coupling elucidates the origin for the optical sideband that is observed repeatedly in monolayers of WSe2 and WS2 but not understood. The theory makes it clear why exciton-plasmon coupling has the right conditions to manifest itself distinctly only in the optical spectra of electron-doped tungsten-based monolayers.

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  • Received 14 June 2017

DOI:https://doi.org/10.1103/PhysRevX.7.041040

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Dinh Van Tuan1,*, Benedikt Scharf2,3,4, Igor Žutić2, and Hanan Dery1,5,†

  • 1Department of Electrical and Computer Engineering, University of Rochester, Rochester, New York 14627, USA
  • 2Department of Physics, University at Buffalo, State University of New York, Buffalo, New York 14260, USA
  • 3Institute for Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany
  • 4Institute for Theoretical Physics and Astrophysics, University of Würzburg, Am Hubland, 97074 Würzburg, Germany
  • 5Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA

  • *vdinh@ur.rochester.edu
  • hanan.dery@rochester.edu

Popular Summary

While atomic particles typically move with little concern for their neighbors, this behavior profoundly changes in certain situations—the particles move together with a shared rhythm known as a collective excitation. Researchers recently discovered a new excitation in monolayer transition-metal dichalcogenides (TMDs)—atomically thin, two-dimensional semiconductors. This excitation (an intervalley plasmon) arises when a TMD is exposed to strong light or electron doping, which increases the coupling between tightly bound electron-hole pairs (or excitons). We offer a theoretical analysis that not only reveals the telltale sign of these plasmons in optical measurements but also explains why they appear only in specific TMDs with a particular preparation of added electrons.

We identify a fascinating pairing between excitons and collective shortwave charge excitations due to electron transitions between the valleys (shortwave plasmons). By renormalization of the electron-hole wave function due to the interaction between ground- and excited-state excitons that is mediated by shortwave plasmons, we elucidate why the unique optical transition band emerges in the spectra of electron-doped WSe2 and WS2, while being conspicuously absent with hole doping, MoSe2 and MoS2. An examination of numerous experiments confirms this scenario.

Given that shortwave plasmons provide a motion of collective charge excitations, they may also profoundly alter the nature of favorable states in other materials, perhaps even turning an ordinary conductor into a superconductor.

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Vol. 7, Iss. 4 — October - December 2017

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