Exciton valley depolarization in monolayer transition-metal dichalcogenides

Min Yang, Cedric Robert, Zhengguang Lu, Dinh Van Tuan, Dmitry Smirnov, Xavier Marie, and Hanan Dery
Phys. Rev. B 101, 115307 – Published 26 March 2020

Abstract

The valley degree of freedom is a sought-after quantum number in monolayer transition-metal dichalcogenides. Similar to optical spin orientation in semiconductors, the helicity of absorbed photons can be relayed to the valley (pseudospin) quantum number of photoexcited electrons and holes. Also similar to the quantum-mechanical spin, the valley quantum number is not a conserved quantity. Valley depolarization of excitons in monolayer transition-metal dichalcogenides due to long-range electron-hole exchange typically takes a few ps at low temperatures. Exceptions to this behavior are monolayers MoSe2 and MoTe2 wherein the depolarization is much faster. We elucidate the enigmatic anomaly of these materials, finding that it originates from Rashba-induced coupling of the dark and bright exciton branches next to their degeneracy point. When photoexcited excitons scatter during their energy relaxation between states next to the degeneracy region, they reach the light cone after losing the initial helicity. The valley depolarization is not as fast in monolayers WSe2, WS2, and MoS2, wherein the degeneracy is absent resulting in negligible Rashba-induced coupling between bright and dark excitons.

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  • Received 6 September 2019
  • Revised 15 January 2020
  • Accepted 10 March 2020

DOI:https://doi.org/10.1103/PhysRevB.101.115307

©2020 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Min Yang1, Cedric Robert2, Zhengguang Lu3,4, Dinh Van Tuan1, Dmitry Smirnov3, Xavier Marie2, and Hanan Dery1,5,*

  • 1Department of Electrical and Computer Engineering, University of Rochester, Rochester, New York 14627, USA
  • 2Université de Toulouse, INSA-CNRS-UPS, LPCNO, 135 Av. Rangueil, 31077 Toulouse, France
  • 3National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA
  • 4Physics Department, Florida State University, Tallahassee, Florida 32306, USA
  • 5Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA

  • *hanan.dery@rochester.edu

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Issue

Vol. 101, Iss. 11 — 15 March 2020

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