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Title: Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides

Abstract

Monolayer transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) semiconductors for application in optoelectronics. Their optical properties are dominated by two series of photo-excited exciton states—A (XA) and B (XB)1,2—that are derived from direct interband transitions near the band extrema. These exciton states have large binding energies and strong optical absorption3,4,5,6, and form an ideal system to investigate many-body effects in low dimensions. Because spin–orbit coupling causes a large splitting between bands of opposite spins, XA and XB are usually treated as spin-polarized Ising excitons, each arising from interactions within a specific set of states induced by interband transitions between pairs of either spin-up or spin-down bands (TA or TB). Here, by using monolayer MoS2 as a prototypical system and solving the first-principles Bethe–Salpeter equations, we demonstrate a strong intravalley exchange interaction between TA and TB, indicating that XA and XB are mixed states instead of pure Ising excitons. Using 2D electronic spectroscopy, we observe that an optical excitation of the lower-energy TA induces a population of the higher-energy TB, manifesting the intravalley exchange interaction. This work elucidates the dynamics of exciton formation in monolayer TMDCs, and sheds light on many-body effects in 2D materials.

Authors:
 [1]; ORCiD logo [2]; ORCiD logo [2];  [1]; ORCiD logo [3]; ORCiD logo [2];  [4]
  1. Univ. of California, Berkeley, CA (United States). Dept. of Chemistry; Kavli Energy Nanoscience Inst. at Berkeley, CA (United States)
  2. Univ. of California, Berkeley, CA (United States). Dept. of Physics; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Materials Sciences Division
  3. National Tsing Hua Univ., Hsinchu (Taiwan). Materials Sciences and Engineering
  4. Univ. of California, Berkeley, CA (United States). Dept. of Chemistry; Kavli Energy Nanoscience Inst. at Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Biophysics and Integrated Bioimaging Division
Publication Date:
Research Org.:
LBNL Materials Project
Sponsoring Org.:
USDOE; USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division
OSTI Identifier:
1530822
DOE Contract Number:  
AC02-05CH11231
Resource Type:
Journal Article
Journal Name:
Nature Physics
Additional Journal Information:
Journal Volume: 15; Journal Issue: 3; Journal ID: ISSN 1745-2473
Country of Publication:
United States
Language:
English

Citation Formats

Guo, Liang, Wu, Meng, Cao, Ting, Monahan, Daniele M., Lee, Yi-Hsien, Louie, Steven G., and Fleming, Graham R. Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides. United States: N. p., 2018. Web. doi:10.1038/s41567-018-0362-y.
Guo, Liang, Wu, Meng, Cao, Ting, Monahan, Daniele M., Lee, Yi-Hsien, Louie, Steven G., & Fleming, Graham R. Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides. United States. doi:10.1038/s41567-018-0362-y.
Guo, Liang, Wu, Meng, Cao, Ting, Monahan, Daniele M., Lee, Yi-Hsien, Louie, Steven G., and Fleming, Graham R. Mon . "Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides". United States. doi:10.1038/s41567-018-0362-y.
@article{osti_1530822,
title = {Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides},
author = {Guo, Liang and Wu, Meng and Cao, Ting and Monahan, Daniele M. and Lee, Yi-Hsien and Louie, Steven G. and Fleming, Graham R.},
abstractNote = {Monolayer transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) semiconductors for application in optoelectronics. Their optical properties are dominated by two series of photo-excited exciton states—A (XA) and B (XB)1,2—that are derived from direct interband transitions near the band extrema. These exciton states have large binding energies and strong optical absorption3,4,5,6, and form an ideal system to investigate many-body effects in low dimensions. Because spin–orbit coupling causes a large splitting between bands of opposite spins, XA and XB are usually treated as spin-polarized Ising excitons, each arising from interactions within a specific set of states induced by interband transitions between pairs of either spin-up or spin-down bands (TA or TB). Here, by using monolayer MoS2 as a prototypical system and solving the first-principles Bethe–Salpeter equations, we demonstrate a strong intravalley exchange interaction between TA and TB, indicating that XA and XB are mixed states instead of pure Ising excitons. Using 2D electronic spectroscopy, we observe that an optical excitation of the lower-energy TA induces a population of the higher-energy TB, manifesting the intravalley exchange interaction. This work elucidates the dynamics of exciton formation in monolayer TMDCs, and sheds light on many-body effects in 2D materials.},
doi = {10.1038/s41567-018-0362-y},
journal = {Nature Physics},
issn = {1745-2473},
number = 3,
volume = 15,
place = {United States},
year = {2018},
month = {12}
}

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