Valley depolarization in monolayer transition-metal dichalcogenides with zone-corner acoustic phonons

Although single-layer transition-metal dichalcogenides with novel valley functionalities are a promising candidate to realize valleytronic devices, the essential understanding of valley depolarization mechanisms is still incomplete. Based on pump-probe experiments performed for MoSe 2 and WSe 2 mono...

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Bibliographic Details
Published in:Nanoscale 2020-11, Vol.12 (44), p.22487-22494
Main Authors: Jeong, Tae-Young, Bae, Soungmin, Lee, Seong-Yeon, Jung, Suyong, Kim, Yong-Hoon, Yee, Ki-Ju
Format: Article
Language:English
Subjects:
Brillouin zones
Chalcogenides
Depolarization
Electron spin
Energy bands
Molybdenum compounds
Monolayers
Phonons
Symmetry
Transition metal compounds
Valleys
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Summary:Although single-layer transition-metal dichalcogenides with novel valley functionalities are a promising candidate to realize valleytronic devices, the essential understanding of valley depolarization mechanisms is still incomplete. Based on pump-probe experiments performed for MoSe 2 and WSe 2 monolayers and corroborating analysis from density functional calculations, we demonstrate that coherent phonons at the K-point of the Brillouin zone can effectively mediate the valley transfer of electron carriers. In the MoSe 2 monolayer case, we identify this mode as the flexural acoustic ZA(K) mode, which has broken inversion symmetry and thus can enable electron spin-flip during valley transfer. On the other hand, in the monolayer WSe 2 case where spin-preserving inter-valley relaxations are preferred, coherent LA(K) phonons with even inversion symmetry are efficiently generated. These findings establish that while the specifics of inter-valley relaxations depend on the spin alignments of energy bands, the K-point phonons should be taken into account as an effective valley depolarization pathway in transition metal dichalcogenide monolayers. We demonstrate that zone-corner phonons in monolayer transition metal dichalcogenides can provide an effective valley depolarization mechanism via strong spin-valley locking behaviors.
ISSN:2040-3364
2040-3372
DOI:10.1039/d0nr04761a