Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS2.


Type
Article
Change log
Authors
Wang, Zilong 
Molina-Sánchez, Alejandro  ORCID logo  https://orcid.org/0000-0001-5121-4058
Altmann, Patrick 
De Fazio, Domenico 
Abstract

In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark excitons, consisting of electrons and holes with opposite spin orientation, have lower energy than A excitons. The transition from bright to dark excitons involves the scattering of electrons from the upper to the lower conduction band at the K point of the Brillouin zone, with detrimental effects for the optoelectronic response of 1L-TMDs, since this reduces their light emission efficiency. Here, we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics in 1L-WS2. This occurs on a sub-ps time scale, and it is significantly dependent on temperature, indicative of phonon-assisted relaxation. Time-dependent ab initio calculations show that intravalley spin-flip scattering occurs on significantly longer time scales only at the K point, while the occupation of states away from the minimum of the conduction band significantly reduces the scattering time. Our results shed light on the scattering processes determining the light emission efficiency in optoelectronic and photonic devices based on 1L-TMDs.

Description
Keywords
Transition metal dichalcogenides, layered materials, optoelectronics, spin and valley dynamics, transient absorption spectroscopy
Journal Title
Nano Lett
Conference Name
Journal ISSN
1530-6984
1530-6992
Volume Title
18
Publisher
American Chemical Society (ACS)
Sponsorship
EPSRC (via University of Manchester) (R119256)
European Commission Horizon 2020 (H2020) Future and Emerging Technologies (FET) (785219)
Engineering and Physical Sciences Research Council (EP/K017144/1)
Engineering and Physical Sciences Research Council (EP/K01711X/1)