Many-Body Effects in Valleytronics: Direct Measurement of Valley Lifetimes in Single-Layer MoS₂

Abstract

Single layer MoS2 is an ideal material for the emerging field of "valleytronics" in which charge carrier momentum can be finely controlled by optical excitation. This system is also known to exhibit strong many-body interactions as observed by tightly bound excitons and trions. Here we report direct measurements of valley relaxation dynamics in single layer MoS2, by using ultrafast transient absorption spectroscopy. Our results show that strong Coulomb interactions significantly impact valley population dynamics. Initial excitation by circularly polarized light creates electron-hole pairs within the K-valley. These excitons coherently couple to dark intervalley excitonic states, which facilitate fast electron valley depolarization. Hole valley relaxation is delayed up to about 10 ps due to nondegeneracy of the valence band spin states. Intervalley biexciton formation reveals the hole valley relaxation dynamics. We observe that biexcitons form with more than an order of magnitude larger binding energy compared to conventional semiconductors. These measurements provide significant insight into valley specific processes in 2D semiconductors. Hence they could be used to suggest routes to design semiconducting materials that enable control of valley polarization.

Intervalley biexcitons and many-body effects in monolayerMoS2

“…This time delay was chosen to avoid contaminating effects of coherent light-matter interaction 7,8 . In contrast to what is expected, we observed strong bleaching peaks not only at A but also at B exciton transitions, and strikingly, these two peaks were present in both spectra measured using different helicities 22 .…”

“…This process can be mediated by flexural phonons 23 , and it can occur during the pump pulse duration (Appendix C). On the other hand, the bleaching peaks that are observed using the opposite probe helicity can be explained by intervalley scattering, which is expected to be very fast due to electron-hole exchange interaction in this material 22,24,25 . In particular, valley excitons with finite in-plane momentum can have exchange interaction that generates an inplane effective magnetic field, around which the exciton pseudospin precesses from K to K valley.…”

Observation of long-lived interlayer excitons in monolayer MoSe2–WSe2 heterostructures

“…A fit to a single exponential decay yields an interlayer exciton lifetime of 1.8 ± 0.3 ns. This timescale is much longer than the intralayer exciton lifetime, which is on the order of tens of ps [24][25][26][27] . By modelling the saturation behaviour using a simplified three-level diagram, the calculated saturation intensity for the interlayer exciton is about 180 times smaller than intralayer (Supplementary Fig.…”

Optical tuning of exciton and trion emissions in monolayer phosphorene

“…Carrier lifetime has been considered to be a very critical parameter in semiconducting materials [33][34][35] . The accurate probing of the carrier lifetime in monolayer phosphorene can greatly help us in understanding its highly excitonic nature and to better explore its optoelectronic applications.…”