Research Seminar of Dr. Zilong Wang from Max Planck Institute

Lecture Time:  09:30 on 22nd July, 2019

Lecture Room: C501 of Science and Innovation Building

Lecture Title: Intravalley spin relaxation dynamics in single-layer  WS2

Zilong Wang

Max Planck Institute

Abstract

Two-dimensional Transition Metal Dichalcogenides (TMDs) have been widely studied  because of the peculiar electronic band structure and the strong excitonic  effects [1]. In these materials the large spin-orbit coupling lifts the spin  degeneracy of the valence (VB) and the conduction band (CB) giving rise to the A  and B interband excitonic transitions. In monolayer WS2, the spins of electrons  in the lowest CB and in the highest VB at K/K’ point of the Brillouin zone are  antiparallel resulting in an intravalley dark exciton state at a lower energy  than the bright exciton, see left panel of Fig.1. On the one hand, the presence  of dark excitons has been revealed indirectly from the observation of anomalous  quenching of the PL emission at low temperature in single-layer WS2 [2]; on the  other hand, however, the intravalley spin-flip process is assumed to occur on a  significantly long time scale, which is usually neglected in theoretical models  describing exciton intra or inter valley scattering processes [3]. Here we use  two-colour helicity-resolved pump-probe spectroscopy to directly resolve the  intravalley spin-flip process of the photoexcited electrons in the CB of  single-layer WS2 [4]. In our experiment, spin-polarized carriers are  photo-injected by a circularly polarized pump beam resonant to the A exciton  transition, while the co-circularly polarized probe pulse is tuned around B  excitonic peak. Our results show that the upper CB states can be quickly  depleted by efficient scattering processes mediated by phonons on a temporal  scale close to our experimental results. Our results shed light on the  intravalley spin relaxation process in single-layer WS2, determining the  formation of the intravalley dark exciton, which we measure to occur on a sub-ps  timescale. The study of dark excitons formation dynamics is important for  designing TMD-based electronic/photonic devices.

References

[1] G. Wang, A. Chernikov, M. M.  Glazov, T. F. Heinz, X. Marie, T. Amand, and B. Urbaszek, “Excitons in  atomically thin transition metal dichalcogenides”, Rev. Mod. Phys. 90, 021001  (2018).

[2] X.-X. Zhang, Y. You, S. Y. F. Zhao  and T. F. Heinz, “Experimental Evidence for Dark Excitons in Monolayer WSe2”,  Phys. Rev. Lett. 115, 257403 (2015).

[3] Y. Song, H. Dery, “Transport Theory  of Monolayer Transition-Metal Dichalcogenides through Symmetry”, Phys. Rev.  Lett. 111, 026601 (2013).

[4] Z. Wang, A. Molina-Sanchez, P.  Altmann, D. Sangalli, D. De Fazio, G. Soavi, U. Sassi, F. Bottegoni, F.  Ciccacci, M. Finazzi, L. Wirtz, A.C. Ferrari, A. Marini, G. Cerullo and S. Dal  Conte, “Intravalley Spin?Flip Relaxation Dynamics in Single-Layer WS2”, Nano  Lett. 18, 6882 (2018).

Biography

 Zilong Wang, received B.S.  degree in physics from Peking University, Beijing in 2010. He received Ph.D.  degree in physics from Nanyang Technological University, Singapore in the year  of 2016. From 2016 to 2018, he was a Post-doctoral researcher in Physics  department of Politecnico di Milano, Italy. And he is currently working in Max  Planck Institute for Quantum Optics in Garching, Germany as a Post-doctoral  researcher. His research interests are ultrafast spectroscopy and  optoelectronics of nanomaterials, such as two-dimensional materials. He is  currently working on the Petahertz electronics in nanomaterials driven by  few-cycle lightwave.