Transient band renormalization and ultrafast polarization-resolved electron dynamics in two-dimensional materials

Two-dimensional (2D) materials exhibit a diverse collection of intriguing electronic phenomena. These include single-particle effects related to new spin and valley physics, as well as exotic many-body interactions that may profoundly influence charge carrier dynamics in these materials. Here, I will discuss time- and angle-resolved hotoemission (TR-ARPES) experiments on single- and bi-layer transition metal dichalcogenides (TMDs), which have been synthesized using van der Waals epitaxy [1]. Such TR-ARPES experiments have previously provided detailed information on hot electron relaxation dynamics in graphene due to directly revealing the time-dependent changes in the material’s band structure and separating electron-phonon interactions in the time domain [2]. I will show how ultrafast band gap tuning [3] and control of the spin- and valley-degrees of freedom can be achieved and observed in pump-probe experiments on semiconducting single-layer TMDs such as MoS 2 and WS 2 [4]. The polarization dependence of excited carriers in bi-layer MoS 2 will be discussed. Recent results on the metallic TMD TaS 2 will also be presented, and I will discuss the observation and analysis of time dependent electron-electron interactions leading to strong band renormalization effects around the Fermi level of 2D TaS 2 . [1] J. A. Miwa, M. Dendzik et al.: Van der Waals Epitaxy of Two-Dimensional MoS 2 -Graphene Heterostructures in Ultrahigh Vacuum. ACS Nano 9, 6502 (2015) [2] J. C. Johannsen, S. Ulstrup et al.: Direct View of Hot Carrier Dynamics in Graphene. Phys. Rev. Lett. 111, 027403 (2013). [3] S. Ulstrup, A. G. Cabo et al.: Ultrafast Band Structure Control of a Two-Dimensional Heterostructure. ACS Nano 10, 6315 (2016). [4] S. Ulstrup, A. G. Cabo et al.: Spin and Valley Control of Free Carriers in Single-Layer WS 2. Physical Review B: Rapid Communications 95, 041405(R) (2017).

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