Spin and Valley Dynamics in 2D van der Waals Materials

• Applicant: Privatdozent Dr. Jens Hübner
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2017 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 391378795

The extremely successful research on layered van der Waals (vdW) materials opened up many new research-lines and potential applications. Here, the versatility of monolayer (ML) MoS2, MoSe2, WS2, and WSe2 stands out due to the direct-bandgap in the visible spectral region located at the K-points of the Brillouin zone. Besides their promise for 2D opto-electronics, ML transition metal dichalcogenides (TMDCs) are interesting for exploiting both the spin and valley pseudospin of electrons and holes for potential applications in (quantum) information processing. The information may be encoded not only by whether an electron (or hole) has spin up or down, but also if it resides in the K+ or K- valley. Strong spin-orbit coupling separates the spin states and crystalline asymmetry leads to valley specific optical selection rules. Thus K+ or K- valleys can be selectively populated and probed using polarized light. The consortium LPCNO Toulouse - LU Hannover will extensively investigate the intrinsic timescales of spin and valley dynamics in ML TMDCs and the mechanisms governing the stability of spin and valley polarization. Both partners have already successfully conducted common research projects in the field of all optical spin noise spectroscopy (SNS). We focus on longer-lived excitations with stable spin and valley states and initialize spin and valley polarization optically for hole and electron doped samples. Doping is achieved chemically or in gated structures already operational at LPCNO. We will study and tune the spin dynamics by 3 different techniques: a) time resolved photoluminescence measurements will monitor the polarization dynamics and transfer during the radiative lifetime of the charged exciton (trion) including the role of spin-forbidden (dark) trions. b) By employing pump-probe measurements developed in Hannover we will probe the spin and valley polarization of resident carriers after recombination. c) For the first time we aim to perform SNS on TMDCs giving access to the long-term, intrinsic polarization dynamics of carriers with the key advantage of accessing the complete temporal dynamics at thermal equilibrium. The LUH group is a world leader in this field and will study the main depolarization mechanisms going from n- to p-type samples for varied carrier concentrations. High quality samples are available to the consortium with optical transition linewidth approaching the homogenous limit in encapsulated samples fabricated at LPCNO. We will study different alloys and materials with spin splittings varying in sign and magnitude. We will investigate the role of localization on the polarization dynamics by going from 2D confinement in MLs to 0D confinement on defects. We will measure the polarization and the charge transfer dynamics in vdW heterostructures. Here, we aim to switch between carrier recombination that is either direct or indirect in real- or momentum-space, by exploiting the band alignment of different TMDC materials.

• DFG Programme: Research Grants
• International Connection: France, Singapore, USA
• Cooperation Partners: Professor Dr. Goki Eda, Ph.D.; Professor Dr. Sefaattin Tongay, Ph.D.; Professor Dr. Bernhard Urbaszek