Magneto-optical spectroscopy of semiconducting transition metal dichalcogenides

• Applicant: Ashish Arora, Ph.D.
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 336082288

Recently, atomically thin layered crystals of semiconducting transition metal dichalcogenides (TMDCs) of the type MX2 (M = Mo, W, ...; X = S, Se, Te) have gained considerable attention because of their extraordinary physical properties. They have large exciton binding energies in the range of tens of meV, a strong spin-orbit splitting of hundreds of meV and exhibit striking effects, such as valley polarization and valley coherence. In 2015, first magneto-optical measurements on these materials have been performed on monolayers and at cryogenic temperatures using unmodulated spectroscopy techniques. Goal of the project is to gain a comprehensive understanding of the unique magneto-optical properties of atomically thin TMDC semiconductors. We will measure the Zeeman splitting of excitons in thin films of TMDCs under magnetic fields up to 2 T and as a function of temperature (10 K - 300 K) with sensitive modulation techniques with about two orders of magnitude better accuracy than the conventionally employed un-modulated magneto-optical methods. We will investigate the magnetic-field-induced valley polarization, which will shed light on the intervalley dynamics in the materials. We will also perform magneto-optical investigations on TMDC films thicker than a monolayer, i.e. bilayers, trilayers, quadruple layers, and up to the bulk limit, for the first time. In that way, we will elucidate the role of quantum confinement and dielectric screening on g-factors of excitons in TMDCs, compared to conventional semiconductor quantum wells. In contrast to W- and Mo-based TMDCs, our magneto-optical experiments on Re-based materials (ReS2 and ReSe2) are expected to yield fundamentally different thickness-dependent results owing to the absence of the indirect to direct band gap transition, which is prominent for Mo or W-based TMDCs. Furthermore, we will measure the g-factor of excitons at temperatures ranging from T = 10 K up to room temperature. In summary, our investigations with elucidate the physical processes, which govern the magneto-optical response in the new class of atomically thin semiconductors. These results are vital to obtain a correct theoretical description with k.p theory and for future real-world valley-based devices, where a low magnetic field is envisioned to be used for the manipulation of the valley degree of freedom.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Dr. Marek Potemski
• Co-Investigator: Privatdozent Dr. Thorsten Deilmann