Patrick Knüppel : Excitons as Probes of Correlations and Magnetism in 2D Semiconductors

The field of two-dimensional materials offers unprecedented opportunities to engineer nanoscale magnetic, electronic, and optical devices. Stacking and twisting of atomically thin layers has become a powerful paradigm for realizing exotic phases of matter. Transition metal dichalcogenide (TMD) moiré heterostructures are particularly promising because they combine strong Coulomb interactions with robust excitons and tunable band topology. At small twist angles, they may host a topological flat band that closely resembles the lowest Landau level but without an external magnetic field. In this talk, I will show how cryogenic optical spectroscopy and tailored device architectures reveal ferromagnetism, correlated insulators, and fractional Chern states in twisted homobilayer TMDs. I will then turn to CrSBr, a layered magnetic semiconductor where excitons also provide a sensitive optical probe of magnetic order. In recent work, we show that magnetic domain walls act as magnetostatic traps for excitons. This enables optical imaging of magnetic domain walls in an antiferromagnet. Looking ahead, this approach could provide a route toward optical readout and control of domain-wall dynamics.

References:

1.     Zeng, Y. et al. Thermodynamic evidence of fractional Chern insulator in moiré MoTe2. Nature622, 69 (2023). https://doi.org/10.1038/s41586-023-06452-3

2.     Knüppel, P. et al. Correlated states controlled by a tunable van Hove singularity in moiré WSe2 bilayers. Nat. Commun. 16, 1959 (2025). https://doi.org/10.1038/s41467-025-57235-5

Biography

Patrick Knüppel obtained his PhD at ETH Zürich, where he explored the interplay of strongly correlated electrons and light in gallium arsenide quantum wells embedded in optical cavities. As a Postdoctoral Fellow at Cornell University, he investigated emergent phases in twisted two-dimensional semiconductors. He is now a Postdoctoral Researcher at the University of Basel, developing scanning-probe instrumentation for the study of 2D magnetic and moiré materials.