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Interfacing van der Waals materials allows manipulating the fundamental symmetries of electron states in solids with unprecedented precision using the individual layers as atomically precise, bottom-up building blocks. The resulting heterostructures have emerged as a versatile, electronically tunable solid-state platform for studying quantum phases with tailored electronic, optical, and spin properties. Here, I will discuss recent examples of intentionally and unintentionally broken symmetries in van der Waals heterointerfaces and their interrogation by non-linear optoelectronic transport [1]. In graphene-based heterostructures, we address gate-tunable, non-linear transport arising from proximity-induced spin-orbit coupling and breaking of the relevant crystal symmetries, which may be used to efficiently manipulate spin-polarized carriers by both optical and electrical means. In epitaxial graphene/Bi2Te2Se heterointerfaces with a commensurate lattice alignment, we find an enhanced helicity-dependent photocurrent, which originates from the peculiar spin-orbit proximity of the commensurate interface [1]. In graphene/WTe2 heterostructures, we demonstrate a current- induced Kerr rotation related to non-linear polarisation of the Fermi surface. We provide a theoretical model based on the non-linear anomalous Hall effect in the heterostructure, whose detection is enabled by the reduced symmetry of the incommensurate heterointerface [2]. Finally, I will outline the use of nanolithography to control the symmetry of van der Waals materials in a top-down approach. In particular, nanoscale superlattices may allow to enhance electronic correlations by flat band formation in a broad class of materials. Such correlated flat bands have non-trivial quantum geometric and topological properties that are addressable in suitable optoelectronic experiments with a resonant optical excitation. References [1] J. Kiemle, P. Zimmermann, A.W. Holleitner, C. Kastl, Light-field and spin-orbit-driven currents in van der Waals materials, Nanophotonics 2020, 9, 2693 - 2708. [2] J. Kiemle, L. Powalla, K. Polyudov, L. Gulati, M. Singh, A. W. Holleitner, M. Burghard, C. Kastl, Gate-Tunable Helical Currents in Commensurate Topological Insulator/Graphene Heterostructures, ACS Nano 2022, 16, 12338-12344. [3] L. Powalla, J. Kiemle, E. J. König, A. P. Schnyder, J. Knolle, K. Kern, A. W. Holleitner, C. Kastl, M. Burghard, Berry curvature-induced local spin polarisation in gated graphene/WTe2 heterostructures, Nat. Commun. 2022, 13, 3152.

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Christoph Kastl, WSI TU München
Seminar Room of the Institute of Physics II
Contact: Erwann/Matteo