Tomasz Smolenski

---

Understanding and controlling strongly correlated many-body systems is one of the central challenges in modern condensed matter physics. Among the most promising experimental platforms for exploring this frontier are van der Waals (vdW) heterostructures based on transition metal dichalcogenide (TMD) monolayers. Owing to their gate tunability, weak dielectric screening, and low carrier effective masses, TMD monolayers readily host correlated electronic phases such as Wigner crystals. The stability of such phases can be further enhanced when two TMD monolayers are combined into a twisted bilayer, where the electronic kinetic energy is further suppressed due to band folding in the periodic moire potential. In all of these structures, the excellent optical properties of TMD monolayers, along with their spin-valley-selective selection rules, provide a unique optical interface to both the charge and spin state of the many-body electron system.

In this talk, I will review our recent ultra-low-temperature magneto-optical investigations of collective electronic phases in TMD-based vdW structures. In the first part, I will demonstrate the formation of novel optical excitations in a TMD monolayer hosting a robust Wigner crystal. These excitations coexist with previously explored exciton-umklapp resonances [1] but, unlike them, arise due to strong attractive exciton-electron interactions that dress the excitons with particle-hole excitations across the many-body Wigner crystal gap. In the second part, I will focus on AA-stacked MoTe2 homobilayers, where strong interlayer hybridization of hole orbitals gives rise to flat topological valence bands supporting robust ferromagnetic metals as well as fractional and integer Chern insulators at various moire filling factors. I will show that the spin state of all these topological ferromagnets can be dynamically reversed by resonantly driving exciton-polaron resonances in the optical absorption spectrum with circularly polarized light [2]. This includes both integer and fractional Chern insulating phases, for which the spin orientation is equivalent to all-optical switching of their many-body Chern number. I will demonstrate that by illuminating the sample with diffraction-limited spot, it is possible to induce such a Chern number flip in selected sample area, which paves the way for optical generation of programmable topological circuits.

[1] T. Smoleński et al., Nature 595, 53–57 (2021)
[2] O. Huber, (…), T. Smoleński, arXiv:2508.19063 (2025)

---

Uni Basel
PH2
Contact: Urban Seifert