Tomasz Smolenski1
ETH Zurich1
When the Coulomb repulsion between itinerant electrons dominates over their kinetic energy, the electrons in solid state materials start to develop strong correlations. A paradigm state of matter that is expected to emerge in this regime is an electronic Wigner crystal, in which the electrons spontaneously break continuous translation symmetry and arrange themselves into a periodic lattice mimicking that of the real crystals. In this talk, I will review our recent experimental explorations of strongly correlated electronic phases in transition metal dichalcogenides (TMD) and their van der Waals heterostructures. In particular, I will present our novel spectroscopic technique allowing us to detect the Wigner crystal in a TMD monolayer through the periodic potential it generates for the excitons [1]. This potential allows the excitons to undergo a Bragg diffraction, which gives rise to a new, Bragg-umklapp transition in the optical excitation spectrum that heralds the presence of a crystalline electronic order.<br/><br/>In the second part of the talk, I will describe magnetic properties of the electrons forming a charge-ordered Mott insulating state in a MoSe2/WS2 bilayer that hosts strong moire potential. In particular, I will show that when such a Mott state is electron-doped, the system exhibits unusual ferromagnetism [2] that is driven not by Coulomb exchange interactions, but arises due to minimization of kinetic energy of itinerant electrons. This observation provides direct evidence for the Nagaoka ferromagnetism in an extended two-dimensional system.<br/><br/>[1] T. Smolenski, P. E. Dolgirev, C. Kuhlenkamp, A. Popert, Y. Shimazaki, P. Back, X. Lu, M. Kroner, K. Watanabe, T. Taniguchi, I. Esterlis, E. Demler, and A. Imamoglu, <i>Signatures of Wigner crystal of electrons in a monolayer semiconductor</i>. Nature <b>595</b>, 53-57 (2021).<br/>[2] L. Ciorciaro, T. Smolenski, I. Morera, N. Kiper, S. Hiestand, M. Kroner, Y. Zhang, K. Watanabe, T. Taniguchi, E. Demler, A. Imamoglu, <i>Kinetic Magnetism in Triangular Moire Materials</i>. arXiv:2305.02150 (2023).