Long range magnetic order in two dimensions (2D) is prohibited by Mermin-Wagner theorem in isotropic Heisenberg model at finite temperatures. However, this limitation can be lifted by magnetic anisotropy and it has led to the recent discovery of 2D Ising ferromagnet in monolayer CrI3 [1] and many other new material platforms have been found since then. Strong interaction between spin, charge and lattice degrees of freedom, makes the identification of ground states, as well as the excitation spectra of these 2D magnets both complex and interesting. For example, researchers have found new so-called spin-orbit entangled excitons with extremely sharp linewidth [2], and in-gap electron-phonon bound states [3, 4]. This complexity calls for the development of novel spectroscopic tools, which can detect these bound states and identify their fermionic and bosonic components. In this talk, I will present our recent studies on 2D antiferromagnetic insulator NiPS3. By using a suite of ultrafast optical spectroscopies, we found a bound state formed by optical phonon and localized electronic states. These states are optically dark at high temperatures and become optically accessible below Néel temperature. These findings open new pathways to coherent control of electronic in-gap states, which are strongly coupled to magnons and phonons.
[1] B. Huang*, G. Clark*, E. Navarro-Moratalla*, D.R. Klein* et al. Nature 546, 270-273 (2017) [2] S. Kang*, K. Kim*, B. H. Kim*, J. Kim* et. al. Nature 583, 785-789 (2020)
[3] E. Ergecen*, B. Ilyas* et al. submitted (2021)
[4] K. Hwangbo*, Q. Zhang* et al. Nature Nanotechnology, 16, 655-660 (2021)
Note: this event will be hybrid. The speaker will be presenting via Zoom, but we will still meet in 276 for whoever wishes to participate in person. The Zoom link will be sent to the Graduate Student and PDRA mailing lists. If you are not on one of those lists and are interested in attending, please email Cat Kengle at ckengle2@illinois.edu for the link.
