Coherent dynamics of collective excitations and Floquet-Bloch manipulation in correlated topological materials

Understanding the emergence of collective excitations in many-body interacting systems has been a crosscutting theme throughout many branches of physics. Developing methods to uncover such excitations in the presence of both strong correlations and topology is a major research effort in modern condensed matter. In this talk, I will demonstrate how femtosecond light-matter interaction can reveal previously unseen emergent phenomena in two purported correlated topological materials – (TaSe₄)₂I and MnBi₂Te₄. In the case of (TaSe₄)₂I, a Weyl charge density wave (CDW) candidate, I will show how we used ultrafast THz emission spectroscopy to discover the existence of a phason that acquires mass by coupling to long-range Coulomb interactions. I will briefly discuss this result in the context of the predicted axion electrodynamics in a Weyl-CDW. For MnBi₂Te₄, a topological antiferromagnet, I will show how we used time-and-angle resolved photoemission spectroscopy (tr-ARPES) to dynamically engineer ‘Floquet-Bloch’ electronic states. This manipulation ultimately revealed the hidden Dirac gap in MnBi₂Te₄. Such studies highlight the usefulness of ultrafast, non-equilibrium spectroscopies to uncover the fundamental physics of correlated topological materials.