Regensburg 2025 – scientific programme
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MA: Fachverband Magnetismus
MA 24: Focus Session: Nonlinear Spectroscopy of Collective Excitations in Quantum Magnets (joint session TT/MA)
MA 24.5: Topical Talk
Wednesday, March 19, 2025, 11:45–12:15, H36
Theory of Nonlinear Spectroscopy of Quantum Magnets — Anubhav Srivastava1, 2, •Stefan Birnkammer1, GiBaik Sim3, Michael Knap1, and Johannes Knolle1, 4 — 1Technical University of Munich, Garching, Germany — 2Indian Institute of Science, Bengaluru, India — 3Hanyang University, Seoul, Korea — 4Imperial College London, London, United Kingdom
Two-dimensional coherent spectroscopy (2DCS) is an established method for probing molecules and has been proposed in the THz regime as a new tool for probing exotic excitations of quantum magnets but the precise nature of coupling between pump field and spin degrees of freedom has remained unclear. Here, we develop a general response theory of 2DCS and show how magneto-electric as well as polarization couplings contribute to 2DCS in addition to the standardly assumed magnetization. We propose experimental protocols to distill individual coupling contributions, for example from exchange-striction or spin current mechanism. We provide example calculations for the paradigmatic twisted Kitaev chain material CoNb2O6 and highlight the crucial role of contributions from cross-coupling between polarization and magnetic nonlinear susceptibilities. Our work paves the way for systematic studies of light-matter couplings in quantum magnets and for establishing 2DCS as a versatile tool for probing fractional excitations of exotic magnetic quantum phases.
Keywords: Two-dimensional Coherent Spectroscopy; Quantum Magnets