Theory of nonlinear spectroscopy of quantum magnets

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Abstract
Two-dimensional coherent spectroscopy (2DCS) is an established method for characterizing molecules and has been proposed in the terahertz regime as a tool for probing exotic excitations of quantum magnets; however, the precise nature of the coupling between pump field and spin degrees of freedom has remained unclear. Here, we develop a general response theory of 2DCS and show how magnetoelectric as well as polarization couplings contribute to 2DCS in addition to the typically assumed magnetization. We propose experimental protocols to probe the presence of polarization couplings, for instance, from exchange-striction or spin-current mechanism, when the electric field couples to quadratic terms in spin operators. We provide example calculations for the paradigmatic twisted Kitaev chain material CoNb 2 O 6 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.
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Funding Information
  • Deutsche Forschungsgemeinschaft (KN1254/1-2)
  • Deutsche Forschungsgemeinschaft (KN1254/2-1)
  • European Research Council
  • Horizon 2020 (851161)
  • European Commission (101169765)
  • National Science Foundation (PHY-2210452)
  • Deutscher Akademischer Austauschdienst France
  • National Research Foundation of Korea (RS-2024-00453943)