Theory of Nonlinear Spectroscopy of Quantum Magnets

TL;DR

This paper develops a response theory for 2D coherent spectroscopy in quantum magnets, revealing how electric and magnetic couplings influence the spectroscopy signals, and proposes experimental methods to identify different coupling mechanisms.

Contribution

It introduces a comprehensive response theory for 2DCS in quantum magnets, including magneto-electric and polarization effects, and suggests experimental protocols to distinguish coupling contributions.

Findings

Response theory incorporating magneto-electric and polarization couplings.

Example calculations for CoNb2O6 demonstrating the theory.

Identification of cross-coupling effects between polarization and magnetic susceptibilities.

Abstract

Two-dimensional coherent spectroscopy (2DCS) is an established method for characterizing molecules and has been proposed in the THz regime as a new 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 magneto-electric as well as polarization couplings contribute to 2DCS in addition to the typically assumed magnetization. We propose experimental protocols to distill individual contributions, for instance from exchange-striction or spin current mechanism, when the electric field couples to terms quadratic in spin operators. We provide example calculations for the paradigmatic twisted Kitaev chain material $\mathrm{CoNb}_{2}\mathrm{O}_{6}$ 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.