Two-photon driven magnon-pair resonance as a signature of spin-nematic order

Abstract

We theoretically study the nonlinear magnetic resonance driven by intense laser or electromagnetic wave in a fully polarized frustrated magnet near a less-visible spin-nematic ordered phase. In general, both magnons and magnon pairs (two-magnon bound state) appear as the low-energy excitation in the saturated state of spin-nematic magnets. Their excitation energies are usually in terahertz (THz) or gigahertz range. Magnon pairs with angular momentum 2$\hbar$ can be excited by the simultaneous absorption of two photons, and such multi-photon processes occur if the applied THz laser is strong enough. We compute laser-driven magnetic dynamics of a frustrated four-spin system with both magnon ($\hbar$) and magnon-pair (2$\hbar$) like excitations which is analogous to a macroscopic frustrated magnet with a spin nematic phase. We estimate the required strength of magnetic field of laser for the realization of two photon absorption, taking into account dissipation effects with the Lindblad equation. We show that intense THz laser with ac magnetic field of 0.1-1.0 Tesla is enough to observe magnon-pair resonance.