Two-photon magnetic resonance as detector of magnon pairs and spin nematic orders*

Spin-nematic (quadrupolar) order is a typical less-visible order in magnetic systems. Proposing an efficient experimental way of detecting spin-nematic (SN) order is important to deepen the understanding of spin nematicity and frustrated magnetism. Several theoretical works have shown that in forced fully polarized states of SN ordered magnets, magnon pairs (molecule of two magnons) generally appear in addition to usual magnons. Moreover, a SN order is usually shown to occur as the result of the Bose-Einstein condensation of magnon pairs in a broad class of SN magnets. Therefore, the detection of magnon pairs can provide an indirect but strong evidence for the emergence of SN order. Since magnon pairs have angular momentum 2hbar, they can be excited through two-photon absorption with strong electromagnetic (EM) waves. Based on these backgrounds, we theoretically consider the magnon-pair resonance driven by strong EM wave in a fully polarized state of SN magnets. Using the quantum master equation for a nano spin model, we predict that magnon-pair resonance peak can be detected with currently available terahertz laser. I explain the essential aspects of our proposal.