Observation of the magnonic Dicke superradiant phase transition
Dasom Kim, Sohail Dasgupta, Xiaoxuan Ma, J.-M. Park, Haotian Wei, Xinwei Li, Liang Luo, Jacques Doumani, Wanting Yang, Di Cheng, Richard H. J. Kim, Henry O. Everitt, Shojiro Kimura, Hiroyuki Nojiri, Jigang Wang, Shixun Cao, Motoaki Bamba, Kaden R. A. Hazzard, Junichiro Kono
Abstract
Two-level atoms ultrastrongly coupled with single-mode cavity photons are predicted to exhibit a quantum phase transition, entering a phase in which both the atomic polarization and the photonic field are finite even without external driving. However, this phenomenon, the superradiant phase transition (SRPT), is forbidden by a no-go theorem due to the existence of the diamagnetic term. Here, we present spectroscopic evidence for a magnonic SRPT in ErFeO 3 , where the role of the photonic mode (two-level atoms) in the photonic SRPT is played by an Fe 3+ magnon mode (Er 3+ spins). The absence of the diamagnetic term in the Fe 3+ -Er 3+ exchange coupling ensures that the no-go theorem does not apply. Ultrabroadband terahertz and gigahertz magnetospectroscopy experiments revealed the signatures of the SRPT in thermal equilibrium, a kink and a softening, respectively, of two spin-magnon hybridized modes at the critical point. Systems near this phase are expected to harbor large-scale squeezing, which will potentially provide a route to next-generation quantum technologies.