A team of researchers has observed the magnonic Dicke superradiant phase transition (SRPT) in ErFeO₃, a material where the role of photonic modes in the traditional SRPT is replaced by magnon modes. In the traditional SRPT, two-level atoms coupled with single-mode cavity photons exhibit a quantum phase transition when the coupling strength exceeds a critical value. However, this phenomenon is forbidden by a no-go theorem due to the presence of a diamagnetic term in the Hamiltonian. In ErFeO₃, the absence of this diamagnetic term in the Fe³⁺-Er³⁺ exchange coupling allows the SRPT to occur. The researchers used terahertz and gigahertz magnetospectroscopy to observe the signatures of the SRPT, including a kink and a softening of two spin-magnon hybridized modes at the critical point. The study demonstrates the magnonic SRPT in thermal equilibrium, providing insights into novel quantum vacuum phenomena and potential applications in quantum information science. The findings highlight the importance of the Dicke model in understanding and predicting quantum phase transitions in magnetic systems. The research also emphasizes the role of magnetic fields in tuning the critical parameters for the SRPT, and the potential for further exploration of quantum vacuum phenomena in the superradiant phase. The study provides a framework for understanding and controlling condensed matter phases through powerful concepts from quantum electrodynamics.A team of researchers has observed the magnonic Dicke superradiant phase transition (SRPT) in ErFeO₃, a material where the role of photonic modes in the traditional SRPT is replaced by magnon modes. In the traditional SRPT, two-level atoms coupled with single-mode cavity photons exhibit a quantum phase transition when the coupling strength exceeds a critical value. However, this phenomenon is forbidden by a no-go theorem due to the presence of a diamagnetic term in the Hamiltonian. In ErFeO₃, the absence of this diamagnetic term in the Fe³⁺-Er³⁺ exchange coupling allows the SRPT to occur. The researchers used terahertz and gigahertz magnetospectroscopy to observe the signatures of the SRPT, including a kink and a softening of two spin-magnon hybridized modes at the critical point. The study demonstrates the magnonic SRPT in thermal equilibrium, providing insights into novel quantum vacuum phenomena and potential applications in quantum information science. The findings highlight the importance of the Dicke model in understanding and predicting quantum phase transitions in magnetic systems. The research also emphasizes the role of magnetic fields in tuning the critical parameters for the SRPT, and the potential for further exploration of quantum vacuum phenomena in the superradiant phase. The study provides a framework for understanding and controlling condensed matter phases through powerful concepts from quantum electrodynamics.