Observation of the Magnonic Dicke Superradiant Phase Transition

Abstract

Two-level atoms coupled with single-mode cavity photons are predicted to exhibit a quantum phase transition when the coupling strength exceeds a critical value, entering a phase in which atomic polarization and photonic field are finite even at zero temperature and 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 in the Hamiltonian. 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. Terahertz and gigahertz magnetospectroscopy experiments revealed the signatures of the SRPT -- a kink and a softening, respectively, of two spin-magnon hybridized modes at the critical point.

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• Multimode Phonon-Polaritons in Lead-Halide Perovskites in the Ultrastrong Coupling Regime
• Gate-tunable single terahertz meta-atom ultrastrong light-matter coupling
• Amplified response of cavity-coupled quantum-critical systems
• Magnetic excitation spectrum and hierarchy of magnetic interactions in ErFeO3
• Controlled bit-flip of period-doubling and discrete time crystalline states in open systems
• Superradiant Quantum Phase Transition in Open Systems: System-Bath Interaction at the Critical Point
• Beyond mean-field dynamics of the Dicke model with non-Markovian dephasing