Observation of the Magnonic Dicke Superradiant Phase Transition

TL;DR

This paper reports spectroscopic evidence of a magnonic superradiant phase transition in ErFeO₃, demonstrating a quantum phase transition driven by magnon-photon coupling that circumvents the traditional no-go theorem.

Contribution

It provides experimental observation of a magnonic SRPT in a solid-state system, where the absence of the diamagnetic term allows the phase transition to occur.

Findings

Detection of a kink in hybridized modes at the critical point

Observation of mode softening indicating phase transition

Evidence of a superradiant phase transition in a magnonic system

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.