Magnonic Superradiant Phase Transition

TL;DR

This paper demonstrates that ErFeO3 undergoes a magnonic superradiant phase transition at low temperatures, driven by ultrastrong coupling between Er spins and Fe magnons, marking a thermal equilibrium SRPT.

Contribution

It models the phase transition in ErFeO3 as a magnonic superradiant phase transition using an extended Dicke model with realistic parameters.

Findings

Identifies ErFeO3 as a system exhibiting equilibrium SRPT.

Shows ultrastrong coupling causes the phase transition.

Uses terahertz magnetospectroscopy data for validation.

Abstract

We show that the low-temperature phase transition in ErFeO3 that occurs at a critical temperature of ~ 4 K can be described as a magnonic version of the superradiant phase transition (SRPT). The role of photons in the quantum-optical SRPT is played by Fe magnons, while that of two-level atoms is played by Er spins. Our spin model, which is reduced to an extended Dicke model, takes into account the short-range, direct exchange interactions between Er spins in addition to the long-range Er-Er interactions mediated by Fe magnons. By using realistic parameters determined by recent terahertz magnetospectroscopy and magnetization experiments, we demonstrate that it is the cooperative, ultrastrong coupling between Er spins and Fe magnons that causes the phase transition. This work thus proves ErFeO3 to be a unique system that exhibits a SRPT in thermal equilibrium, in contrast to previous observations of laser-driven non-equilibrium SRPTs.