Magnonic superradiant phase transition

Motoaki Bamba,Junichiro Kono,Xinwei Li,Nicolas Marquez Peraca

doi:10.1038/s42005-021-00785-z

Motoaki Bamba, Junichiro Kono + Show 2 more

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https://doi.org/10.1038/s42005-021-00785-z

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Abstract

In the superradiant phase transition (SRPT), coherent light and matter fields are expected to appear spontaneously in a coupled light–matter system in thermal equilibrium. However, such an equilibrium SRPT is forbidden in the case of charge-based light–matter coupling, known as no-go theorems. Here, we show that the low-temperature phase transition of ErFeO3 at a critical temperature of approximately 4 K is an equilibrium SRPT achieved through coupling between Fe3+ magnons and Er3+ spins. By verifying the efficacy of our spin model using realistic parameters evaluated via terahertz magnetospectroscopy and magnetization experiments, we demonstrate that the cooperative, ultrastrong magnon–spin coupling causes the phase transition. In contrast to prior studies on laser-driven non-equilibrium SRPTs in atomic systems, the magnonic SRPT in ErFeO3 occurs in thermal equilibrium in accordance with the originally envisioned SRPT, thereby yielding a unique ground state of a hybrid system in the ultrastrong coupling regime.

Highlights

In the superradiant phase transition (SRPT), coherent light and matter fields are expected to appear spontaneously in a coupled light–matter system in thermal equilibrium
Considering that the magnon excitation in this Fe3þ subsystem corresponds to the photon excitation in the electromagnetic vacuum, the rotation of the Fe3þ AFM vector indicates the spontaneous appearance of magnons, corresponding to the appearance of photons in the ordinary SRPT, in thermal equilibrium
In this study, using an ErFeO3 spin model phase diagrams obtained via magnetization reproducing both measurements[26] the and terahertz magnetospectroscopy results[31], we derived an extended

Summary

Introduction

In the superradiant phase transition (SRPT), coherent light and matter fields are expected to appear spontaneously in a coupled light–matter system in thermal equilibrium. 1234567890():,; In 1973, it was proposed[1,2] that photon and matter fields spontaneously appear in thermal equilibrium as a static transverse electromagnetic field and a static polarization, respectively, when the photon–matter coupling strength exceeds a certain threshold, entering the so-called ultrastrong coupling regime[3,4,5]. This phenomenon is known as the superradiant phase transition (SRPT) or Dicke phase transition, as the Dicke model was used in the theoretical calculations[1,2], having been originally developed to describe the superradiance phenomena[6]. The present work shows theoretically that the phase transition in erbium orthoferrite (ErFeO3) with a critical temperature Tc of ~4 K, known as the low-temperature phase transition (LTPT), is a magnonic SRPT, that is, an SRPT in which the Er3þ spins cooperatively couple with the Fe3þ magnonic field (spin-wave field) instead of with a photonic field as in the originally proposed

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