Superradiant Phase Transition—Bridging Quantum Optics, Thermodynamics, and Condensed Matter Physics

In 1973, physicists theoretically predicted a phenomenon called the “superradiant phase transition” (SRPT), which is caused by a particular type of light–matter interaction giving the so-called Dicke cooperativity. At high temperature (in normal phase), we find incoherent (random) electromagnetic polarization (or electric current) in matters, which is caused by thermal motion of particles, and thus incoherent electromagnetic wave (thermal radiation) is also obtained. By decreasing the temperature, we usually find that the intensity of the thermal radiation is simply decreased. In contrast, when the SRPT occurs, at temperature below the critical temperature (in superradiant phase), we can find a spontaneous appearance of a coherent static (i.e., temporally non-oscillating) electromagnetic field and a coherent static electromagnetic polarization (or coherent persistent current) in matter. Such a superradiant phase is stabilized owing to ultrastrong light-matter coupling. Unfortunately, there have been no experimental demonstrations of such an SRPT yet.

However, this phenomenon is not exclusive to light–matter interactions; it can also appear in strongly coupled matter–matter systems. In our previous study, which was published in Science in 2018, we experimentally observed “magnonic” Dicke cooperativity in a compound called erbium orthoferrite (ErFeO3). We took light out of the system Hamiltonian (system energy) and mimicked it with spin waves (magnons) on the iron (Fe) atoms. In this case, the electron paramagnetic resonance of the erbium (Er) spins was the electromagnetic transition of the atoms in the SRPT story.

Because the magnonic Dicke cooperativity we observed was somewhat controversial as evidence of the magnonic SRPT, we recently conducted a theoretical analysis based on our results for the ErFeO3 system. In our latest study, we proved that the ultrastrong coupling between Fe magnons and Er spins was responsible for a phase transition of ErFeO3, whereas Er–Er interactions also contribute to it partially.

Our findings mark the first time that magnonic SRPT was confirmed in a physical system. They pave the way for understanding and hopefully realizing the photonic SRPT, which represents a key phenomenon that bridges quantum optics, thermodynamics, and solid-state physics. It might be leveraged for noise-robust quantum technologies and for the generation of coherent light (like lasers) directly from heat, helping us save large amounts of energy.

Motoaki Bamba, Xinwei Li, Nicolas Marquez Peraca, and Junichiro Kono
“Magnonic Superradiant Phase Transition”
Communications Physics 5, 3 (2022)