Observation of Dicke cooperativity between strongly coupled phonons and crystal-field excitations in a rare-earth orthoferrite

Abstract: Collective interactions between localized electronic excitations and the crystal lattice are central to many emergent phenomena in materials, including ferroelectricity and quantum magnetism. Despite its importance, the scaling behavior of such cooperativity remains largely unexplored. Here, we report the direct observation of Dicke-type cooperativity arising from the coupled phonons and non-degenerate crystal-field transitions (CFE), namely a pseudo-Jahn-Teller effect1, in the rare-earth orthoferrite, ErFeO3. Using magneto-Raman spectroscopy, we uncover strongly coupled spin, lattice, and orbital excitations. By varying the temperature, we identify the phonon-CFE coupling strength scales as sqrt(N), where N represents the effective ground-state population, which is a hallmark of Dicke cooperativity. Our findings verified the cooperative nature of the interaction between local Jahn-Teller ions and long-range phonons, offering a pathway for tailoring electronic and vibrational properties of materials through population control.