Persistent quantum vibronic dynamics in a 5d¹ double perovskite oxide

Quantum entanglement between the spin, orbital, and lattice degrees of freedom in condensed matter systems can emerge due to an interplay between spin-orbit and vibronic interactions. Heavy transition metal ions decorated on a face-centered cubic lattice, for example, in 5d¹ double perovskites, are particularly suited to support these quantum entangled states, whereas direct evidence has not yet been presented. In this work, we report additional peaks in the low-energy spectra of a 5d¹ double perovskite, Ba₂CaReO₆, which cannot be explained by adopting a purely classical description of lattice vibrations. Instead, our theoretical analysis demonstrates that these spectroscopic signatures are characteristic of orbital-lattice entangled states in Ba₂CaReO₆. Crucially, both theory and experiment demonstrate that these quantum-entangled states persist to low temperatures, despite the onset of multipolar order.