Dynamical Disorder of Spin-Induced Jahn-Teller Orbitals with the Insulator-Metal Transition in Cobaltites

Using elastic and inelastic neutron scattering, we investigated the evolution of the local atomic structure and lattice dynamics of La_1-xSr_xCoO₃ (x=0–0.5) as it crosses over with x from an insulator to a ferromagnetic metal (FMM). Our pair density function analysis indicates that, in the paramagnetic insulating phase for all x, spin activation of Co³⁺ ions induces local static Jahn-Teller (JT) distortions. The size of the JT lattice increases almost linearly with x. However, in the FMM phase, static JT distortions are absent for x≤30%. This coincides with narrowing of ℏω=22 and 24 meV modes in the phonon spectrum which we argue is due to localized dynamical JT fluctuations. For x>30%, static JT distortions reappear along with broadening of the phonon modes because of weakened charge-lattice interactions.