Dynamical disorder of spin-induced Jahn-Teller orbitals with the insulator-metal transition in cobaltates

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Using elastic and inelastic neutron scattering, we investigated the evolution of the local atomic structure and lattice dynamics of La(1-x)Sr(x)CoO3 (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 (PMI) 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 hw=22 and 24 meV modes in the phonon spectrum in 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.