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

TL;DR

This study investigates how local atomic structures and lattice dynamics evolve in La(1-x)Sr(x)CoO3 as it transitions from an insulator to a ferromagnetic metal, revealing the role of Jahn-Teller distortions and dynamical fluctuations.

Contribution

It provides new insights into the interplay between static and dynamic Jahn-Teller distortions across the insulator-metal transition in cobaltates.

Findings

Static Jahn-Teller distortions are present in the insulating phase.

Dynamical Jahn-Teller fluctuations occur in the ferromagnetic metallic phase.

Static distortions reappear at higher Sr doping levels.

Abstract

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.