# Interacting chains of orbital polarons in "Colossal" magnetoresistive La1-xSrxMnO3 revealed by spin and lattice dynamics

## TL;DR

This study investigates the spin and lattice dynamics of La1-xSrxMnO3 to understand the origin of colossal magnetoresistance, revealing orbital polarons and their chains as key features influencing magnetic and electronic properties.

## Contribution

It introduces a new interpretation of spin dynamics in terms of orbital polarons and identifies their chain structures as crucial for CMR in La1-xSrxMnO3.

## Key findings

- Observation of discrete magnetic energy spectra indicating orbital polarons
- Identification of 2D and 3D orbital polarons at different doping levels
- Evidence for chains of orbital polarons influencing magnetic and transport properties

## Abstract

The origin of the effect of colossal magneto-resistance (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudo-cubic La1-xSrxMnO3 along the Mn-O-Mn bond direction at four x doping values (x < 0.5) and various temperatures and report a new lattice dynamics study at x0=0.2, representative of the optimal doping for CMR. We propose an interpretation of the spin dynamics in terms of orbital polarons. This picture is supported by the observation of a discrete magnetic energy spectrum Enmag (q) characteristic of the internal excitations of "orbital polarons" defined by Mn3+ neighbors surrounding a Mn4+ center with a hole. Because of its hopping, the hole mixes up dynamically all the possible orbital configurations of its surrounding Mn3+ sites with degenerate energies. The Enmag values indicate a lift of orbital degeneracy by phonon excitations. The number n varies with the spatial dimension D of the polaron and the q-range determines its size. At x=0.125 and x=0.3 the spectrum reveals 2D polarons coupled by exchange and 3D "free" polarons respectively, with sizes l=1.67a < 2a in all bond directions. At x0=0.2, the spin and the lattice dynamics provide evidence for chains of orbital polarons of size l=2a with a periodic distribution over ~ 3a and an interaction energy ~ 3 meV. At T < Tc the charges propagate together with the longitudinal acoustic phonons along the chains enhancing their ferromagnetic character. The phase separation between metallic and ferromagnetic chains in a non-metallic matrix may be crucial for CMR.

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Source: https://tomesphere.com/paper/1705.01476