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.

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.