Uncorrelated and correlated nanoscale lattice distortions in the paramagnetic phase of magnetoresistive manganites

TL;DR

This study reveals that uncorrelated lattice distortions are present in both paramagnetic phases of manganites, but correlated distortions only occur in the orthorhombic phase and are linked to insulating behavior.

Contribution

It provides new insights into the different roles of uncorrelated and correlated lattice distortions in the paramagnetic phases of magnetoresistive manganites.

Findings

Uncorrelated distortions are present in both phases without anomaly at the transition.

Correlated polaronic distortions are only in the orthorhombic phase.

Correlated distortions are likely responsible for insulating behavior.

Abstract

Neutron scattering measurements on a magnetoresistive manganite La$_{0.75}$(Ca$_{0.45}$Sr$_{0.55}$)$_{0.25}$MnO$_3$ show that uncorrelated dynamic polaronic lattice distortions are present in both the orthorhombic (O) and rhombohedral (R) paramagnetic phases. The uncorrelated distortions do not exhibit any significant anomaly at the O-to-R transition. Thus, both the paramagnetic phases are inhomogeneous on the nanometer scale, as confirmed further by strong damping of the acoustic phonons and by the anomalous Debye-Waller factors in these phases. In contrast, recent x-ray measurements and our neutron data show that polaronic correlations are present only in the O phase. In optimally doped manganites, the R phase is metallic, while the O paramagnetic state is insulating (or semiconducting). These measurements therefore strongly suggest that the {\it correlated} lattice distortions are primarily responsible for the insulating character of the paramagnetic state in magnetoresistive manganites.