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

Neutron scattering measurements on a magnetoresistive manganite ${\mathrm{La}}_{0.75}{({\mathrm{Ca}}_{0.45}{\mathrm{Sr}}_{0.55})}_{0.25}{\mathrm{MnO}}_{3}$ show that uncorrelated dynamic polaronic latice distortions are present in both the orthorhombic $(O)$ and rhombohedral $(R)$ paramagnetic phases. The uncorrelated distortions do not exhibit any significant anomaly at the $O\text{\char21{}}R$ 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\char21{}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 correlated lattice distortions are primarily responsible for the insulating character of the paramagnetic state in magnetoresistive manganites.