Polaronic Orbital Polarization In A Layered Colossal Magnetoresistive Manganite

The striking anisotropy observed in Huang scattering distributions near the intense Bragg reflections of La1.2Sr1.8Mn2O7 is shown to be the result of orbitally polarized polarons in the paramagnetic insulating state above T-C. X-ray single-crystal diffuse scattering patterns from this bilayered colossal magnetoresistive manganite are calculated in terms of the polaronic local structure and compared with experimental measurements. At 300 K, the polaronic e(g) electrons occupy "out-of-plane" (i.e., 3z(2)-r(2)) orbitals, leading to MnO6 octahedra that are Jahn-Teller elongated along the c axis, perpendicular to the perovskite layers. Between 300 K and T-C, however, the "orbital polarization" is shown to shift into the a-b plane (i.e., 3x(2)-r(2) and 3y(2)-r(2) orbitals), allowing the formation of nanoscale polaron correlations above T-C.