Polaronic orbital polarization in a layered colossal magnetoresistive manganite

The striking anisotropy observed in Huang scattering distributions near the intense Bragg reflections of La_1.2Sr_1.8Mn₂O₇ 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., 3z²-r²) 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., 3x²-r² and 3y²-r² orbitals), allowing the formation of nanoscale polaron correlations above T_C.