Novel Orbital Ordering Induced By Anisotropic Stress In A Manganite Thin Film

A novel structure of orbital ordering is found in a Nd0.5Sr0.5MnO3 thin film, which exhibits a clear first-order transition, by synchrotron x-ray diffraction measurements. Lattice parameters vary drastically at the metal-insulator transition at 170 K (=T-MI), and superlattice reflections appear below 140 K (=T-CO). The electronic structure between T-MI and T-CO is identified as A-type antiferromagnetic with a d(x2-y2) ferro-orbital ordering. The new type of antiferro-orbital ordering characterized by the wave vector (1/4 1/4 1/2) in cubic notation emerges below T-CO. The accommodation of the large lattice distortion at the first-order phase transition and the appearance of the novel orbital ordering are brought about by the anisotropy in the substrate, a new parameter for the phase control.