A lightly doped perovskite mangantite La_{0.88}Sr_{0.12}MnO_3 exhibits a phase transition at T_{OO}=145 K from a ferromagnetic metal (T_C=172 K) to a novel ferromagnetic insulator.We identify that the key parameter in the transition is the orbital degree of freedom in e_g electrons. By utilizing the resonant x-ray scattering technique, orbital ordering is directly detected below T_{OO}, in spite of a significant diminution of the cooperative Jahn-Teller distortion. The new experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present experimental and theoretical studies uncover a crucial role of the orbital degree in the metal-insulator transition in lightly doped manganites.
@article{arxiv.cond-mat/9901148,
title = {Transition between two ferromagnetic states driven by orbital ordering in La_{0.88}Sr_{0.12}MnO_3},
author = {Y. Endoh and K. Hirota and S. Ishihara and S. Okamoto and Y. Murakami and A. Nishizawa and T. Fukuda and H. Kimura and H. Nojiri and K. Kaneko and S. Maekawa},
journal= {arXiv preprint arXiv:cond-mat/9901148},
year = {2019}
}