  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.

Strongly Correlated Electrons;Materials Science