Orbital polarization in manganese oxides

Abstract

We study the role of orbital degrees of freedom in perovskite manganites, which are well known as colossal-magnetoresistance (CMR) compounds. The double degeneracy of the $e_g$ orbitals is treated with isospin. Orbital polarization is essential to explain the variety of phases in these compounds and their physical properties. Especially important is the fact that the orbital is aligned as $d_{x^2-y^2}$ in the metallic layered antiferromagnetic state, which explains the experimentally observed quasi-two-dimensional transport and absence of spin canting. A large orbital fluctuation in the ferromagnetic state is also predicted.