Novel type of orbital ordering: complex orbitals in doped Mott insulators

Abstract

An orbital ordering, often observed in Mott insulators with orbital degeneracy, is usually supposed to disappear with doping, e.g. in the ferromagnetic metallic phase of manganites. We propose that the orbital ordering of a novel type may exist in such situation: there may occur ferro orbital ordering of complex orbitals (linear superposition of basic orbitals d_x^2-y^2 and d_z^2 with complex coefficients). Despite the perfect orbital ordering, such state still retains cubic symmetry and thus would not induce any structural distortion. This novel state can resolve many problems in the physics of CMR manganites and can also exist in other doped Mott insulators with Jahn-Teller ions.