A theory is developed to explain various types of electronic collective behaviors in doped manganites $R_{1-x}{X}_x{\mathrm{MnO}}_3(R=\mathrm{La},$ Pr, Nd, etc. and $X=\mathrm{Ca},$ Sr, Ba, etc.). Starting from a realistic electronic model, we derive an effective Hamiltonian by utilizing the projection perturbation techniques and develop a spin-charge-orbital coherent-state theory, in which the Jahn-Teller effect and the orbital degeneracy of $e_g$ electrons in Mn ions are taken into account. Physically, the experimentally observed charge-ordering state and electronic phase separation are two macroscopic quantum phenomena with opposite physical mechanisms, and their physical origins are elucidated in this theory. The interplay of the Jahn-Teller effect, the lattice distortion, as well as the double-exchange mechanism leads to different magnetic structures and to different charge-ordering patterns and phase separation.

• Received 10 June 1999

DOI:https://doi.org/10.1103/PhysRevB.61.9532