Phase separation and charge ordering in doped manganite perovskites: Projection perturbation and mean-field approaches

A theory is developed to explain various types of electronic collective behaviors in doped maganites R1-xXxMnO3 (R = La, Pr, Nd, etc. and X = 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 stale 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.