Interplay of spin, orbital, and charge degrees of freedom in half-doped manganites

Energy-band-structure calculations of the ground-state orbital and charge orders for various magnetic structures have been performed in the tight-binding approximation for manganites of the La0.5Ca0.5MnO3 type with 16 Mn ions in the unit cell. The arising inhomogeneous charge distribution was accounted for self-consistently by assuming a linear dependence of Jahn-Teller (JT) splitting of the e(g) level on its occupation for each of the ions. This dependence promotes the phases with charge separation and the charge degree of freedom becomes an essential factor responsible for stabilization of such phases as a ground state. Among them are the CE structure and also a peculiar ferrimagnetic structure with magnetization twice as low as that for the saturated ferromagnet. The existence and the width of the gap in the electron spectrum of the CE phase depend on the relation between Hund exchange and JT splitting, so that the JT effect stabilizes the insulating state. A finite character of Hund coupling also favors stabilization of the CE phase in the region of reasonable values of the interatomic exchange and hopping integrals.