Charge, spin, and orbital degrees of freedom in manganites: Mean-field slave-boson approach

The phase diagram of La1-xCaxMnO3 is studied using a mean-field slave-boson formulation for e(g) electrons in two orbitals and classical t(2g) spins S oriented in the magnetic configurations of the A, B, C, and G phases as a function of x. The manganese ions are in a mixed valent state of two magnetic configurations Mn4+ and Mn3+, with all 3d electrons coupled ferromagnetically via Hund's rule. The multiple occupancy of the e(g) levels is excluded at each site by a large Coulomb energy. The e(g) electrons hop between Mn sites with amplitude t giving rise to the ferromagnetic double exchange, which competes with the antiferromagnetic superexchange J between the t(2g) spins of the ions. The e(g) electrons couple to the lattice degrees of freedom via the Jahn-Teller effect. The model has only two parameters, namely, J/t and the Jahn-Teller energy. We obtain that only the A and C phases can exhibit a Jahn-Teller distortion.