HYBRIDIZATION IMPURITY MODEL FOR MN IONS - II

Manganese ions in a mixed valent state of two magnetic L configuration s, Mn4+ and Mn3+, play an important role in the magnetoresistance of L aMnO3-based systems. We consider a Mn impurity with the Mn4+ represent ed by a spin S-3/2 (three localized d electrons in the t(2g) orbitals with their spins ferromagnetically coupled) and the Mn3+ configuration having an additional localized d electron in one of the e(g) orbitals to form a total spin (S + 1/2). The e(g) electron hybridizes with the conduction electrons and the multiple occupancy of the eg level is ex cluded by a large Coulomb energy. This gives rise to a quadrupolar Kon do effect,which compensates the orbital degrees of freedom into a quad rupolar singlet, and interferes with the usual spin Kondo effect. We c alculate the ground slate energy, valence, charge susceptibility and t he quadrupolar susceptibility as a function of the energy of the e(g) level in zero-magnetic field and for the spin-polarized Limit (ferroma gnetic lattice). The methods used are a mean-field slave-boson formula tion, the noncrossing diagram approximation (NCA) and the Bethe ansatz . Both, the charge and quadrupolar susceptibilities, are larger in the spin-polarized lattice, indicating that this state is more favorable for charge order and lattice distortions.