HYBRIDIZATION MODEL FOR A PAIR OF INTERMEDIATE VALENCE MANGANESE IMPURITIES

Manganese ions in a mixed-valent state of two magnetic configurations, Mn4+ and Mn3+, play an important role in the magnetoresistance of LaM nO3-based systems. We describe each Mn impurity with the Mn4+ represen ted by a spin S = 3/2 (three localized d electrons in the t(2g) orbita ls with their spins ferromagnetically coupled) and the Mn3+ configurat ion having an additional localized d electron in one of the e(g) orbit als 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 excluded by a large Coulomb energy at each site. This gives rise to a quadrupolar Kondo effect; which compensates the orbital degrees of fr eedom into a quadrupolar singlet, and interferes with the usual spin K ondo effect. We consider a pair of such manganese ions and allow the e (g) electrons to hop between the two sites. Hence, bonding and antibon ding levels are formed giving rise to the ferromagnetic double-exchang e mechanism. We study the interaction between the impurities in the in teger-valent and the mixed-valent regimes. In the integer-valent limit we renormalize the interactions using the vertex function in the lead ing logarithmic approximation. Two neighbouring impurities with the sa me integer valence interact ferromagnetically. Mn3+ ions have in addit ion a quadrupolar Kondo effect. In the intermediate valence regime we calculate the ground state energy, the valence, the population differe nce between the bonding and antibonding states, the charge susceptibil ity, the quadrupolar susceptibility and the response to a charge imbal ance between the two sites as a function of the energy of the e(g) lev el in zero magnetic field and for the spin-polarized limit (ferromagne tic lattice) using a mean-field slave-boson formulation. The results i ndicate that the intersite hopping suppresses charge order and lattice distortions.