Hybridization model for a pair of intermediate valence manganese impurities

Abstract

Manganese ions in a mixed-valent state of two magnetic configurations, and , play an important role in the magnetoresistance of -based systems. We describe each Mn impurity with the represented by a spin S = 3/2 (three localized d electrons in the orbitals with their spins ferromagnetically coupled) and the configuration having an additional localized d electron in one of the orbitals to form a total spin (S + 1/2). The electron hybridizes with the conduction electrons and the multiple occupancy of the 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 freedom into a quadrupolar singlet, and interferes with the usual spin Kondo effect. We consider a pair of such manganese ions and allow the electrons to hop between the two sites. Hence, bonding and antibonding levels are formed giving rise to the ferromagnetic double-exchange mechanism. We study the interaction between the impurities in the integer-valent and the mixed-valent regimes. In the integer-valent limit we renormalize the interactions using the vertex function in the leading logarithmic approximation. Two neighbouring impurities with the same integer valence interact ferromagnetically. ions have in addition a quadrupolar Kondo effect. In the intermediate valence regime we calculate the ground state energy, the valence, the population difference between the bonding and antibonding states, the charge susceptibility, the quadrupolar susceptibility and the response to a charge imbalance between the two sites as a function of the energy of the level in zero magnetic field and for the spin-polarized limit (ferromagnetic lattice) using a mean-field slave-boson formulation. The results indicate that the intersite hopping suppresses charge order and lattice distortions.