Interplay between double-exchange, superexchange, and Lifshitz localization in doped manganites -: art. no. 104430

Considering the disorder caused in manganites by the substitution Mn-->Fe or Ga, we accomplish a systematic study of doped manganites begun in previous papers. To this end, a disordered model is formulated and solved using the variational mean-field technique. The subtle interplay between double exchange, superexchange, and disorder causes similar effects on the dependence of T-C on the percentage of Mn substitution in the cases considered. Yet, in La2/3Ca1/3Mn1-yGayO3 our results suggest a quantum critical point (QCP) for yapproximate to0.1-0.2, associated to the localization of the electronic states of the conduction band. In the case of LaxCaxMn1-yFeyO3 (with x=1/3,3/8) no such QCP is expected.