Coexistence of large and small polarons in manganites

Abstract

The interplay of the electron-phonon interaction and of the double- and superexchange magnetic effects is analyzed in the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ perovskites with $0<x<0.5.$ By using an analytical variational scheme that allows us to treat the electron-phonon interaction in a fully quantum manner it is shown that in the intermediate electron-phonon coupling regime a charge-density instability occurs near the metal-insulator transition, induced by either temperature or hole doping. The system segregates in antiferromagnetic or paramagnetic small polarons and ferromagnetic large polaron domains characterized by different values of the lattice distortions that are satisfactory compared with those experimentally observed.