Competing Ferromagnetic and Charge-Ordered States in Models for Manganites: The Origin of the Colossal Magnetoresistance Effect

The one-orbital model for manganites with cooperative phonons and superexchange coupling J_AF is investigated via large-scale Monte Carlo simulations. The results for two orbitals are also briefly discussed. Focusing on the electron density n=0.75, a regime of competition between ferromagnetic metallic and charge-ordered (CO) insulating states is identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects are observed. The CMR is associated with the development of short-distance correlations among polarons, above the spin ordering temperatures, resembling the charge arrangement of the low-temperature CO state.