Unbiased Monte Carlo simulations of realistic models: Colossal magnetoresistive manganites

We investigate the one-orbital model for manganites with cooperative phonons and superexchange coupling JAF via large-scale Monte Carlo (MC) simulations. Results for two-orbitals are also briefly discussed. Focusing on an the realistic electronic density n=0.75, a regime of competition between ferromagnetic (FM) metallic and charge-ordered (CO) insulating states was identified in the finite temperature phase diagram. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects were observed. The CMR magnitude is much larger than recently reported when randomly distributed polarons form the competing insulator. The appearance of 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. We present calculations of charge-charge correlations as well as typical Monte Carlo snapshots to support this view.