Abstract
Using Monte Carlo simulations and the two eg-orbital model for manganites, the stability of the CE and A-type antiferromagnetic insulating states is analyzed when quenched disorder in the superexchange JAF between the t2g localized spins and in the on-site energies is introduced. At vanishing or small values of the electron-Jahn-Teller phonon coupling, the previously hinted "fragility" of these insulating states is studied in
detail, focusing on their charge transport properties. This fragility is here found to induce a rapid transition from the insulator to a poor metallic state upon the introduction of disorder. A possible qualitative explanation
is presented based on the close proximity in energy of ferromagnetic metallic phases, and also on percolative ideas valid at large disorder strength. The scenario is compared with previously discussed insulator-to-metal
transitions in other contexts. This particularly severe effect of disorder must be present in other materials as well, in cases involving phases that arise as a compromise between very different tendencies, as it probably
occurs with striped states in the cuprates.
