Abstract
Using Monte Carlo simulations and the two $e_{\rm g}$-orbital model for manganites, the stability of
the CE and A-type antiferromagnetic insulating states is analyzed when quenched disorder
in the superexchange $J_{\rm AF}$ between the $t_{\rm 2g}$ 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.
It is argued that the effect unveiled here has unique properties that may define a new
class of giant effects in complex oxides.
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 occurs with striped
states in the cuprates.
