  We present a simplified model for the colossal magnetoresistance in doped
manganites by exactly solving a double-exchange model (with Ising-like local
spins) and quenched binary disorder within dynamical mean field theory. We
examine the magnetic properties and the electrical and thermal transport. Our
solution illustrates three different physical regimes: (i) a weak-disorder
regime, where the system acts like a renormalized double-exchange system (which
is insufficient to describe the behavior in the manganites); (ii) a
strong-disorder regime, where the system is described by strong-coupling
physics about an insulating phase (which is the most favorable for large
magnetoresistance); and (iii) a transition region of moderate disorder, where
both double-exchange and strong-coupling effects are important. We use the
thermopower as a stringent test for the applicability of this model to the
manganites and find that the model is unable to properly account for the sign
change of the thermopower seen in experiment.
