Nanoscale Phase Coexistence and Percolative Quantum Transport

We study the nanoscale phase coexistence of ferromagnetic metallic (FMM) and antiferromagnetic insulating (AFI) regions by including the effect of AF superexchange and weak disorder in the double exchange model. We use a new Monte Carlo technique, mapping on the disordered spin-fermion problem to an effective short range spin model, with self-consistently computed exchange constants. We recover `cluster coexistence' as seen earlier in exact simulation of small systems. The much larger sizes, $\sim 32 \times 32$, accessible with our technique, allows us to study the cluster distribution for varying electron density, disorder, and temperature. We track the magnetic structure, obtain the density of states, with its `pseudogap' features, and, for the first time, provide a fully microscopic estimate of the resistivity in a phase coexistence regime, comparing it with the `percolation' scenario.