  Interest in many strongly spin-orbit coupled 5d-transition metal oxide
insulators stems from mapping their electronic structures to a J=1/2 Mott
phase. One of the hopes is to establish their Mott parent states and explore
these systems' potential of realizing novel electronic states upon carrier
doping. However, once doped, little is understood regarding the role of their
reduced Coulomb interaction U relative to their strongly correlated 3d-electron
cousins. Here we show that, upon hole-doping a candidate J=1/2 Mott insulator,
carriers remain localized within a nanoscale phase separated ground state. A
percolative metal-insulator transition occurs with interplay between localized
and itinerant regions, stabilizing an antiferromagnetic metallic phase beyond
the critical region. Our results demonstrate a surprising parallel between
doped 5d- and 3d-electron Mott systems and suggest either through the near
degeneracy of nearby electronic phases or direct carrier localization that U is
essential to the carrier response of this doped spin-orbit Mott insulator.
