  It was found that, although isovalent, Rh substituted for Ir in Sr$_2$IrO$_4$
may trap one electron inducing effective hole doping of Ir sites. Transport and
thermoelectric measurements on Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$ single crystals
presented here reveal the existence of an electron-like contribution to
transport, in addition to the hole-doped one. As no electron band shows up in
ARPES measurements, this points to the possibility that this hidden electron
may delocalize in disordered clusters.

Strongly Correlated Electrons