  The Ruddlesden-Popper (RP) series of iridates (Srn+1IrnO3n+1) have been the
subject of much recent attention due to the anticipation of emergent physics
arising from the cooperative action of spin-orbit (SO) driven band splitting
and Coulomb interactions[1-3]. However an ongoing debate over the role of
correlations in the formation of the charge gap and a lack of understanding of
the effects of doping on the low energy electronic structure have hindered
experimental progress in realizing many of the predicted states[4-8] including
possible high-Tc superconductivity[7,9]. Using scanning tunneling spectroscopy
we map out the spatially resolved density of states in the n=2 RP member,
Sr3Ir2O7 (Ir327). We show that the Ir327 parent compound, argued to exist only
as a weakly correlated band insulator in fact possesses a substantial ~130meV
charge excitation gap driven by an interplay between structure, SO coupling and
correlations. A critical component in distinguishing the intrinsic electronic
character within the inhomogeneous textured electronic structure is our
identification of the signature of missing apical oxygen defects, which play a
critical role in many of the layered oxides. Our measurements combined with
insights from calculations reveal how apical oxygen vacancies transfer spectral
weight from higher energies to the gap energies thereby revealing a path toward
obtaining metallic electronic states from the parent-insulating states in the
iridates.
