Atomic scale impact of La doping on the electronic structure in bilayer iridate Sr₃Ir₂O₇

Strongly correlated electronic systems exhibit exotic states created by intricate interplay between strong spin-orbit coupling, Coulomb repulsion, and crystal field. Doped marginal Mott insulators are crucial to study at the nanoscale because of the inherent local nature of the interactions and disorder. Here, we use scanning tunneling microscopy (STM) to study La-doped bilayer strontium iridate Sr₃Ir₂O₇ (Ir327), a member of Ruddlesden-Popper series of perovskite iridates. By inhomogeneously supplying electrons, La doping creates nanoscale phase separation and drives the crystal through the metal/Mott-insulator transition. The crystal structure of Ir327 is composed of stacked bilayers of corner-sharing IrO₆ octahedra, while La atoms are introduced into Sr sites. By analyzing STM topography and spectroscopy, we conclude that (a) La-dopants preferentially dope the middle Sr-O plane in each bilayer and (b) La atoms in the middle plane are responsible for in-gap spectral weight transfer and impurity states. We report observation of charge or orbital order in the outer Sr-O planes. No similar order has been observed in the pristine or Ru-doped Ir327. We explain the results in terms of spatial interplay between strong local interactions and effect of the dopants.