Particularly in Sr2 IrO 4, the interplay between spin-orbit coupling, bandwidth and on-site Coulomb repulsion stabilizes a J eff = 1/2 spin-orbital entangled insulating state at low temperatures. Whether this insulating phase is Mott-or Slater-type, has been under intense debate. We address this issue via spatially resolved imaging and spectroscopic studies of the Sr 2 IrO 4 surface using scanning tunneling microscopy/spectroscopy (STM/S). STS results clearly illustrate the opening of an insulating gap (150 ∼ 250âmeV) below the Néel temperature (T N), in qualitative agreement with our density-functional theory (DFT) calculations. More importantly, the temperature dependence of the gap is qualitatively consistent with our DFT + dynamical mean field theory (DMFT) results, both showing a continuous transition from a gapped insulating ground state to a non-gap phase as temperatures approach T N. These results indicate a significant Slater character of gap formation, thus suggesting that Sr 2 IrO 4 is a uniquely correlated system, where Slater and Mott-Hubbard-type behaviors coexist.
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