  Layered 5d transition metal oxides (TMOs) have attracted significant interest
in recent years because of the rich physical properties induced by the
interplay between spin-orbit coupling, bandwidth and on-site Coulomb repulsion.
In Sr2IrO4, this interplay opens a gap near the Fermi energy and stabilizes a
Jeff=1/2 spin-orbital entangled insulating state at low temperatures. Whether
this metal-insulating transition (MIT) is Mott-type (electronic-correlation
driven) or Slater-type (magnetic-order driven) has been under intense debate.
We address this issue via spatially resolved imaging and spectroscopic studies
of the Sr2IrO4 surface using scanning tunneling microscopy/spectroscopy
(STM/S). The STS results clearly illustrate the opening of the (~150-250 meV)
insulating gap at low temperatures, in qualitative agreement with our
density-functional theory (DFT) calculations. More importantly, the measured
temperature dependence of the gap width coupled with our DFT+dynamical mean
field theory (DMFT) results strongly support the Slater-type MIT scenario in
Sr2IrO4. The STS data further reveal a pseudogap structure above the Neel
temperature, presumably related to the presence of antiferromagnetic
fluctuations.
