Microscopic Evidence for Slater-Type Metal-Insulator Transition in Sr$_{2}$IrO$_{4}$

The interplay between spin-orbit coupling, bandwidth and on-site coulomb repulsion in layered 5d transition metal oxides (TMO) acquired much interest recently. In Sr$_{2}$IrO$_{4}$, the interplay opens a gap near the Fermi energy and stabilizes a J$_{\mathrm{eff}}=$1/2 spin-orbital entangled insulating state at low temperatures. However, whether this metal-insulating transition (MIT) is Mott-type (electronic-correlation driven) or slater-type (magnetic order driven) is still under hot debate. In this presentation, we give, for the first time, the atomic resolved structure of Sr$_{2}$IrO$_{4}$ surface in real space by using scanning tunneling microscopy. Tunneling spectroscopic results illustrate the gap opening of Sr$_{2}$IrO$_{4}$ at low temperatures with the gap size of 250 mV, indicating the metal to insulator transition. More importantly, the pair of peaks around gap in spectra suggests the quasi-particle coherent excitation, implying the Slater-type insulating state. This is further confirmed by temperature dependent measurements and density functional theory calculations.