Direct visualization of spin-dependent orbital geometry on the Na2IrO3 surface with ultra-high resolution

Abstract

The honeycomb iridate Na2IrO3, as a candidate for the Kitaev model, has drawn increasing attention in recent years. It is a rare example of a strongly correlated, topologically nontrivial band structure that may have protected quantum spin Hall states. The nature of its intriguing insulating phase and magnetic order is still under debate. In the present work, we combine low-temperature scanning tunneling microscopy/spectroscopy and density functional theory calculations to show that Na2IrO3 exhibits a band gap of 420 meV at 77 K, indicating a novel relativistic Mott insulator rather than Slater-like states. In addition, it is demonstrated that the Ir-O-Ir bonds and the subtle local density of states variation of Ir atoms induced by spin correlations can be imaged in real space in ultra-high resolution utilizing a spin-polarized oxygen-functionalized scanning tunneling microscopy tip. The direct observation of the zigzag Ir-O-Ir bonds at 77 K strongly dictates the zigzag magnetic ordering below T N ≈ 15 K because of the strong spin-orbit interactions that lock the lattice and magnetic moments.