Metallic conductivity on Na-deficient structural domain walls in the spin-orbit Mott insulator Na${}_2$IrO${}_3$

Honeycomb Na${}_2$IrO${}_3$ is a prototype spin-orbit Mott insulator and Kitaev magnet. We report a combined structural and electrical resistivity study of Na${}_2$IrO${}_3$ single crystals. Laue back-scattering diffraction indicates twinning with $\pm {120}^{\circ }$ rotation around the $c^{\ast }$-axis while scanning electron microscopy displays nanothin lines parallel to all three b-axis orientations of twin domains. Energy dispersive x-ray analysis line-scans across such domain walls indicate no change of the Ir signal intensity, i.e. intact honeycomb layers, while the Na intensity is reduced down to $\sim 2\mathrm{/}3$ of its original value at the domain walls, implying significant hole doping. Utilizing focused-ion-beam micro-sectioning, the temperature dependence of the electrical resistance of individual domain walls is studied. It demonstrates the tuning through the metal-insulator transition into a correlated-metal ground state by increasing hole doping.