Honeycomb latticeNa2IrO3at high pressures: A robust spin-orbit Mott insulator

The honeycomb iridate ${\mathrm{Na}}₂{\mathrm{IrO}}₃$ has received much attention as a candidate to realize a quantum spin liquid state, but the nature of its insulating state remains controversial. We found that the material exhibits structural transitions at 3 and 10 GPa. The former is accompanied by 166-meV suppression of the activation gap, but the energies for the low-lying interband transitions change by less than 10 meV. This can be reconciled in a picture in which the application of high pressure barely shifts the electronic bands, but rather merely broadens them. First-principles calculations uncover a strong correlation between the band gap and the $\ensuremath{\beta}$ angle of the monoclinic structure, indicating non-negligible interlayer coupling. These results offer clear evidence for a spin-orbit Mott insulating state in ${\mathrm{Na}}₂{\mathrm{IrO}}₃$ and are inconsistent with the quasimolecular orbital model.