High-pressure versus isoelectronic doping effect on the honeycomb iridate Na${}_2$IrO${}_3$

Abstract

We study the effect of isoelectronic doping and external pressure in tuning the ground state of the honeycomb iridate Na${}_2$IrO${}_3$ by combining optical spectroscopy with synchrotron x-ray diffraction measurements on single crystals. The obtained optical conductivity of Na${}_2$IrO${}_3$ is discussed in terms of a Mott insulating picture versus the formation of quasimolecular orbitals and in terms of Kitaev-interactions. With increasing Li content $x$, (Na${}_{1-x}$Li${}_x$)${}_2$IrO${}_3$ moves deeper into the Mott insulating regime and there are indications that up to a doping level of 24\% the compound comes closer to the Kitaev-limit. The optical conductivity spectrum of single crystalline $\hat{I}\pm$-Li${}_2$IrO${}_3$ does not follow the trends observed for the series up to $x=0.24$. There are strong indications that $\hat{I}\pm$-Li${}_2$IrO${}_3$ is less close to the Kitaev-limit compared to Na${}_2$IrO${}_3$ and closer to the quasimolecular orbital picture. Except for the pressure-induced hardening of the phonon modes, the optical properties of Na${}_2$IrO${}_3$ seem to be robust against external pressure. Possible explanations of the unexpected evolution of the optical conductivity with isolectronic doping and the drastic change between $x=0.24$ and $x=1$ are given by comparing the pressure-induced changes of lattice parameters and the optical conductivity with the corresponding changes induced by doping.

12 pages, 6 figures, accepted for publication in Phys. Rev. B