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

TL;DR

This study investigates how isoelectronic doping and external pressure influence the electronic and structural properties of honeycomb iridate Na$_2$IrO$_3$, revealing doping-induced shifts towards the Kitaev-limit and robustness of optical properties under pressure.

Contribution

It provides a comparative analysis of doping and pressure effects on Na$_2$IrO$_3$, highlighting the doping-induced approach to the Kitaev-limit and the stability of optical properties under pressure.

Findings

Doping with Li moves Na$_2$IrO$_3$ closer to the Kitaev-limit.

External pressure mainly causes phonon mode hardening, with minimal impact on optical properties.

$ ext{Na}_2 ext{IrO}_3$ remains robust under pressure, unlike doping effects.

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 $\alpha$-Li$_2$IrO$_3$ does not follow the trends observed for the series up to $x=0.24$. There are strong indications that $\alpha$-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.