Electronic excitations in $\gamma$-Li$_2$IrO$_3$

TL;DR

This paper combines density functional theory and exact diagonalization to study the electronic excitations and optical properties of $ ext{Li}_2 ext{IrO}_3$ compounds, revealing the microscopic origins of optical transitions and anisotropic behavior.

Contribution

It provides a detailed analysis of electronic excitations in $ ext{Li}_2 ext{IrO}_3$ using combined computational methods, highlighting the role of $j_ ext{eff}$ states and deviations from the Kitaev model.

Findings

Optical excitations mainly involve $j_ ext{eff}$=1/2 and 3/2 states.

Anisotropic optical behavior explained by microscopic transitions.

Deviations from pure Kitaev physics influence low-frequency transitions.

Abstract

We investigate the electronic properties of the three-dimensional stripyhoneycomb $\gamma$-Li$_2$IrO$_3$ via relativistic density functional theory calculations as well as exact diagonalization of finite clusters and explore the details of the optical conductivity. Our analysis of this quantity reveals the microscopic origin of the experimentally observed (i) optical transitions and (ii) anisotropic behavior along the various polarization directions. In particular we find that the optical excitations are overall dominated by transitions between $j_\text{eff}$ = 1/2 and 3/2 states and the weight of transitions between $j_\text{eff}$ = 1/2 states at low frequencies can be correlated to deviations from a pure Kitaev description. We furthermore reanalyze within this approach the electronic excitations in the known two-dimensional honeycomb systems $\alpha$-Li$_2$IrO$_3$ and Na$_2$IrO$_3$ and discuss the results in comparison to $\gamma$-Li$_2$IrO$_3$.