Optical Signatures of Spin-Orbit Exciton in Bandwidth Controlled Sr$_2$IrO$_4$ Epitaxial Films via High-Concentration Ca and Ba Doping

TL;DR

This study explores how chemical doping in Sr$_2$IrO$_4$ thin films affects their electronic and optical properties, revealing a complex interplay between spin-orbit excitons and inter-site transitions through experimental and theoretical analysis.

Contribution

It demonstrates the origin of optical conductivity features in doped Sr$_2$IrO$_4$ as due to coupling between spin-orbit excitons and inter-site transitions, supported by multi-orbital Hubbard model calculations.

Findings

Optical spectra shift to lower energies with increased doping.

Two-peak optical conductivity arises from exciton and inter-site transition overlap.

Experimental results align with Fano-like coupling in theoretical models.

Abstract

We have investigated the electronic and optical properties of (Sr$_{1-x}$Ca$_{x}$)$_2$IrO$_4$ (x= 0 - 0.375) and (Sr$_{1-y}$Ba$_y$)$_2$IrO$_4$ (y= 0 - 0.375) epitaxial thin-films, in which the bandwidth is systematically tuned via chemical substitutions of Sr ions by Ca and Ba. Transport measurements indicate that the thin-film series exhibits insulating behavior, similar to the J$_{eff}$= 1/2 spin-orbit Mott insulator Sr$_2$IrO$_4$. As the average A-site ionic radius increases from (Sr$_{1-x}$Ca$_{x}$)$_2$IrO$_4$ to (Sr$_{1-y}$Ba$_y$)$_2$IrO$_4$, optical conductivity spectra in the near-infrared region shift to lower energies, which cannot be explained by the simple picture of well-separated J$_{eff}$= 1/2 and J$_{eff}$= 3/2 bands. We suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically-forbidden spin-orbit exciton and the inter-site optical transitions within the J$_{eff}$= 1/2 band. Our experimental results are consistent with this interpretation as implemented by a multi-orbital Hubbard model calculation: namely, incorporating a strong Fano-like coupling between the spin-orbit exciton and inter-site d-d transitions within the J$_{eff}$= 1/2 band.