Optical signatures of spin-orbit exciton in bandwidth-controlled Sr2IrO4 epitaxial films via high-concentration Ca and Ba doping

Abstract

We have investigated the electronic and optical properties of ${(\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{a}}_{x})}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ ($x=0--0.375$) and ${(\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}y}\mathrm{B}{\mathrm{a}}_{y})}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ ($y=0--0.375$) 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 ${J}_{\mathrm{eff}}=1/2$ spin-orbit Mott insulator $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$. As the average A-site ionic radius increases from ${(\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{a}}_{x})}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ to ${(\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}y}\mathrm{B}{\mathrm{a}}_{y})}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$, optical conductivity spectra in the near-infrared region shift to lower energies, which cannot be explained by the simple picture of well-separated ${J}_{\mathrm{eff}}=1/2$ and ${J}_{\mathrm{eff}}=3/2$ 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 intersite optical transitions within the ${J}_{\mathrm{eff}}=1/2$ band. Our experimental results are consistent with this interpretation as implemented by a multiorbital Hubbard model calculation: namely, incorporating a strong Fano-like coupling between the spin-orbit exciton and intersite $d\text{\ensuremath{-}}d$ transitions within the ${J}_{\mathrm{eff}}=1/2$ band.