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

We have investigated the electronic and optical properties of (Sr1-xCax)(2)IrO4 (x = 0-0.375) and (Sr1-yBay)(2)IrO4 (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 Sr2IrO4. As the average A-site ionic radius increases from (Sr1-xCax)(2)IrO4 to (Sr1-yBay)(2)IrO4, 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 intersite optical transitions within the J(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-d transitions within the J(eff) = 1/2 band.