We have investigated the electronic and optical properties of (Sr1−xCax)2IrO4 (x = 0–0.375) and (Sr1−yBay)2IrO4 (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 Jeff = 1/2 spin-orbit Mott insulator Sr2IrO4. As the average A-site ionic radius increases from (Sr1−xCax)2IrO4 to (Sr1−yBay)2IrO4, optical conductivity spectra in the near-infrared region shift to lower energies, which cannot be explained by the simple picture of well-separated Jeff = 1/2 and Jeff = 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 Jeff = 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 Jeff = 1/2 band.
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We acknowledge the support of National Science Foundation Grants No. DMR-1454200 for thin-film synthesis and characterizations, No. DMR-1265162 and No. DMR-1712101 for target synthesis, and No. DMR-1262261 for infrared spectroscopy.
Souri, Maryam; Kim, B. H.; Gruenewald, John H.; Connell, John G.; Thompson, J.; Nichols, J.; Terzic, Jsaminka; Min, B. I.; Cao, Gang; Brill, Joseph W.; and Seo, Sung S. Ambrose, "Optical Signatures of Spin-Orbit Exciton in Bandwidth-Controlled Sr2IrO4 Epitaxial Films via High-Concentration Ca and Ba Doping" (2017). Physics and Astronomy Faculty Publications. 525.