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arXiv · 1706.03529

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

M. Souri·B. H. Kim·J. H. Gruenewald·J. G. Connell·J. Thompson·J. Nichols·J. Terzic·B. I. Min·G. Cao·J. W. Brill·A. Seo
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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.

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M. Souri, B. H. Kim, J. H. Gruenewald, J. G. Connell, J. Thompson, J. Nichols, J. Terzic, B. I. Min, G. Cao, J. W. Brill, A. Seo. 2017-06-12. Optical Signatures of Spin-Orbit Exciton in Bandwidth Controlled Sr$_2$IrO$_4$ Epitaxial Films via High-Concentration Ca and Ba Doping. https://doi.org/10.1103/physrevb.95.235125

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