Optical signatures of spin-orbit exciton in bandwidth-controlled $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ epitaxial films via high-concentration Ca and Ba doping

Optical signatures of spin-orbit exciton in bandwidth-controlled $S r_{2} Ir O_{4}$ 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, and A. Seo

Phys. Rev. B 95, 235125 – Published 14 June 2017

Abstract

We have investigated the electronic and optical properties of ${(S r_{1 - x} C a_{x})}_{2} Ir O_{4}$ ( $x = 0 - 0.375$ ) and ${(S r_{1 - y} B a_{y})}_{2} Ir O_{4}$ ( $y = 0 - 0.375$ ) 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 $J_{eff} = 1 / 2$ spin-orbit Mott insulator $S r_{2} Ir O_{4}$ . As the average A-site ionic radius increases from ${(S r_{1 - x} C a_{x})}_{2} Ir O_{4}$ to ${(S r_{1 - y} B a_{y})}_{2} Ir O_{4}$ , optical conductivity spectra in the near-infrared region shift to lower energies, which cannot be explained by the simple picture of well-separated $J_{eff} = 1 / 2$ 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.

Received 10 April 2017

DOI:https://doi.org/10.1103/PhysRevB.95.235125

Physics Subject Headings (PhySH)

Electronic structure Excitons Optical conductivity Spin-orbit coupling

Iridates Strongly correlated systems Thin films

Electron-correlation calculations Hubbard model Optical spectroscopy Physical vapor deposition X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

M. Souri¹, B. H. Kim^2,3, J. H. Gruenewald¹, J. G. Connell¹, J. Thompson¹, J. Nichols¹, J. Terzic^1,4, B. I. Min², G. Cao^1,4, J. W. Brill¹, and A. Seo^1,*

¹Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, USA
²Department of Physics, PCTP, Pohang University of Science and Technology, Pohang 790-784, Korea
³iTHES Research Group and Computational Condensed Matter Physics Laboratory, RIKEN, Wako, Saitama 351-0198, Japan
⁴Department of Physics, University of Colorado at Boulder, Boulder, Colorado 80309, USA

^*a.seo@uky.edu

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Vol. 95, Iss. 23 — 15 June 2017

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Figure 1
(a) X-ray $θ − 2 θ$ scans of the epitaxial ${(S r_{1 - x} A_{x})}_{2} Ir O_{4}$ (A: Ca, Ba) thin films grown on STO substrates, where only the ( $00 l$ )-diffraction peaks of the films ( $l = 4, 8, 12, 16, 24$ ) are visible. (b) The enlarged scans near (0012) reflections of the films and the (002) reflections of the substrates. The peaks from the substrates are labeled with the filled diamond (♦) symbols.
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Figure 2
(a) Reciprocal space map around the (103) reflection of the STO substrates with the (1118) reflection of the ${(S r_{1 - x} C a_{x})}_{2} Ir O_{4}$ and ${(S r_{1 - y} B a_{y})}_{2} Ir O_{4}$ films. (b) The in-plane (left axis) and out-of-plane (right axis) lattice parameters of the ${(S r_{1 - x} C a_{x})}_{2} Ir O_{4}$ and ${(S r_{1 - y} B a_{y})}_{2} Ir O_{4}$ thin films obtained from the reciprocal space maps and x-ray diffraction scans, respectively, as a function of average A-site ionic radius in the ${(S r_{1 - x} A_{x})}_{2} Ir O_{4}$ thin films (A: Ca, Ba). The solid red and blue squares represent the in-plane and out-of-plane lattice parameters, respectively. (c) Normalized resistivity versus temperature of the ${(S r_{1 - x} C a_{x})}_{2} Ir O_{4}$ and ${(S r_{1 - y} B a_{y})}_{2} Ir O_{4}$ thin films on STO substrates, which indicates that all the films are insulators.
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Figure 3
Optical conductivity spectra $[σ_{1} (ω)]$ of the ${(S r_{1 - x} C a_{x})}_{2} Ir O_{4}$ and ${(S r_{1 - y} B a_{y})}_{2} Ir O_{4}$ thin films at room temperature with the minimal set of Lorentz oscillators. The experimental spectra are shown by thick olive, cyan, blue, magenta, and orange curves from $(S r_{0.75} C a_{0.25})_{2} Ir O_{4}$ to $(S r_{0.75} B a_{0.25})_{2} Ir O_{4}$ , respectively. The fit oscillators are shown by thin green, red, blue, orange, and gray curves. The black curves show the total spectra of the fit, which match the experimental spectra. Note that the β peak position shows a significant red shift as it goes from ${(S r_{1 - x} C a_{x})}_{2} Ir O_{4}$ to ${(S r_{1 - y} B a_{y})}_{2} Ir O_{4}$ . The spectra are shifted vertically for clarity.
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Figure 4
(a) The α and β peak positions ( $ω_{α}$ and $ω_{β}$ ) as a function of average A-site ionic radius for the ${(S r_{1 - x} A_{x})}_{2} Ir O_{4}$ thin films (A: Ca, Ba) obtained from Lorentz oscillator fitting. (b) The ratio of the spectral weight of the α to β peaks versus average A-site ionic radius from ${(S r_{1 - x} C a_{x})}_{2} Ir O_{4}$ to ${(S r_{1 - y} B a_{y})}_{2} Ir O_{4}$ .
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Figure 5
(a) Calculated spectral weight of spin-orbit exciton and (b) $σ_{1} (ω)$ at different values of the electronic hopping integral ( $t$ ). The spectra in panel (b) are shifted vertically for clarity. (c) Schematic diagram representing the Fano-like resonance between the intersite optical transitions of the $J_{eff} = 1 / 2$ state and the spin-orbit exciton, which creates a two-peak-like structure in the final optical spectrum. The red and blue arrows are electrons with up and down spins in the $J_{eff} = 3 / 2$ and $J_{eff} = 1 / 2$ states, indicating the intersite optical transitions and the on-site spin-orbit exciton.
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Physical Review B

covering condensed matter and materials physics

Optical signatures of spin-orbit exciton in bandwidth-controlled $S r_{2} Ir O_{4}$ 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, and A. Seo

Phys. Rev. B 95, 235125 – Published 14 June 2017

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Physical Review B

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Optical signatures of spin-orbit exciton in bandwidth-controlled Sr2IrO4 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, and A. Seo

Phys. Rev. B 95, 235125 – Published 14 June 2017

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Optical signatures of spin-orbit exciton in bandwidth-controlled $S r_{2} Ir O_{4}$ epitaxial films via high-concentration Ca and Ba doping