Insulating nature of iridium oxide Ca₅Ir₃O₁₂ probed by synchrotron-radiation-based infrared spectroscopy

Ca₅Ir₃O₁₂ shows semiconducting behavior, although it is expected to have metallic conduction because of its 1/6-filled band. To investigate the insulating nature of Ca₅Ir₃O₁₂, synchrotron-radiation-based infrared spectroscopy and electrical resistivity measurements were performed. Optical conductivity obtained using Kramers-Kroenig transformation is compared with ab initio calculations based on density-functional and random-phase-approximation calculations, and their differences are carefully discussed. A detailed analysis of the temperature dependence of electrical resistivity along the c-axis reveals that ln ρ ∝ T^-2/3. This temperature dependence indicates that the electrical conduction of Ca₅Ir₃O₁₂ along the c-axis follows Efros-Shklovskii variable-range hopping. The optical conductivity with polarization parallel to the c-axis shows no clear onset at 1,300 cm^-1(0.16 eV) of the energy gap obtained by assuming the Arrhenius law. This feature of the optical conductivity is consistent with the fact that Ca₅Ir₃O₁₂ follows Efros-Shklovskii variable-range hopping. (author)