Infrared refractive indices of LaAlO_3, LaGaO_3, and NdGaO_3

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Abstract
We discuss the experimental and theoretical studies of the infrared optical constants of three perovskite materials: LaAlO3, LaGaO3, and NdGaO3. These materials are commonly used as substrates for depositing high-temperature superconducting films. The transmittance and the reflectance are measured with a Fourier-transform infrared spectrometer. At frequencies from 1000 to 10 000 cm−1, for which the substrates are transparent, the refractive index is extracted directly from the measured transmittance and reflectance. Kramers–Kronig relations are employed to yield the refractive index of these materials at frequencies from 100 to 1000 cm−1. We also use a Lorentz phonon oscillator model to obtain the refractive index by fitting the measured data to the model function. The advantages and limitations of each method are discussed. The refractive index for the LaAlO3 substrate is also obtained at 10 and 78 K with a cryogenic accessory. The results are compared with those reported by other research groups. This research provides information necessary for the quantitative study of the optical properties of thin superconducting films.