Anisotropic Optical Conductivity of Nd_2-xCe_xCuO₄ Thin Films

Abstract

Optical conductivity spectra σ₁(ω) of Nd_2-xCe_xCuO₄ thin films, measured by the reflectance-transmittance method (R-T method) which has been proposed to investigate far-infrared spectroscopy, are investigated based on the anisotropic pairing model. Precise measurements of the frequency-dependent conductivity σ₁(ω) enable us to examine quantitatively the nature of the superconducting gap through infrared properties for the first time in high-T_c superconductors. We show that the behavior of optical conductivity σ₁(ω) is consistent with the anisotropic superconducting gap and is well explained by the formula for d-wave pairing in the far-infrared region. Our results suggest that the electron-doped cuprate superconductors Nd_2-xCe_xCuO₄ have nodes in the superconducting gap.