Far-Infrared Reflectance and Transmittance Studies of YBa2Cu3O7-x Single-Crystal Thin Films

Abstract

A new method of estimating the optical constants of thin films has been developed and applied to YBa2Cu3O7-x epitaxial single-crystal thin films (Tc=90 K) grown on MgO substrates to determine the far-infrared optical conductivity σab(ω), which is the response when an unpolarized excitation is applied parallel to the ab-plane in multidomain (twinned) single crystals of YBa2Cu3O7-x. In this method, both the transmittance spectra T(ω) and reflectance spectra R(ω) are measured as a function of the wave number ω, and the results are substituted into a set of coupled equations which exactly describe R(ω) and T(ω) of thin films on substrates, where the complex refractive index N=n+ik of the thin films is an unknown parameter. The coupled equations are numerically solved using the Newton method, and the values of n and k are determined as a function of ω. We have determined σab(ω) of YBa2Cu3O7-x thin films for ω=50–250 cm-1 at T=34–97 K. Analysis of the temperature dependence of σab(ω) based on the two-fluid model suggests that the symmetry of the superconducting pairing state of the specimen is d-wave.