Far-Infrared Study of Bi-Sr-Ca-Cu-O Thin Films on MgO Substrates

Far-infrared transmission measurements of Bi-Sr-Ca-Cu-O superconducting thin films deposited on MgO substrates have been made in the range from 50 to 300 cm -1 at temperatures between 7 and 174 K. Transmittance spectra cannot be explained by the Leplae theory of a conventional superconductor in the superconducting state, while transmittance spectra were fully explained by the Drude theory in the normal state. The discrepancy of a theoretical spectrum with the experimental spectrum is ascribed to a large portion of the real part of the optical conductivity below the so-called energy gap ∼400 cm -1 . Because these extra optical conductivities are proportional to their DC conductivities in many specimens, this extra optical conductivity is ascribed to an electronic excitation due to a low-lying energy gap and/or residual normal conductivity below 400 cm -1 .