Far infrared evidence: The real part of the optical conductivity below the superconducting energy gap of BiSrCaCuO thin films

Abstract Transmission spectroscopy of BiSrCaCuO thin films has been carried out in the range from 50 to 300 cm −1 at temperatures between 7 and 174K. The transmittance spectra of the normal state can be explained by the Drude theory. However, the transmittance spectra of the superconducting state cannot be explained by the Leplae theory of the conventional superconductor. The discrepancy is ascribed to the real part of the optical conductivity due to an electronic excitation. Far-infrared transmittance spectra strongly suggest the existence of a low lying superconducting energy gap and/or a residual normal conduction below 400cm −1 in these specimens.