IN A CLEAN HIGH-TC SUPERCONDUCTOR YOU DO NOT SEE THE GAP

The frequency-dependent conductivity of laser-deposited YBa2Cu3O7- thin films shows an onset of midinfrared absorption at 140 cm-1 and structure in the 400500-cm-1 region. These low-energy absorptions occur both above and below Tc, making them unlikely to be the superconducting gap in the usual BCS sense. The absorption across the gap is weak because the high-Tc materials are in the clean limit; this weak absorption is masked by the midinfrared absorption. © 1990 The American Physical Society.