In a clean high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">T</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">c</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>superconductor you do not see the gap

The frequency-dependent conductivity of laser-deposited ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin films shows an onset of midinfrared absorption at \ensuremath{\sim}140 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and structure in the 400--500-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ region. These low-energy absorptions occur both above and below ${T}_{c}$, making them unlikely to be the superconducting gap in the usual BCS sense. The absorption across the gap is weak because the high-${T}_{c}$ materials are in the clean limit; this weak absorption is masked by the midinfrared absorption.