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Far-infrared conductivity of yba2Cu3O7–y

We review experimental data on the optical conductivity of YBa2Cu3O7–y Kramers-Kronig analysis shows that in the mid infrared range the optical conductivity is typical of perovskites with a rich phonon structure in the oxygen bond stretching and bending region. In the far infrared a depression of conductivity sets in below the superconducting transition temperature that we associate with the presence of a superconducting energy gap. In all the samples we have looked at there is residual conductivity in the gap.