Infrared properties of YBa2Cu3O7-δ

We review recent experimental data on the infrared properties of YBa2Cu3O7-δ ceramics and single crystals. Kramers-Kronig analysis shows that the normal state far-infrared optical conductivity is dominated by a Drude response from free carriers with a plasma frequency of the order of 1.0 eV and a relaxation rate of 0.37 eV (300 cm-1 at 100 K. A strong mid-infrared band centered between 0.2 and 0.5 eV is seen in all the samples. Below the superconducting transition temperatures the far-infrared conductivity is depressed below 400 cm-1 but this is not a true energy gap since there remains, even in the best oriented crystal samples, a residual absorption.