Dependence of the infrared properties of single-domain YBa2Cu3O7-y on oxygen content.

The first polarized infrared-reflectivity measurements of untwinned crystals of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}} as a function of {ital y} are reported. We separate the chain and plane contributions to the infrared conductivity, and find that for crystals with {ital T}{sub {ital c}}{approx gt}50 K the prominent normal state midinfrared feature is associated with the chains. A threshold in the plane conductivity at {similar to}500 cm{sup {minus}1} has a strong temperature dependence just below {ital T}{sub {ital c}} suggesting an association with the superconducting gap. Its temperature dependence above and below {ital T}{sub {ital c}} is quite similar to that of relevant NMR data, and is suggestive of gap development in the normal state.