Optical conductivity in the superconducting state of layered cuprate oxides

The optical conductivity for very low frequencies is investigated in the superconducting state of YBa₂Cu₃O₇ using a layered structure model that some pairing interaction generates superconductivity in the CuO₂-plane layers and the interlayer transfer interaction induces weak superconductivity in the CuO-chain layers. The optical conductivity has contributions from the plane and chain layers which show a striking difference in the temperature dependence below T_c. The optical conductivity of the plane layers has a peak just below T_c, which is similar to the Hebel-Slichter peak in the nuclear relaxarion rate, and decrease rapidly below the temperature of the peak, while the optical conductivity of the chain layers monotonically increases with desreasing temperature. The temperature dependence of the optical conductivity is compared with that of the nuclear relaxation rate.