Zn substitution effects on the $a$-, $b$-, and $c$-axis optical conductivity spectra have been investigated for optimally annealed ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ crystals. It was revealed that a very small amount of Zn seriously affects the low-energy charge dynamics in both normal and superconducting states. In addition to the increase of scattering rates, Zn substitution introduces additional features in the conductivity spectra and reduces the in-plane anisotropy by suppressing the plasma frequency in the $b$ direction in the normal state. In the superconducting state, a very strong anisotropic pair-breaking effect was found.

• Received 24 November 1997

DOI:https://doi.org/10.1103/PhysRevB.57.R11081