Zn-substitution effects on the optical conductivity in YBa₂Cu₃O_7-δ crystals: Strong pair breaking and reduction of in-plane anisotropy

Zn substitution effects on the a-, b-, and c-axis optical conductivity spectra have been investigated for optimally annealed YBa₂Cu₃O_7-δ 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.