The Zn-substitution effects on the c-axis optical spectra have been investigated over T-range for YBa2Cu3Oy with y equals 6.6 and 6.9. In the highly doped crystal a large number of unpaired carriers forms a Drude-like spectrum at low frequencies, which buries the gap feature in the conductivity spectrum, keeping the maximum gap frequency unchanged. It strongly suggests an anisotropic superconducting gap. In the underdoped crystal, the anomalous peak around 400 cm-1 which appears in the limited doping and temperature ranges almost disappears by 1.3% Zn-substitution. Considering that the reported strong Zn-effect on T1-1-behavior, this spectral anomaly could be related to the spin gap formation. On the other hand, the low frequency conductivity does not show a remarkable change by Zn-substitution, which indicates less effect of Zn on the interlayer hopping along the c-axis.
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