Effect of Ni impurities on the optical properties ofYBa2Cu3O6+y

The optical properties of twinned single crystals of ${\mathrm{YBa}}_2({\mathrm{Cu}}_{1-x}{\mathrm{Ni}}_x{)}_3{\mathrm{O}}_{6+y}$ have been determined for a nominally pure system and for Ni concentrations of $x=0.0075\mathrm{}$ and $0.014$ with oxygen dopings of $y=0.60\mathrm{}$ (underdoped) and 0.95 (optimal) over a wide frequency range above and below $T_c.$ The optical conductivity of the twinned materials shows a rapid increase in the residual conductivity at low frequency with increasing Ni concentration, and an unusual feature appears below $T_c$ at $\approx 300\hspace{0.5em}{\mathrm{cm}}^{-1}\hspace{0.5em}\left(37\mathrm{}\hspace{0.5em}\mathrm{meV}\right),$ which is absent in the pure system and is not sensitive upon oxygen content. In a detwinned crystal of the optimally doped system this Ni-induced feature is observed only along the b axis, suggesting that Ni is doping into the chains. Since the feature is observed in the normal state, it is not associated with the superconducting transition. The free-carrier response in the ${\mathrm{CuO}}_2$ planes is quite different in the optimally doped and underdoped materials. The presence of Ni in these materials has little effect upon the in-plane dynamics, but the low-frequency conductivity along just the chains is destroyed. This suggests that Ni acts either as a localization site, or an impurity scatterer and strong pair breaker, destroying the superconductivity along the chains and strongly reducing the anisotropy of the system at low frequency. The feature at $\approx 300\hspace{0.5em}{\mathrm{cm}}^{-1}$ along the chain direction is most likely due to the Ni impurities acting as strong scattering barriers, resulting in thermally activated hopping. However, the proximity of this feature to the estimated value of the c-axis pseudogap in the underdoped materials also suggests that strong scattering out of the chains may be coupling to the c-axis dynamics.