Effect of Ni impurities on the optical properties of YBa2Cu3O6+gamma

The optical properties of twinned single crystals of YBa2(Cu1-xNix)(3)O6+y have been determined for a nominally pure system and for Ni concentrations of x=0.0075 and 0.014 with oxygen dopings of y=0.60 (underdoped) and 0.95 ( optimal) over a wide frequency range above and below T-c. The optical condu ctivity of the twinned materials shows a rapid increase in the residual con ductivity at low frequency with increasing Ni concentration, and an unusual feature appears below T-c at approximate to 300 cm(-1) (37 meV), which is absent in the pure system and is not sensitive upon oxygen content. In a de twinned crystal of the optimally doped system this Ni-induced feature is ob served only along the b axis, suggesting that Ni is doping into the chains. Since the feature is observed in the normal stare, it is not associated wi th the superconducting transition. The free-carrier response in the CuO2 pl anes is quite different in the optimally doped and underdoped materials The presence of Ni in these materials has little effect upon the in-plane dyna mics, 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 t he chains and strongly reducing the anisotropy of the system at low frequen cy. The feature at approximate to 300 cm(-1) along the chain direction is m ost likely due to the Ni impurities acting as strong scattering barriers, r esulting in thermally activated hopping. However, the proximity of this fea ture to the estimated value of the c-axis pseudogap in the underdoped mater ials also suggests that strong scattering out of the chains may be coupling to the c-axis dynamics.