Zn and Ni doping effects on Tc and spin gap behavior in YBa 2Cu 4O 8 and YBa 2Cu 3O 6.6

Abstract

The existence of local moments in both Zn- and Ni-doped YBa 2Cu 4O 8 (Y1248) is confirmed by a Curie-law T-dependence of NMR linewidth and the Cu relaxation at low T. The reduction of Cu-NQR signal intensity in Ni-doped Y1248 suggests that, in contrast to the case of Ni-doped YBa 2Cu 3O 7 (Y1237), the Cu spin correlation around Ni is destroyed. This effect may be responsible for the larger reduction of T c by Ni-doping than in Y1237. For Cu sites which are away from an impurity by two lattice constants, 1/ T 1 T continues to increase below T ∗ , at which it has a maximum in the non-doped material. The effect in reducing T ∗ , which is sometimes called the spin gap temperature, is much more pronounced in Zn-doped samples, suggesting that the reduction of T ∗ is mainly due to the induction of magnetic moments in the d x 2− y 2 orbital, which may modify Cu spin fluctuation.