Why does Ni suppress superconductivity in La_1.85Sr_0.15Cu_1−y Ni _y O₄?

Abstract

The role played by Ni impurities in destroying the superconducting state of La2_~Sr~CuO4 is still unclear.This is partly due to the lack of clear experimental evidence on the effect of Ni substitution upon the atomic and electronic structure of this cuprate.To provide this information, we performed Ni /(-edge polarized XAFS measurements on oriented powder.A special magnetic alignment geometry allowed us to measure pure ~ and aAb-polarized XAFS in identical experimental conditions.We found that the NiO6 octahedra are contracted along the t-axis by 0.16(A) relative to the CuO6 octahedra in Lal.~sSro.~sCuO4,dragging La atoms and causing the observed macroscopic t-axis contraction.This contraction reflects the non-Jahn-Teller 3d a state of Ni in this compound (similar to that in La2NiO4) and indicates that Ni does not "trap" hole carders but rather, probably, "repels" them.We suggest that this repulsion is the mechanism behind T~ suppression.