Zn and Ni doping effects on the low-energy spin excitations in La1.85Sr0.15CuO4

Impurity effects of Zn and Ni on the low-energy spin excitations were systematically studied in optimally doped La1.85Sr0.15Cu1-yAyO4 (A=Zn,Ni) by neutron scattering. Impurity-free La1.85Sr0.15CuO4 shows a "spin gap" of ω=4meV below Tc in the antiferromagnetic incommensurate spin excitation. In Zn:y=0.004, the spin excitation shows a spin gap of 3meV below Tc. In Zn:y=0.008 and Zn:y=0.011, however, the dynamical susceptibility χ″ at ω=3meV decreases below Tc and increases again at lower temperature, indicating an in-gap state. In Zn:y=0.017, the low-energy spin state remains unchanged with decreasing temperature, and elastic magnetic peaks appear below ∼20K and then exponentially grow. These results suggest that Zn induces a novel in-gap spin state, which becomes dominant and more static with increasing Zn. As for Ni:y=0.009 and Ni:y=0.018, the low-energy excitations below ω=3meV and 2meV disappear below Tc. The temperature dependence of χ″ at ω=3meV, however, shows no upturn in contrast with Zn:y=0.008 and Zn:y=0.011, indicating the absence of an in-gap state. In Ni:y=0.029, though the magnetic signals were observed also at ω=0meV, they showed no temperature dependence, implying that there is no static component. Furthermore, as ω increases, the magnetic peak width broadens and the peak position shifts toward the magnetic zone center (ππ). We interpret the impurity effects as follows: Zn locally makes a nonsuperconducting island exhibiting the in-gap state in the superconducting sea with the spin gap. Zn reduces the superconducting volume fraction, thus suppressing Tc. On the other hand, Ni primarily affects the superconducting sea, and the spin excitations become more dispersive and broaden with increasing energy, which is recognized as a consequence of the reduction of energy scale of spin excitations. We believe that the reduction of energy scale is relevant to the suppression of Tc.