  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 4meV
below Tc in the antiferromagnetic(AF) 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 magnetic signals at 3meV decrease below
Tc and increase 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 then exponentially
increase. As for Ni:y=0.009 and Ni:y=0.018, the low-energy excitations below
3meV and 2meV disappear below Tc. The temperature dependence at 3meV, however,
shows no upturn in constrast with Zn:y=0.008 and Zn:y=0.011, indicating the
absence of in-gap state. In Ni:y=0.029, the magnetic signals were observed also
at 0meV. Thus the spin gap closes with increasing Ni. Furthermore, as omega
increases, the magnetic peak width broadens and the peak position, i.e.
incommensurability, shifts toward the magnetic zone center (pi pi). We
interpret the impurity effects as follows: Zn locally makes a
non-superconducting 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.
