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Comment on: ``Using Ni Substitution and $^{17}$O NMR to Probe the Susceptibility $χ'(q)$ in Cuprates''

In a recent letter (PRL 79, 2117 (1997)), Bobroff et al. presented novel $^{17}$O NMR measurements for YBa_2(Cu_{1-x}Ni_x)_3O_{6+y} which offer a new probe of the momentum dependence of the static spin susceptibility $χ'(q)$. They analyzed the $^{17}$O line width by assuming a Gaussian form for $χ'$, and came to the conclusion that the correlation length $ξ$ is independent of temperature $T$. Furthermore, they claim that a Lorentzian form of $χ'$ yields the same result. We will demonstrate that the Lorentzian and Gaussian form of $χ'$ actually yield qualitatively different results for the $^{17}$O line width and its dependence on $ξ$. We show that their data when combined with $T_{2G}$ data from Takigawa require that $ξ$ be T-dependent. Furthermore, assuming a Gaussian form of $χ'$ leads to the unphysical result that $ξ$ increases with $T$. In contrast, the Lorentzian form of $χ'$ yields that $ξ$ decreases with increasing $T$ and is thus fully compatible with the experimental results.