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

TL;DR

This paper critically examines recent $^{17}$O NMR measurements in cuprates, demonstrating that the assumed form of the spin susceptibility $ ext{chi}'(q)$ significantly affects the inferred temperature dependence of the correlation length $ ext{xi}$, favoring a Lorentzian over a Gaussian form.

Contribution

It clarifies the impact of the assumed $ ext{chi}'(q)$ form on the interpretation of NMR data and shows that a Lorentzian form aligns with experimental results, unlike a Gaussian.

Findings

Gaussian form implies unphysical T-increasing $ ext{xi}$

Lorentzian form indicates $ ext{xi}$ decreases with T

Data supports T-dependent $ ext{xi}$ consistent with Lorentzian

Abstract

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 $\chi'(q)$. They analyzed the $^{17}$O line width by assuming a Gaussian form for $\chi'$, and came to the conclusion that the correlation length $\xi$ is independent of temperature $T$. Furthermore, they claim that a Lorentzian form of $\chi'$ yields the same result. We will demonstrate that the Lorentzian and Gaussian form of $\chi'$ actually yield qualitatively different results for the $^{17}$O line width and its dependence on $\xi$. We show that their data when combined with $T_{2G}$ data from Takigawa require that $\xi$ be T-dependent. Furthermore, assuming a Gaussian form of $\chi'$ leads to the unphysical result that $\xi$ increases with $T$. In contrast, the Lorentzian form of $\chi'$ yields that $\xi$ decreases with increasing $T$ and is thus fully compatible with the experimental results.