Small tip angle NMR as a probe of electron-mediated nuclear spin-spin couplings in YBa₂Cu₃O₇

We develop the theory and application of small tip angle NMR techniques that can be used to measure couplings between nuclear spins and multiple neighboring spins. We employ the techniques to measure indirect (electron-mediated) nuclear spin-spin couplings between neighboring ^63,65Cu and ¹⁷O nuclear spins in the high-temperature superconductor YBa₂Cu₃O₇. Predictions for the values of these couplings can be obtained from the existing phenomenological models of electronic spin susceptibility x(q,ω) and hyperfine couplings that have been used in attempts to understand the wide body of YBa₂Cu₃O₇ NMR data gathered to date. We find that the measured couplings are incompatible with these models.