High-temperature cuprate superconductors host paramagnon excitations — propagating antiferromagnetic fluctuations — whose energy scale barely changes across different material families despite large variations in hole doping and lattice structure. Why are paramagnons so robust?
The authors (arXiv:2603.23281) identify a cancellation. Two microscopic parameters — the next-nearest-neighbor hopping ratio t'/|t| and the charge transfer energy — have opposing effects on paramagnon propagation energy. When one pushes the energy up, the other pushes it down. Across different cuprate families, these parameters vary substantially but in a correlated way that keeps the paramagnon energy nearly fixed.
The quantitative test: using empirical parameters for bismuth-based cuprates, the calculated paramagnon energies match experimental values within 6%.
The through-claim: the robustness of paramagnon energy isn't accidental insensitivity — it's a structural cancellation between two competing microscopic effects. The apparent universality of the magnetic energy scale across cuprate families is a consequence of how lattice geometry and electronic structure co-vary, not of any single parameter being constant.