Feedback effect on high-energy magnetic fluctuations in the model high-temperature superconductor HgBa ₂CuO _4+δ observed by electronic Raman scattering

We use electronic Raman scattering to study the model single-layer cuprate superconductor HgBa ₂CuO _4+δ. In an overdoped sample, we observe a pronounced amplitude enhancement of a high-energy peak related to two-magnon excitations in insulating cuprates upon cooling below the critical temperature T _c. This effect is accompanied by the appearance of the superconducting gap and a pairing peak above the gap in the Raman spectrum, and it can be understood as a hitherto-undetected feedback effect on the high-energy magnetic fluctuations due to the Cooper pairing interaction. This implies a direct involvement of the high-energy magnetic fluctuations in the pairing mechanism. All of these effects occur already above T _c in two underdoped samples, demonstrating a related feedback mechanism associated with the pseudogap.