Feedback Effect on High-Energy Magnetic Fluctuations in the Model High-Temperature Superconductor HgBa2CuO4+δ Observed by Electronic Raman Scattering

We use electronic Raman scattering to study the model single-layer cuprate superconductor HgBa2CuO4+delta. 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.