  We use electronic Raman scattering to study the model single-layer cuprate
superconductor HgBa2CuO4+d. 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 Tc. 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
consequence of feedback of the Cooper pairing interaction on the high-energy
magnetic fluctuations. All of these effects occur already above Tc in two
underdoped samples, demonstrating a related feedback mechanism associated with
the pseudogap.
