Doping-dependent photon scattering resonance in the model high-temperature superconductor HgBa₂CuO_4+δ revealed by Raman scattering and optical ellipsometry

We study the model high-temperature superconductor HgBa₂CuO _4+δ with electronic Raman scattering and optical ellipsometry over a wide doping range. The dependence of the resonant Raman cross section on the incident photon energy changes drastically as a function of doping, in a manner that corresponds to a rearrangement of the interband optical transitions seen with ellipsometry. This doping-dependent Raman resonance allows us to reconcile the apparent discrepancy between Raman and x-ray detection of magnetic fluctuations in superconducting cuprates. Intriguingly, the strongest variation occurs across the doping level where the antinodal superconducting gap appears to reach its maximum.