Paramagnons and high-temperature superconductivity in mercury-based cuprates

High-temperature superconductivity in the cuprates is realized upon doping antiferromagnetic parent compounds, in which spin excitations have a bandwidth of a few hundred meV. At such high energies, paramagnons and two-magnon excitations are known to persist well into and beyond the superconducting doping range [1,2,3], but it remains unclear to what extent they contribute to Cooper pairing [4] or how the magnetic and superconducting energy scales are related to each other. We have used resonant inelastic photon (Raman, X-ray) scattering to study the first two members of the Hg-family of cuprates, HgBa₂CuO_4+d and HgBa₂CaCu₂O_6+d, which have nearly identical crystal structure in the charge-reservoir layers but different electronic environment in the quintessential Cu-O layers. We find that the latter compound, which has higher T_c and larger superconducting gap, also has considerably higher magnetic excitation energies.
[1] Nat. Phys. 7, 725 (2011).
[2] Nat. Mater. 12, 1019 (2013).
[3] Phys. Rev. Lett. 111, 187001 (2013).
[4] Phys. Rev. Lett. 108, 227003 (2012).