New insight into high-Tc cuprates offered by photon scattering

High-Tc cuprate superconductors are well-known for their antiferromagnetism inherited from the Mott-insulating parent compounds, and for their variety of symmetry-broken phases that develop upon charge-carrier doping. Visible (Raman) and invisible (X-ray) photon scattering experiments, in particular those that utilize resonant enhancement, allow us to probe ordering tendencies and dynamic correlations with excellent sensitivity. In this talk, I will present two recent studies that apply these combined methods to the first two members of the Hg-family of cuprates, Hg1201 and Hg1212. In the first study, we demonstrate that short-range charge order exists in both Hg1201 and Hg1212. The most robust charge order is found at an underdoping level that is prone to strong variations in both electronic and lattice degrees of freedom, implying that the charge order is an emergent phenomenon driven by a more fundamental instability. In the second study, we reveal a "magnon isotope effect" on the superconductivity, by showing that the single- and bi-magnon energies undergo significant increases from Hg1201 to Hg1212 that correlate with the rise in the superconducting gap energy and Tc. Together with superconductivity’s feedback on the spin excitations, these results support a magnetic pairing mechanism.