ICQM member Yuan Li publishes an article in Physical Review Letters resolving a major debate in the high-Tc cuprates
High-Tc superconductivity in the copper-oxide materials ('cuprates') is widely believed to be 'magnetically driven'. Almost every experimental technique available to condensed matter physicists has been applied to these materials to unveil the magnetism therein. Along with the advancement of the resonant inelastic x-ray scattering (RIXS) technique in recent years, a debate has arisen concerning its observation of strong magnetic excitations throughout the phase diagram, which is in contrast with the lack of similar signals seen by Raman and neutron scattering at high doping. Whether or not the signal observed by RIXS at high doping represents true magnetic excitations is crucial for understanding the superconducting mechanism, because the lack of such excitations would imply that the superconductivity may, at least in part, not be so different from the familiar BCS type.
In the article 'Doping-Dependent Photon Scattering Resonance in the Model High-Temperature Superconductor HgBa2CuO4+d Revealed by Raman Scattering and Optical Ellipsometry', Dr. Yuan Li and his collaborators report a systematic study of a model high-Tc compound using both Raman scattering and optical ellipsometry. The former technique probes magnetic excitations using inelastic scattering of light, and the latter measures the coupling between light and electronic transitions in materials. The combined data set indicates that the widely accepted picture, that magnetic-excitation signals seen by Raman scattering would vanish at high doping, is primarily due to a loss of resonance enhancement at high doping, where optically active electronic transitions occur only at rather different energies than at low doping. In fact, the magnetic signal can be recovered throughout the phase diagram by choosing proper incident photon wavelengths that match the doping evolution of the electronic transitions, as confirmed by their experiments. This resolves the discrepancy between existing RIXS and Raman scattering results, lending more credibility to the magnetically-driven-superconductivity viewpoint.