Polarimetric Resonant Inelastic X-ray Scattering of a newly discovered mode in a cuprate high-temeprature superconductor.

Resonant Inelastic X-ray Scattering (RIXS) has been a premier tool in the study of cuprate high-temperature superconductors. Spectrometers with increasing energy resolution near the Cu-L3 edge have been able to detect and study several states and excitations below 1eV energy-loss, including magnon, and paramagnon excitations, and static charge order and its dynamic excitations. With the advent of energy resolution better than 35 meV at the Cu L3-edge, it has been possible to study the bond stretching phonon mode at ~70meV and its coupling to the electronic charge order. We recently performed Cu L3 RIXS on a cuprate superconductor and discovered a never before reported mode with sub 1eV energy. There are many possibilities for its origin, including charge, magnetic or lattice excitations. We propose to investigate the physical origin of this mode by leveraging the unique capability of ERIXS to resolve the polarization of the scattered photon.