Detection of a two phonon process in a cuprate superconductor via polarimetric RIXS

Recent advances in Resonant Inelastic X-ray Scattering (RIXS), such as increased energy resolution near the Cu-L3 edge, have enabled the study of the electron-phonon coupling associated with the bond-stretching mode at ~70meV in underdoped cuprate superconductors and, in particular, the mode’s coupling to the electronic charge order. In this work, we detect and investigate an unexplored mode at ~140meV in an underdoped Bi₂Sr₂CaCu₂O_8+δ cuprate superconductor. The mode’s occurrence at approximately twice the energy of the single phonon hints at a two-phonon process. However, the 140meV energy scale is similar to the charge order gap reported from Raman scattering and could also be the result of excitations across the pseudogap. Utilizing high energy resolution momentum mapping and polarimetric RIXS, we demonstrate that this mode originates from a two-phonon process.