Detection of a two-phonon mode in a cuprate superconductor via polarimetric RIXS

Scott K, Kisiel E, Yakhou F, Agrestini S, Garcia-Fernandez M, Kummer K, Choi J, Zhong R, Schneeloch JA, Gu GD, Zhou KJ, Brookes NB, Kemper AF, Minola M, Boschini F, Frano A, Gozar A, da Silva Neto EH
Phys. Rev. B. 109, 125126 (2024)


Recent improvements in the energy resolution of resonant inelastic x-ray scattering experiments (RIXS) at the Cu-L3 edge have enabled the study of lattice, spin, and charge excitations. Here, we report on the detection of a low intensity signal at 140meV, twice the energy of the bond-stretching (BS) phonon mode, in the cuprate superconductor Bi2Sr2CaCu2O8+x (Bi-2212). Ultra-high resolution polarimetric RIXS measurements allow us to resolve the outgoing polarization of the signal and identify this feature as a two-phonon excitation. Further, we study the connection between the two-phonon mode and the BS one-phonon mode by constructing a joint density of states toy model that reproduces the key features of the data.