The role of high-energy optical phonons in cuprate superconductors has recently drawn new attention. While the BCS theory evidently fails to describe the physics of these materials, phonons could assist spin fluctuations in providing the glue for Cooper pairs, significantly enhancing the critical temperature [1]. This proposal has recently been supported by the observation of a strong electron phonon coupling in some cuprates, comparable to the superexchange energy governing the magnetic properties [2,3]. Using Resonant Inelastic X-ray Scattering (RIXS) at the oxygen K-edge we have studied how the coupling between Fermi electrons and the highest-energy phonon branches is affected by the screening of free carriers in NdBa₂Cu₃O_7-x, from the strongly under-doped to the optimally doped region. We have found a strong renormalization of bond-stretching phonons, while bond-bending ones seem to be less affected by the addition of free carriers, in agreement with the theoretical results [1].

[1] Johnston, S., et al. Physical Review B 82 (2010): 064513.
[2] Braicovich, Lucio, et al., Phys. Rev. Research 2 (2020): 023231.
[3] Peng, Y. Y., et al. Phys. Rev. Letters 125 (2020): 097002.