Charge density fluctuations uncover the relevance of electron-phonon coupling in cuprate superconductors

Electron–phonon coupling (EPC) is a key interaction governing lattice dynamics, charge transport, and collective electronic phases in quantum materials. In several families of unconventional superconductors, including transition-metal dichalcogenides and kagome metals, growing evidence points to a cooperative role of EPC and dynamic charge density fluctuations (CDF) in stabilizing superconductivity. Yet, how the strength of EPC evolves across phase diagrams and relates to superconducting properties in strongly correlated systems remains an open question. Here we investigate the interplay between phonons and the CDF recently identified in cuprate superconductors. Using resonant inelastic x-ray scattering, we track the dispersion and intensity of bond-stretching phonons in YBa 2 Cu 3 O 7− δ over wide ranges of doping, temperature, and momentum. We observe that both the phonon softening at the CDF wave vector and the effective charge–lattice response probed by the RIXS phonon intensity are maximized near p = 0.19, where superconducting properties are optimal and CDF intensity is strongest. These results identify dynamic charge density fluctuations, rather than static charge density waves, as the dominant source of phonon renormalization in cuprates, and establish a direct correlation between the EPC-related phonon renormalization and the superconducting dome. More broadly, our measurements emphasize EPC as a doping dependent property of correlated materials, shaped by the electronic environment in which lattice vibrations are embedded.