Charge density waves (CDWs) are established as a universal feature in the phase diagram of the cuprates, yet their origin and relation to superconductivity (SC) remain unclear. The electron-phonon interaction (EPI), pivotal to both conventional CDWs and SC, holds here an enigmatic role. Pronounced anomalies in the low temperature phonon spectra constitute experimental manifestations of the EPI, however a rigorous understanding of these observations is missing.

In this talk, investigations of the phonon dispersion and EPI in YBa₂Cu₃O_6+x, a cuprate with minimal chemical disorder, will be presented through the lens of inelastic x-ray scattering. Low energy spectral weight appears in the phonon spectra at low temperatures and rapidly grows in the superconducting state [1]. The spectral changes are observed only in reciprocal space periodicities describing the CDW modulations. Surprisingly, these are strongest at the wave vector of the long-ranged 3D-CDW phase appearing when SC is suppressed with field or strain [2,3], despite the fact that this is absent in the measurement conditions. We propose that these observations are rooted in a hybridization between phonons and collective CDW excitations, reflected in the lattice dynamics even in the absence of long-range order and boosted by the emergence of SC. Insights offered by the magnetic field and uniaxial pressure dependence will be discussed, demonstrating further the deeply original interplay of CDW and SC in the cuprates.

[1] S.M. Souliou et al., Commun. Phys 8, 362 (2025)

[2] S. Gerber et al., Science 350, 949-952 (2015)

[3] H.-H. Kim et al., Science 362, 1040-1044 (2018)