Giant phonon anomalies and central peak due to charge density wave formation in YBa₂Cu₃O_6.6

We have used inelastic x-ray scattering to monitor low-energy phonons in YBa₂Cu₃O_6.6. Phonons in a narrow range of momentum space around the Charge Density Wave ordering vector exhibit extremely large superconductivity-induced renormalizations. This implies, that the electron-phonon interaction is sufficient to generate various anomalies in electronic spectra, but does not contribute significantly to Cooper pairing. In addition, a "central peak" suggests that the gradual onset of a spatially inhomogeneous CDW domain with decreasing temperature is a generic feature of the underdoped cuprates.