Phonon Signature of Charge Inhomogeneity in High Temperature   Superconductors YBa2Cu3O{6+x}
  Temperature and composition dependences of high-energy longitudinal optical (LO) phonons in YBa2Cu3O{6+x}, studied by inelastic neutron scattering measurements, provide clear evidence for temperature dependent spatial inhomogeneity of doped charges. The measurements indicate that charge doping increases the volume fraction of the charged regions, while the local charge density remains unchanged. For the optimally doped sample the charge distribution changes sharply near the superconducting transition temperature, with stronger charge inhomogeneity below Tc. The remarkable magnitude of the phonon response to charge suggests strong involvement of phonons in charge dynamics. 
Y. Petrov, T. Egami, R. J. McQueeney, M. Yethiraj, H. A. Mook, and F.   Dogan