  We report a detailed Raman scattering study of the lattice dynamics in
detwinned single crystals of the underdoped high temperature superconductor
YBa2Cu3O6+x (x=0.75, 0.6, 0.55 and 0.45). Whereas at room temperature the
phonon spectra of these compounds are similar to that of optimally doped
YBa2Cu3O6.99, additional Raman-active modes appear upon cooling below ~170-200
K in underdoped crystals. The temperature dependence of these new features
indicates that they are associated with the incommensurate charge density wave
state recently discovered using synchrotron x-ray scattering techniques on the
same single crystals. Raman scattering has thus the potential to explore the
evolution of this state under extreme conditions.
