Lattice dynamical signature of charge density wave formation in underdoped YBa 2 Cu 3 O 6 + x

We report a detailed Raman scattering study of the lattice dynamics in detwinned single crystals of the underdoped high-temperature superconductor ${\text{YBa}}_{2}{\text{Cu}}_{3}{O}_{6+x}$ ($x=0.75$, 0.6, 0.55, and 0.45). Whereas at room temperature the phonon spectra of these compounds are similar to that of optimally doped YBa${}_{2}$${\text{Cu}}_{3}$O${}_{6.99}$, additional Raman-active modes appear upon cooling below $\ensuremath{\sim}$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 thus has the potential to explore the evolution of this state under extreme conditions.