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Lattice dynamical signature of charge density wave formation in underdoped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>YBa</mml:mtext><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mtext>Cu</mml:mtext><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>6</mml:mn><mml:mo>+</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>
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 …