New collective mode in YBa2Cu3O6+x observed by time-domain reflectometry

We report the observation of coherent oscillations associated with charge-density-wave (CDW) order in the underdoped cuprate superconductor YBa2Cu3O6+x by time-resolved optical reflectivity. Oscillations with frequency 1.87 THz onset at approximately 105 and 130 K for dopings of x = 0.67 (ortho-VIII) and x = 0.75 (ortho-III), respectively. Upon cooling below the superconducting critical temperature (T-c), the oscillation amplitude is enhanced, the phase shifts by pi, and the frequency softens by delta nu/nu approximate to 7%. A biquadratically coupled Landau-Ginzburg model qualitatively describes this behavior as arising from competition between superconducting and CDW orders.