## A new collective mode in YBCO observed by time-domain reflectometry    [PDF]

J. P. Hinton, J. D. Koralek, Y. M. Lu, A. Vishwanath, J. Orenstein, D. A. Bonn, W. N. Hardy, Ruixing Liang
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 K 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~7%. A bi-quadratically coupled Landau-Ginzburg model qualitatively describes this behavior as arising from competition between superconducting and CDW orders.
View original: http://arxiv.org/abs/1305.1361