A new collective mode in YBCO observed by time-domain reflectometry

TL;DR

This paper reports the discovery of a new collective mode in YBCO related to charge density wave order, observed via time-domain reflectometry, revealing interactions between CDW and superconducting states.

Contribution

It introduces a novel collective mode in YBCO and models its behavior as a competition between superconducting and charge density wave orders.

Findings

Oscillations at 1.87 THz appear below 105-130 K.

Oscillation amplitude increases and phase shifts below T_c.

Frequency softens by approximately 7% below T_c.

Abstract

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.