Looking for pair density waves in YBa2Cu3O7-x via RIXS

The saga of HTS has been recently enriched by the detection of the pair density wave (PDW) order. Such emerging order characterized by Coopers pairs with finite momentum has been directly observed for the first time by STM in 2016. In agreement with theory, STM experiments determined the PDW wavelength to be twice that of CDW. However a modulation with a wavevector Q=Q_CDW/2 has not been observed by X-ray scattering experiments so far. A recent RIXS measurement on underdoped YBCO seems to solve this contradiction: by isolating the quasi-elastic contribution of the RIXS spectra along the (H,0) direction from the background, we revealed both a CDW peak at Q_CDW≈0.32 rlu and a second peak at Q≈Q_CDW/2. Here we propose to study with RIXS the latter peak, as function of temperature T and doping p, in YBCO thin films. The T and p evolution of the peak will tell us if this peak is the long-searched PDW peak, possibly giving hints on its connection with the CDW and the SC states.

高温超导体（High Temperature Superconductor，HTS）的研究历程近日因对密度波（Pair Density Wave，PDW）序的探测而得到拓展。这种以有限动量库珀对（Cooper Pair）为特征的新兴序，已于2016年首次通过扫描隧道显微镜（Scanning Tunneling Microscope，STM）直接观测到。与理论预言一致，STM实验测得PDW的波长为电荷密度波（Charge Density Wave，CDW）的两倍。但迄今为止，X射线散射实验尚未观测到波矢满足Q=Q_CDW/2的调制信号。近期一项针对欠掺杂钇钡铜氧（Yttrium Barium Copper Oxide，YBCO）的共振非弹性X射线散射（Resonant Inelastic X-ray Scattering，RIXS）测量似乎化解了这一矛盾：通过从背景信号中分离出RIXS谱沿(H,0)方向的准弹性贡献，我们同时观测到了位于Q_CDW≈0.32相对晶格单位（reciprocal lattice units，rlu）处的CDW峰，以及另一个位于Q≈Q_CDW/2处的次级峰。本研究拟利用RIXS对YBCO薄膜中的该次级峰展开研究，探究其随温度T与掺杂浓度p的演化规律。通过分析该峰的温度与掺杂依赖行为，我们将得以判断其是否为长期以来学界追寻的PDW峰，并由此为其与CDW及超导（Superconductivity，SC）态之间的关联提供线索。