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.

高温超导电性（High-Temperature Superconductivity, HTS）的研究历程近日因配对密度波（Pair Density Wave, PDW）序的发现而得到新的充实。这种以有限动量库珀对为特征的新兴序，于2016年首次通过扫描隧道显微镜（Scanning Tunneling Microscope, STM）直接观测到。
理论预言与STM实验结果一致，即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Q=Q_CDW/2的调制信号。
近期一项针对欠掺杂YBCO的共振非弹性X射线散射（Resonant Inelastic X-ray Scattering, RIXS）测量似乎解决了这一矛盾：通过从背景中分离出RIXS谱沿(H,0)方向的准弹性贡献，我们同时观测到了Q_CDW≈0.32倒易晶格单位（reciprocal lattice units, rlu）处的CDW峰，以及Q≈Q_CDW/2处的第二个峰。
本研究拟利用RIXS对YBCO薄膜中的该第二峰开展温度T与掺杂浓度p依赖的研究。通过分析该峰随T和p的演化规律，我们将能够判断该峰是否为长期以来备受追寻的PDW峰，并有望揭示其与CDW及超导电性（Superconductivity, SC）态之间的关联。