[论文解读] Charge and spin order dichotomy in NdNiO$_2$ driven by SrTiO$_3$ capping layer

Krieger, G., Martinelli, L.+11 more|arXiv (Cornell University)|Dec 6, 2021

Chemical and Physical Properties of Materials被引用 7

一句话总结

本研究揭示，NdNiO₂薄膜中的SrTiO₃顶盖层可抑制电荷序并稳定磁激发态，而未覆盖的薄膜则在(1/3, 0)波矢处表现出明显的电荷序峰，并伴随Ni-3d/Nd-5d杂化增强。顶盖层的存在抑制了电荷序并促进了磁序相干性，表明界面化学——可能涉及氢的引入——在调控无限层镍氧化物中电子相态方面起着关键作用。

ABSTRACT

Superconductivity in infinite-layer nickelates holds exciting analogies with that of cuprates, with similar structures and $3d$-electron count. Using resonant inelastic x-ray scattering (RIXS) we studied electronic and magnetic excitations and charge density correlations in Nd$_{1-x}$Sr$_{x}$NiO$_2$ thin films with and without an SrTiO$_3$ capping layer. We observe dispersing magnons only in the capped samples, progressively dampened at higher doping. In addition, we detect an elastic resonant scattering peak in the uncapped $x=0$ compound at wave vector (1/3,0), remindful of the charge order signal in hole doped cuprates. The peak weakens at $x=0.05$ and disappears in the superconducting $x=0.20$ film. The uncapped samples also present a higher degree of Ni$3d$-Nd$5d$ hybridization and a smaller anisotropy of the Ni$3d$ occupation with respect to the capped samples. The role of the capping on the possible hydrogen incorporation or on other mechanisms responsible for the electronic reconstruction far from the interface remains to be understood.

研究动机与目标

• 研究SrTiO₃顶盖层对Nd₁₋ₓSrₓNiO₂薄膜中电子序与磁序的影响。
• 确定界面化学（特别是氢的引入）在调控电荷与自旋关联中的作用。
• 阐明未覆盖NdNiO₂薄膜中电荷序的起源及其在覆盖样品中的抑制机制。
• 评估Ni-3d/Nd-5d杂化在无限层镍氧化物中对自旋与电荷序的影响。
• 评估顶盖层是否改变拓扑还原动力学，从而影响电子重构。

提出的方法

• 在Ni L₃边进行共振非弹性X射线散射（RIXS），以探测Nd₁₋ₓSrₓNiO₂薄膜中的自旋与电荷激发。
• 采用掠入射几何（10°入射角）以增强表面灵敏度，并检测弹性散射特征。
• 在x = 0.00、0.05和0.20掺杂水平下，对比STO覆盖与未覆盖样品，以分离顶盖层的影响。
• 分析能量损失与动量分辨的RIXS图谱，以识别色散磁激发态与电荷序峰。
• 将实验RIXS数据与NdNiO₂H及NdNiO₂的DFT计算结果相关联，评估氢引入的影响。
• 通过Nd M₅边RIXS研究，探测Nd-5d态对电荷调制的间接贡献。

Figure 1: Summary of the RIXS results for uncapped and STO capped NSNO(0) films. (a,c): energy-loss/in-plane-momentum scattering intensity maps along the high symmetry directions indicated in the insets, excited at incident photon energy $\sim$ 852.5 eV (Ni 1+ XAS peak) using $\pi$ polarization. (b,

实验结果

研究问题

• RQ1 SrTiO₃顶盖层如何影响NdNiO₂薄膜中电荷序的出现？
• RQ2 Ni-3d/Nd-5d杂化在未覆盖NdNiO₂中稳定电荷序方面起何作用？
• RQ3 为何仅在STO覆盖的NdNiO₂薄膜中观察到色散磁激发态，而未覆盖样品中未观测到？
• RQ4 来自CaH₂还原过程的氢引入在多大程度上影响电荷与自旋序？
• RQ5 覆盖与未覆盖薄膜在拓扑还原过程中的电子重构有何不同？

主要发现

• 在未覆盖的NdNiO₂（x=0）中，于波矢(1/3, 0)处观察到明显的弹性共振散射峰，表明其电荷序特征与空穴掺杂铜氧化物相似。
• 该电荷序峰在x=0.05时减弱，并在超导态的x=0.20薄膜中完全消失，表明其与超导性存在竞争关系。
• 色散磁激发态仅在STO覆盖样品中被检测到，且其强度随掺杂浓度升高而逐渐衰减。
• 未覆盖样品表现出更强的Ni-3d/Nd-5d杂化，且Ni-3d轨道占据的各向异性低于覆盖样品。
• 电荷序峰与Ni 1+ XAS能量共振，表明3d⁸R态与电荷调制之间存在关联。
• Nd M₅边RIXS证实了Nd-5d态参与电荷调制，支持稀土态在电荷序中的作用。

Figure 2: (a) Quasi-elastic scattering intensity vs. the in-plane transferred momentum along the (10) direction for different doping in uncapped NSNO(x). Data acquired at 20 K and grazing incidence with $\pi$ polarization. Corresponding data along the (11) direction (in orange) is presented for comp