[论文解读] Electronic structure of compressively strained thin film La$_2$PrNi$_2$O$_7$

Bai Yang Wang, Yong Zhong+17 more|ArXiv.org|Apr 23, 2025

Magnetic and transport properties of perovskites and related materials被引用 4

一句话总结

本地在位ARPES研究考察了压缩应变的La2PrNi2O7薄膜，比较实验带结构偏移与DFT预测，并揭示强重整化的3d_z2带和3d_x2-y2的非平凡kz色散，对应变和压强对镍酸盐超导性的影响具有启示意义。

ABSTRACT

The discovery of superconductivity in the bulk nickelates under high pressure is a major advance in physics. The recent observation of superconductivity at ambient pressure in compressively strained bilayer nickelate thin films has now enabled direct characterization of the superconducting phase through angle resolved photoemission spectroscopy (ARPES). Here we present an in-situ ARPES study of compressively strained La$_2$PrNi$_2$O$_7$ films grown by oxide molecular beam epitaxy, and the ozone treated counterparts with an onset T$_c$ of 40 K, supplemented with results from pulsed laser deposition films with similar T$_c$. We resolve a systematic strain-driven electronic band shift with respect to that of bulk crystals, in qualitative agreement with density functional theory (DFT) calculations. However, the strongly renormalized flat 3$d_{z2}$ band shifts a factor of 5-10 smaller than anticipated by DFT. Furthermore, it stays ~70 meV below the Fermi level, contradicting the expectation that superconductivity results from the high density of states of this band at the Fermi level. We also observed a non-trivial k$_z$ dispersion of the cuprate-like 3$d_{x2-y2}$ band. Combined with results from both X-ray diffraction and DFT, we suggest that the strained films are under ~5 GPa effective pressure, considerably larger than the naïve expectation from the DFT relaxed structure. Finally, the ~70 meV energy position is intriguingly close to the collective mode coupling more prominently seen in thin films, in the energy range of both oxygen related phonons and the maximum of the spin excitation spectrum.

研究动机与目标

• 研究压缩应变如何影响镍酸盐薄膜的电子结构及其与超导性的关系。
• 在应变条件下表征能带偏移并与DFT预测进行比较。
• 评估Ni 3d轨道带（dz2和dx2-y2）在与超导潜力相关的作用。
• 从结构与电子数据推断应变薄膜中的有效压力。

提出的方法

• 在氧化物分子束外延生长的La2PrNi2O7薄膜及经臭氧处理的对照样品上进行原位ARPES测量。
• 将观测到的能带偏移与密度泛函理论(DFT)计算进行比较。
• 分析cuprate样的3d_x2-y2带的kz色散。
• 将X射线衍射数据与DFT结合以推断应变/压力效应。
• 将约70 meV的能量位置与可能的集体模耦合联系起来。

实验结果

研究问题

• RQ1 压缩应变如何相对于体相晶体改变La2PrNi2O7的电子能带？
• RQ2 强烈重整化的3d_z2带是否符合或与这些薄膜的超导性预期背离？
• RQ3 应变下3d_x2-y2带的kz色散的性质是什么？
• RQ4 应变薄膜经历的有效压力是多少，与观测到的电子结构变化有何关系？
• RQ5 近70 meV的能量位置是否与对超导性相关的声子或自旋激发模相关？

主要发现

• 压缩应变的La2PrNi2O7薄膜相对于体相晶体显示出系统性的应变驱动的能带偏移， qualitatively与DFT一致。
• 强烈重整化的3d_z2带比DFT预测的偏移小5–10倍，且位于费米能级约70 meV以下。
• 3d_x2-y2带表现出与CuO2型特征相符的非平凡kz色散。
• 综合XRD与DFT数据表明薄膜承受约5 GPa的有效压力，超过简单的松弛结构估计。
• 约70 meV的能量位置接近与氧相关的声子及自旋激发谱的能量范围，暗示可能的耦合。