[论文解读] The s$^\pm$-Wave Pairing and the Destructive Role of Apical-Oxygen Deficiencies in La$_3$Ni$_2$O$_7$ Under Pressure

Yubo Liu, Jia‐Wei Mei+3 more|PubMed|Jul 19, 2023

Magnetic and transport properties of perovskites and related materials参考文献 19被引用 20

一句话总结

本文使用多轨道RPA（包括用于缺陷的实空间RPA）在压力下证明La3Ni2O7中的s±波配对，伽玛口袋对嵌嵌至关重要，并且证明顶氧缺陷会诱发局部磁矩，抑制Tc。

ABSTRACT

Recently, the bilayer perovskite nickelate La_{3}Ni_{2}O_{7} has been reported to show evidence of high-temperature superconductivity (SC) under a moderate pressure of about 14 GPa. To investigate the superconducting mechanism, pairing symmetry, and the role of apical-oxygen deficiencies in this material, we perform a random-phase approximation based study on a bilayer model consisting of the d_{x^{2}-y^{2}} and d_{3z^{2}-r^{2}} orbitals of Ni atoms in both the pristine crystal and the crystal with apical-oxygen deficiencies. Our analysis reveals an s^{±}-wave pairing symmetry driven by spin fluctuations. The crucial role of pressure lies in that it induces the emergence of the γ pocket, which is involved in the strongest Fermi-surface nesting. We further found the emergence of local moments in the vicinity of apical-oxygen deficiencies, which significantly suppresses the T_{c}. Therefore, it is possible to significantly enhance the T_{c} by eliminating oxygen deficiencies during the synthesis of the samples.

研究动机与目标

• 确定在加压下La3Ni2O7的配对机制与对称性。
• 确定压力如何通过费米面嵌套和γ口袋促成超导。
• 评估顶氧缺陷对Tc和超导的影响。
• 解释双层镍酸盐中主导配对的轨道贡献。

提出的方法

• 从DFT构建双层两轨道（d_x2-y2, d3z2-r2）紧毗宾模型。
• 应用多轨道RPA以获得重整化的自旋和电荷易变性。
• 通过线性化的缝隙方程和最大的特征值λ 来确定主导配对通道。
• 使用实空间RPA研究顶氧缺陷随机分布的系统。
• 分析压力下γ口袋的出现如何影响费米面嵌套与配对。
• 将配对对称性与费米面囊上的符号结构（s±）联系起来。

Figure 1: (Color online) (a) Band structure along the high symmetry lines. (b) FS in the first Brillouin zone. The three pockets are labeled by $\alpha,\beta$ and $\gamma$ , respectively. The colors in (a) and (b) indicate the orbital component.

实验结果

研究问题

• RQ1 在中等压力下La3Ni2O7 的配对机制及最终对称性是什么？
• RQ2 压力诱导的γ口袋如何影响费米面嵌套和Tc？
• RQ3 顶氧缺陷对磁性涨落和超导性的影响？
• RQ4 双层镍酸盐中哪种轨道特征主导超导配对？

主要发现

• 在原始晶体中，自旋涨落偏好纯净晶体中的s±波配对对称性。
• γ口袋在压力下出现并参与最强的费米面嵌套，增强超导。
• 主要配对涉及层间d3z2−r2轨道，具有显著的轨道贡献引导间隙结构。
• α和γ口袋的超导间隙符号与β口袋相反，与s±配对一致。
• 顶氧缺陷诱发局部磁矩，抑制Tc，降低临界作用强度Uc^(s)，并且在现实δ 下对典型U可能导致超导被破坏。
• 实空间RPA显示Tc 抑制随缺陷浓度δ 增大而增强，在所选U=1.16 eV时，大约δ>3%时Tc几乎消失。

Figure 2: (Color online) (a) The distribution of the largest eigenvalue $\chi(\mathbf{q})$ of the RPA-renormalized spin susceptibility matrix in the Brillouin zone. This distribution peaks at three unequivalent momenta, which are marked as $\mathbf{Q}_{1}$ , $\mathbf{Q}_{2}$ , and $\mathbf{Q}_{3}$ r