通过$^{17}$O-NMR揭示双层镍酸盐La$_{3}$Ni$_{2}$O$_{7}$中层间自旋单重态形成与SDW序的轨道选择性共存及反常自旋重构
Orbital-Selective Coexistence of Interlayer Spin-Singlet Formation and SDW Order with Anomalous Spin Reconfiguration in Bilayer Nickelate La$_{3}$Ni$_{2}$O$_{7}$ Revealed by $^{17}$O-NMR
AI总结：
该研究通过$^{17}$O-NMR揭示双层镍酸盐La$_3$Ni$_2$O$_7$中，层间自旋单重态形成与SDW序呈轨道选择性共存，且存在反常自旋重构，深化了对其轨道选择性本质的认识。
AI中文摘要：
利用位点选择性$^{17}$O-NMR测量，对双层镍酸盐La$_3$Ni$_2$O$_7$的内顶点O(1)、外顶点O(2)和平面O(3,4)位点的自旋密度波（SDW）序自旋结构进行了研究。在$T_{\rm SDW}$（=150 K）以下，所有平面O(3,4)位点的峰因有限内磁场的出现而显著展宽，而O(2)位点仍无（或可忽略的小）内磁场，这些结果与具有单一自旋无（或大小自旋）条纹的公度SDW序一致。对于桥接NiO$_2$平面的O(1)位点，内磁场在$T_{\rm SDW}$以下几乎抵消，表明相邻平面间存在反平行自旋构型。然而，在$T_\text{A}$（~115 K）以下，尽管面内SDW序仍保持稳定，O(1)位点的光谱却消失了，这意味着尽管NiO$_2$平面间存在预期的强层间自旋耦合，但通过Ni--O(1)--Ni键的反平行自旋构型在$T_{\rm A}$以下并非特别稳定。最重要的是，我们强调，与$d_{3z^2-r^2}$轨道具有强共价性的O(2)位点处的局部自旋磁化率极小，表明桥接NiO$_2$平面的Ni-$d_{3z^2-r^2}$轨道中存在充分发展的层间自旋单重态形成。这些发现为层间自旋单重态形成及通过连接NiO$_2$平面的Ni--O(1)--Ni键轨道发生的反常自旋重构提供了新认识，这些特征是双层镍酸盐La$_3$Ni$_2$O$_7$的轨道选择性本质的体现。
英文摘要：
The spin structure of the spin density wave (SDW) order in the bilayer nickelate La$_3$Ni$_2$O$_7$ has been investigated using site-selective $^{17}$O-NMR measurements on the inner apical O(1), outer apical O(2), and planar O(3,4) sites. Below $T_{\rm SDW}$ (= 150 K), the peak of all planar O(3,4) sites significantly broadens due to the emergence of a finite internal magnetic field, whereas O(2) sites remain with no (or a negligibly small) internal field. These results are consistent with commensurate SDW order with a single spin-spinless (or large-tiny spin) stripe. As for the O(1) sites that bridge the NiO$_2$ planes, the internal field is nearly canceled below $T_{\rm SDW}$, indicating an antiparallel spin configuration between adjacent planes. However, below $T_\text{A}$ ($\sim$ 115 K), the spectrum of the O(1) site disappears even though the in-plane SDW order remains robust, implying that the antiparallel spin configuration through the Ni--O(1)--Ni bond is not particularly stable below $T_{\rm A}$, despite the expected strong interlayer spin coupling between the NiO$_2$ planes. Above all, we emphasize that the local spin susceptibility is extremely small at the O(2) site that has a strong covalency with the $d_{3z^2-r^2}$ orbital, indicating a well-developed interlayer spin-singlet formation in the Ni-$d_{3z^2-r^2}$ orbitals bridging the NiO$_2$ planes. These findings shed new light on the interlayer spin-singlet formation and the anomalous spin reconfiguration through the $\text{Ni--O(1)--Ni}$ bonding orbitals connecting the NiO$_2$ planes, which characterize the orbital-selective nature of the bilayer nickelate La$_3$Ni$_2$O$_7$.
