[论文解读] Neutron Scattering Studies on the High-$T_c$ Superconductor La$_3$Ni$_2$O$_{7-δ}$ at Ambient Pressure

Tao Xie, Mengwu Huo+11 more|arXiv (Cornell University)|Jan 23, 2024

Magnetic and transport properties of perovskites and related materials被引用 5

一句话总结

弹性与非弹性中子散射在常压下的多晶La3Ni2O7-δ中发现没有到10 K的长_range磁序，且在约45 meV附近出现非色散自旋激发，与双条纹AFM样态中的强层间耦合一致。

ABSTRACT

After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctuations have been widely believed to play a crucial role in forming the superconducting Cooper pairs. The recent discovery of high-temperature superconductivity near 80 K in the bilayer nickelate La$_3$Ni$_2$O$_7$ under pressure provides a new platform to elucidate the origins of high-temperature superconductivity. We perform elastic and inelastic neutron scattering studies on a polycrystalline sample of La$_3$Ni$_2$O$_{7-δ}$ at ambient pressure. No magnetic order can be identified down to 10 K. The absence of long-range magnetic order in neutron diffraction measurements may be ascribed to the smallness of the magnetic moment. However, we observe a weak flat spin-fluctuation signal at $\sim$ 45 meV in the inelastic scattering spectra. The observed spin excitations could be interpreted as a result of strong interlayer and weak intralayer magnetic couplings for stripe-type antiferromagnetic orders. Our results provide crucial information on the spin dynamics and are thus important for understanding the superconductivity in La$_3$Ni$_2$O$_7$.

研究动机与目标

• 研究La3Ni2O7-δ在常压与低温下是否存在磁序。
• 利用弹性和非弹性中子散射表征自旋涨落及其能量/动量依赖性。
• 估算磁交换耦合并评估可能的磁基态。
• 将观测到的自旋动力学与理论模型及DFT声子计算进行比较，以理解耦合机制。
• 将结果放在镍酸盐高-Tc超导性及其与铜酸盐/铁砟磷矿的广义背景中加以解读。

提出的方法

• 通过中子粉末衍射在10 K和160 K下搜索长程磁序。
• 采用具有多种入射能Ei的非弹性中子散射，以绘制自旋和声子激发。
• 基于SpinW的自旋激发谱建模以提取交换耦合。
• 进行DFT声子计算以计算色散关系和态密度以便与INS比较。
• 通过高低温INS光谱的差分去除磁性贡献。

Figure 1: Structure, electronic occupancy, and bulk electrical transport, magnetic and structural characterization of the La 3 Ni 2 O 7-δ polycrystalline sample. (a) Crystal structure of La 3 Ni 2 O 7-δ in the $Amam$ space group. The open blue circle represents an oxygen vacancy. (b) Schematic of th

实验结果

研究问题

• RQ1 La3Ni2O7-δ在常压下是否在10 K甚至更低温度处存在长程磁序？
• RQ2 在La3Ni2O7-δ的常压下，自旋涨落及其能量尺度的本质是什么？
• RQ3 磁交换耦合的量级及层内与层间的等级排序如何？
• RQ4 是否可以用如交替双条纹AFM等磁结构来解释观测到的自旋激发？

主要发现

• 未检测到磁Bragg峰；在10 K以下未观察到长程磁序。
• 在非弹性散射谱中出现以约45 meV为中心的弱平坦自旋涨落信号。
• 自旋激发谱可很好地用交替双条纹AFM序局描述，具有强层间耦合与弱层内耦合（SJ⊥ ≈ 36 meV; SJ1 ≈ 5; SJ2 ≈ -2; SJ4 ≈ 1）。
• 观测到的声子模式具有色散与非色散特征；DFT声子色散和DOS大致与INS特性吻合。
• 强层间磁交换在自旋动力学中占主导，与铜酸盐及铁基超导体存在显著差异。

Figure 2: INS spectra of La 3 Ni 2 O 7-δ with different incident energies at low temperature. (a) INS spectrum collected at PELICAN with $E_{i}$ = 3.7 meV. (b) and (c) are INS spectra measured at MERLIN with $E_{i}$ = 22.6 and 160 meV, respectively. (d) INS spectrum up to 350 meV obtained at MAPS. (