本文通过第一性原理计算研究了新合成的双无限层氧氟化物La₃Ni₂O₅F的结构稳定性、电子结构与磁性，并探讨了化学压力与外延应变的影响。声子谱证实了该晶体结构的动力学稳定性，其电子结构呈现高度二维的类铜氧化物费米面，主要由Ni-d_(x²-y²)态主导，伴随稀土离子自掺杂导致的~d^(1.2)有效填充，电荷转移能接近铜氧化物区间。化学压力和外延应变下，这类电子特征保持显著稳健，镍-d_(x²-y²)填充仅发生适度变化。自旋极化计算揭示出多种面内与面间自旋排列的近简并磁构型，压应变进一步增强磁阻挫，却不显著改变电子结构。因此，La₃Ni₂O₅F被确立为有前景的铜氧化物类似物，晶格工程可有效精细调控其电子与磁学性质。
本文利用功能重正化群方法，研究了双层镍酸盐La₃Ni₂O₇在常压与高压晶体结构下自旋密度波序与超导的竞争。通过对比两种结构的弱耦合多轨道模型，发现随着Hund耦合增大，主导不稳定性从超导转变为具有特征波矢Q₁≈(π/2,π/2)的自旋密度波，与实验一致。令人意外的是，常压与高压结构的非相互作用磁化率和fRG主导不稳定性几乎相同，表明压力下超导的出现不能仅由低能电子结构变化解释。进一步分析表明，抑制正交畸变是关键因素：当体系趋近四方极限时，对称性相关的自旋密度波涨落近乎简并，从而阻碍长程磁有序并增强配对相互作用。这些结果揭示了晶格对称性是调控双层镍酸盐中竞争有序态的核心参量，并提示通过单轴应变降低正交畸变有望在常压下实现体超导。
11 条关联记录
Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Rud-dlesden-Popper nickelate La3Ni2O7, the atomic structure and electronic band structure of the trilayer nickelate La4Ni3O10 under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P21/a space group to the tetragonal I4/mmm around 12.6–13.4 GPa is identified, accompanied by a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni $$3{d_{{z^2}}}$$orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La4Ni3O10. The trilayer nickelate La4Ni3O10 shows some similarities with the bilayer La3Ni2O7 and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates.
Very recently, a new superconductor with Tc = 80 K has been reported in nickelate (La3Ni2O7) at around 15–40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with Tc above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement under low-temperature and high-pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results were carried out at room temperature. This encouraged us to carry out in situ high-pressure and low-temperature synchrotron XRD experiments to determine which phase is responsible for the high Tc state. In addition to the phase transition from the orthorhombic Amam structure to the orthorhombic Fmmm structure, a tetragonal phase with the space group of I4/mmm was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in La3Ni2O7. The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the eg orbitals (3dz2 and 3dx2–y2) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between Tc and this structural evolution, especially Ni–O octahedra regularity and the in-plane Ni–O–Ni bonding angles, is analyzed. This work sheds new light to identify what is the most likely phase responsible for superconductivity in double-layered nickelate.
139 La-NMR/NQR measurements of La 3 Ni 2 O 7-δ (δ∼0.0 and δ∼0.08), and La 4 Ni 3 O 10 have been performed. 139 La-NMR and transport and magnetic studies have also been carried out for Tl(La 2 Sr 2 )Ni 2 O 9 . Anomalous temperature ( T ) dependence of the longitudinal relaxation rates 1/ T 1 has been found at temperatures T A ∼(140–150 K) for all the systems. In Tl(La 2 Sr 2 )Ni 2 O 9 , there exists a transition to a magnetically ordered state at T ∼20 K. The large broadening of the NMR spectra observed for Tl(La 2 Sr 2 )Ni 2 O 9 below 20 K indicates that the system is in the charge ordered state in the temperature region between 20 K and T A with localized magnetic moments at Ni sites. The NQR intensity of La 4 Ni 3 O 10 begins to decrease rapidly with decreasing T at T A ∼140 K and almost disappears at T ∼120 K (wipeout). By arguing results of the present experimental studies, we propose that in all the systems with NiO 2 planes studied here exhibit similar type transitions to the charge ordered states at temperatures T A , all of which are in the narrow T region around 140 K. It has also been found that all the systems exhibit resistivity anomalies in the T region of (450–550) K, which suggests that they have a tendency of similar type changes or transitions of their electronic states at the temperatures, too.
14 条关联记录
研究人员采用脉冲激光沉积结合氢化钙拓扑还原法，成功制备了一系列无限层结构 Nd₁₋ₓEuₓNiO₂ 薄膜，掺杂范围扩展至 x=0–0.7。电输运测量揭示在 0.2≤x≤0.5 区间存在一个超导穹顶，其掺杂宽度大于分子束外延制备的样品，与化学溶液法相当。其中 x=0.3 的薄膜表现出最优超导转变温度约 31 K，显著高于其他真空外延技术所得数值，表明脉冲激光沉积是制备高质量、高转变温度该类镍氧化物超导薄膜的有效途径。磁输运实验在欠掺杂和过掺杂区域均观测到强健的磁场增强与重入超导行为，这归因于 Eu²⁺ 局域磁矩在外场下的极化产生内部交换场，部分补偿外场所致；仅靠 Jaccarino-Peter 效应不足以完全解释该现象，暗示存在其他机制。在刚高于起始超导转变温度的低温区，霍尔电阻呈现非线性特征，且未伴随明显的磁滞，这可能源于磁性杂质散射。这些结果突出了磁性稀土 Eu²⁺ 离子在赋予无限层镍氧化物奇异物理属性中的关键作用。
The ambient-pressure superconductivity in La3Ni2O7 thin films via compressive epitaxial strain provides a highly accessible platform for diverse characterization techniques, facilitating the studies of high-temperature superconductivity. Here, we systematically map the phase diagram and reveal the superconducting dome with an electron-hole crossover in compressively strained La3−𝑥Sr𝑥Ni2O7−𝛿 thin films by simultaneously tuning Sr doping and oxygen content. The maximum transition temperature (𝑇𝑐) coincides with an anomalous sign change in the Hall coefficient (𝑅𝐻), reminiscent of electron-doped cuprates, which may signal a Fermi surface reconstruction. Beyond the superconducting dome, a ln1/𝑇 insulating regime and a 𝑇-linear resistivity regime are also resolved, resembling behaviors observed in cuprates and infinite-layer nickelates. This work reveals a dome-shaped relationship between 𝑇𝑐 and 𝑅𝐻 and establishes a key framework for understanding unconventional superconductivity in nickelate systems.
9 条关联记录