通过第一性原理密度泛函理论计算，系统研究了双层Ruddlesden-Popper型镍酸盐La₃Ni₂O₇薄膜中四价元素替代的电子掺杂效应。研究发现，与铜氧化物不同，铈（Ce）掺杂难以有效向低能带引入电子载流子，而锆（Zr）、铪（Hf）和钍（Th）可作为高效电子掺杂剂。这些元素替代能显著增大Ni-dz²轨道间的层间跳跃积分t⊥，可能增强层间超交换耦合J⊥，进而潜在提高超导转变温度Tc。利用约束随机相位近似评估了相互作用参数，发现电子掺杂使得低能轨道（包括Ni-dx²-y²和dz²及其杂化氧轨道）的占据增加，并改变了层内与层间轨道的电子填充比例。结构分析显示，掺杂剂离子半径的差异导致Ni-O键长变化，其中Zr和Hf引起晶格收缩，而Th掺杂效果最强。结果表明，Zr、Hf、Th是实现La₃Ni₂O₇电子掺杂的候选材料，为澄清该体系中电子配对机制的争议提供了新途径。
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通过结合密度泛函理论和动力学平均场理论，研究了无限层镍酸盐LaNiO₂在电子掺杂和空穴掺杂下电子结构、磁性与关联效应的演化。结果显示，由于稀土5d态的存在，Ni-d_{x²-y²}带的自掺杂效应呈现显著不对称：空穴掺杂强烈抑制自掺杂，而电子掺杂虽增大稀土5d电子口袋尺寸，却未有效空穴掺杂Ni-d_{x²-y²}带。这一差异直接影响磁性响应——空穴掺杂迅速压制反铁磁序，电子掺杂则使反铁磁态保持为基态。尽管存在这些差异，两种掺杂下的电子关联均由Ni-d_{x²-y²}轨道主导，表明单带描述在电子和空穴掺杂区均可能适用。
High-temperature superconductivity was discovered in the pressurized nickelate La3Ni2O7 which has a unique bilayer structure and mixed valence state of nickel. The properties at ambient pressure contain crucial information of the fundamental interactions and bosons mediating superconducting pairing. Here, using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we identified that Ni 3 $${d}_{{x}^{2}-{y}^{2}}$$, Ni 3 $${d}_{{z}^{2}}$$, and ligand oxygen 2p orbitals dominate the low-energy physics with a small charge-transfer energy. Well-defined optical-like magnetic excitations soften into quasi-static spin-density-wave ordering, evidencing the strong electronic correlation and rich magnetic properties. Based on an effective Heisenberg spin model, we extract a much stronger inter-layer effective magnetic superexchange than the intra-layer ones and propose two viable magnetic structures. Our findings emphasize that the Ni 3 $${d}_{{z}^{2}}$$orbital bonding within the bilayer induces novel electronic and magnetic excitations, setting the stage for further exploration of La3Ni2O7 superconductor.
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Trilayer Ruddlesden-Popper phase La4Ni3O10 has been observed with Tc of ∼30 K at high pressure in a recent experiment, which further expanded the family of nickelate superconductors. In this study, we explored the effects of electronic correlations in La4Ni3O10 using density functional theory plus dynamical mean-field theory at ambient and high pressures. Our derived spectral functions and Fermi surface of the ambient pressure phase are nicely consistent with the experimental results by angle-resolved photoemission spectroscopy, which emphasized the importance of electronic correlations in La4Ni3O10. We also found the electronic correlations in pressurized La4Ni3O10 are both orbital-dependent and layer-dependent due to the presence of Hund’s rule coupling. There is a competition between the Hund’s rule coupling and the crystal-field splitting, and therefore, the Ni–O layers with weaker crystal-field splitting energy would have stronger electronic correlations.
The discovery of superconductivity with a critical temperature of about 80 K in La3Ni2O7 single crystals under pressure has received enormous attention. La3Ni2O7 is not superconducting under ambient pressure but exhibits a transition at T ∗ ≃ 115 K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of superconductivity and other instabilities. Here, our optical study shows that La3Ni2O7 features strong electronic correlations which significantly reduce the electron’s kinetic energy and place this system in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands at the Fermi level. The transition at T ∗ removes the Drude component exhibiting non-Fermi liquid behavior, whereas the one with Fermi-liquid behavior is barely affected. These observations in combination with theoretical results suggest that the Fermi surface dominated by the Ni- $${d}_{3{z}^{2}-{r}^{2}}$$orbital is removed due to the transition at T ∗. Our experimental results provide pivotal information for understanding the transition at T ∗ and superconductivity in La3Ni2O7.
We employ a density-functional theory plus dynamical mean-field theory framework to investigate the correlated electronic structure of the alternating single-layer–trilayer (1313) polymorph of La3Ni2O7, which becomes superconducting under pressure. At ambient pressure, the single layer is in a Mott-insulating regime and the low-energy physics is dominated by the trilayer block. Under pressure, the gap in the single-layer block closes due to orbital-selective physics, enabling charge transfer into the trilayer block. This change in effective doping of the trilayer block could be linked to the higher 𝑇𝑐 obtained in La3Ni2O7−1313 (∼80 K) when compared to the nominal trilayer La4Ni3O10 compound (∼30 K). We conclude that correlation-driven layer differentiation is crucial in the La3Ni2O7−1313 polymorph and that its low-energy physics aligns closely with the trilayer La4Ni3O10 compound (in spite of the apparent differences in nominal filling) rather than with the conventional bilayer La3Ni2O7.