Paper

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.

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 La3⁢Ni2⁢O7, 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 La3⁢Ni2⁢O7−1313 (∼80 K) when compared to the nominal trilayer La4⁢Ni3⁢O10 compound (∼30 K). We conclude that correlation-driven layer differentiation is crucial in the La3⁢Ni2⁢O7−1313 polymorph and that its low-energy physics aligns closely with the trilayer La4⁢Ni3⁢O10 compound (in spite of the apparent differences in nominal filling) rather than with the conventional bilayer La3⁢Ni2⁢O7.

研究人员利用偏振超快泵浦-探测光谱，对比研究了常压下双层La₃Ni₂O₇和三层La₄Ni₃O₁₀单晶的正常态电子动力学。两种材料均表现出密度波转变并伴随准粒子弛豫瓶颈的打开，但其电子响应具有显著不同的对称性：三层La₄Ni₃O₁₀在整个温区内保持光学各向同性，而双层La₃Ni₂O₇在低温下则展现出明显的二重（C₂）旋转对称性破缺，即电子向列性。这种向列性体现在准粒子慢弛豫动力学和有效能隙尺度的各向异性中，并且其在115 K以下与一个次级各向同性序相互竞争，导致向列信号随温度呈非单调变化。该工作揭示了双层镍酸盐中电子向列涨落的存在，而三层体系中并未出现这一现象，暗示了电子向列性与La₃Ni₂O₇高压超导配对之间可能存在的紧密关联，为深入理解这类镍基超导体的微观机制提供了关键信息。

基于对三层镍酸盐La₄Ni₃O₁₀高压超导的发现，该研究利用密度泛函理论结合动力学平均场理论（DFT+DMFT）设计并计算了钴基类似物La₄Co₂NiO₈Cl₂。通过将高压相La₄Co₃O₁₀内层Co替换为Ni并掺入Cl实现电子掺杂，该化合物获得了与超导La₄Ni₃O₁₀相似的晶体结构及强关联电子特征：外层Co轨道呈现强有效质量增强和非费米液体行为，内层Ni则表现为弱关联费米液体；在M点附近出现源于外层Co轨道且接近费米能级的平带；同时具有显著的轨道选择性以及高自旋与低自旋态混合的局域自旋涨落。这些特性与La₄Ni₃O₁₀的关键电子态高度吻合，表明La₄Co₂NiO₈Cl₂有望成为钴基层状化合物中实现高温超导电性的候选材料，为后续实验探索提供了理论依据。

It has been recently shown that under pressure trilayer Ruddlesden-Popper nickelate La4⁢Ni3⁢O10 (LNO) becomes superconducting below a critical temperature ≈20 K, in addition to the infinite-layer and bilayer systems. Motivated by this observation, we explore the effects of electron correlations on its electronic structure and magnetic properties using the advanced density functional theory plus dynamical mean-field theory approach. Our results for the normal-state electronic structure and correlation effects in LNO show much in common with the infinite-layer and bilayer nickelates, with remarkable site- and orbital-dependent renormalizations of the Ni 3⁢𝑑 bands and notable incoherence of the Ni 𝑑3⁢𝑧2−𝑟2 states, caused by correlation effects. Our analysis of the Fermi surface and magnetic correlations suggests the emergence of competing spin and charge stripe states, implying the importance of in-plane spin fluctuations to explain superconductivity in this material.

Motivated by the recent observation of superconductivity in the pressurized trilayer Ruddlesden-Popper (RP) nickelate La4⁢Ni3⁢O10, we explore its structural, electronic, and magnetic properties as a function of hydrostatic pressure from first-principles calculations. We find that an orthorhombic (monoclinic)-to-tetragonal transition under pressure takes place concomitantly with the onset of superconductivity. The electronic structure of La4⁢Ni3⁢O10 can be understood using a molecular trimer basis wherein 𝑛 molecular subbands arise as the 𝑑𝑧2 orbitals hybridize strongly along the 𝑐 axis within the trilayer. The magnetic tendencies indicate that the ground state at ambient pressure is formed by nonmagnetic inner planes and stripe-ordered outer planes that are antiferromagnetically coupled along the 𝑐 axis, resulting in an unusual ↑, 0, ↓ stacking that is consistent with the spin density wave model previously suggested by neutron diffraction. Such a state is destabilized at the pressure where superconductivity arises. Despite the presence of 𝑑𝑧2 states at the Fermi level, the 𝑑𝑥2−𝑦2 orbitals also play a key role in the electronic structure of La4⁢Ni3⁢O10. This active role of the 𝑑𝑥2−𝑦2 states in the low-energy physics of the trilayer RP nickelate, together with the distinct electronic behavior of the inner and outer planes, resembles the physics of multilayer cuprates.

Signatures of superconductivity under pressure have recently been reported in the bilayer La3⁢Ni2⁢O7 and trilayer La4⁢Ni3⁢O10 Ruddlesden-Popper (RP) nickelates with the general chemical formula La𝑛+1⁢Ni𝑛⁢O3⁢𝑛+1 (𝑛 = number of perovskite layers along the 𝑐-axis). The emergence of superconductivity is always concomitant with a structural transition in which the octahedral tilts are suppressed, bringing the apical Ni-O-Ni angle to 180∘ and causing an increase in the out-of-plane 𝑑𝑧2 orbital overlap. Due to this strong interlayer coupling, a flat band of pure 𝑑𝑧2 character crosses the Fermi level. Here, using first-principles calculations, we explore biaxial strain (both compressive and tensile) as a means to mimic the electronic structure characteristics of RP nickelates (up to 𝑛=5) under hydrostatic pressure. Our findings highlight that strain enables the decoupling of the structural and electronic structure effects obtained under hydrostatic pressure: While compressive strain brings the apical Ni-O-Ni angle closer to 180∘, it shifts the 𝑑𝑧2 flat bands away from the Fermi energy, giving rise to a more cupratelike electronic structure. In contrast, tensile strain reduces the apical Ni-O-Ni angle (to values of ∼160∘), but it recovers the flat 𝑑𝑧2 band at the Fermi level appearing in the bilayer and trilayer RP nickelates under pressure. Overall, strain represents a promising way to tune the electronic structure of RP nickelates and could be an alternative route to achieve superconductivity at ambient pressure in this family of materials.