该文档主要展示了与镍基超导体相关的重要文献摘要。
The electronic structure of various nickel oxides with nickel valence varying from \(1^+\) to \(3^+\) was investigated with the aim to find similarities and differences to the isoelectronic cuprates. Only if the \(\text{Ni}\) ions are forced into a planar coordination with the \(O\) ions can a \(S=1/2\) magnetic insulator be realized with the difficult \(\text{Ni}^+\) oxidation state and possibly doped with low spin (\(S=0\)) \(\text{Ni}^{2+}\) holes directly analogous to the superconducting cuprates. The more common \(\text{Ni}^{3+}\) oxidation state cannot be used to make a parent magnetic insulator as it forms rather as localized \(S=1\) \(\text{Ni}^{2+}\) embedded in a sea of itinerant \(O\) holes. Strong coupling of these holes to the localized spins via \(2p −3d\) hybridization leads to a heavy-fermion system with a large Kondo temperature, which was confirmed experimentally for \(\text{LaNiO}_3\).
研究了不同镍氧化物的电子结构，其中镍的价态从\(1^+\)到\(3^+\)不等，目的是寻找其与等电子的铜氧化物之间的相似性和差异性。只有当镍离子被强制与氧离子形成平面配位时，才能实现具有难以实现的\(S=1/2\)磁性绝缘体的\(\text{Ni}^+\)氧化态，并可能直接掺杂低自旋(\(S=0\))的\(\text{Ni}^{2+}\)空穴，这与超导铜氧化物的情况完全类似。而更常见的\(\text{Ni}^{3+}\)氧化态无法用来制备母体磁性绝缘体，因为它更倾向于形成局部化的\(S=1\) \(\text{Ni}^{2+}\)嵌入在一群巡游的氧空穴中。这些空穴通过\(2p −3d\)杂化与局部化自旋强烈耦合，导致形成具有较大Kondo温度的重费米子系统，这一点已在\(\text{LaNiO}_3\)的实验中得到验证。
The discovery of unconventional superconductivity in \((\text{La},\text{Ba})_{2}\text{CuO}_{4}\) has motivated the study of compounds with similar crystal and electronic structure, with the aim of finding additional superconductors and understanding the origins of copper oxide superconductivity. Isostructural examples include bulk superconducting \(\text{Sr}_{2}\text{RuO}_{4}\) and surface-electron-doped \(\text{Sr}_{2}\text{IrO}_4\), which exhibits spectroscopic signatures consistent with a superconducting gap, although a zero-resistance state has not yet been observed. This approach has also led to the theoretical investigation of nickelates, as well as thin-film heterostructures designed to host superconductivity. One such structure is the \(\text{LaAlO}_3\)/\(\text{LaNiO}_3\) superlattice, which has been recently proposed for the creation of an artificially layered nickelate heterostructure with a singly occupied band. The absence of superconductivity observed in previous related experiments has been attributed, at least in part, to incomplete polarization of the \(e_g\) orbitals. Here we report the observation of superconductivity in an infinite-layer nickelate that is isostructural to infinite-layer copper oxides. Using soft-chemistry topotactic reduction, \(\text{NdNiO}_2\) and \(\text{Nd}_{0.8}\text{Sr}_{0.2}\text{NiO}_2\) single-crystal thin films are synthesized by reducing the perovskite precursor phase. Whereas \(\text{NdNiO}_2\) exhibits a resistive upturn at low temperature, measurements of the resistivity, critical current density and magnetic-field response of \(\text{Nd}_{0.8}\text{Sr}_{0.2}\text{NiO}_{2}\) indicate a superconducting transition temperature of about \(9\) to \(15\) kelvin. Because this compound is a member of a series of reduced layered nickelate crystal structures, these results suggest the possibility of a family of nickelate superconductors analogous to copper oxides and pnictides.
在 \((\text{La},\text{Ba})_{2}\text{CuO}_{4}\) 中发现的非常规超导性激发了对具有类似晶体和电子结构的化合物的研究，目的是寻找更多的超导体并理解铜氧化物超导性的起源。等结构的例子包括体超导的 \(\text{Sr}_{2}\text{RuO}_{4}\) 和表面电子掺杂的 \(\text{Sr}_{2}\text{IrO}_4\)，后者表现出与超导能隙一致的光谱特征，尽管尚未观察到零电阻态。这种方法还促使了对镍氧化物的理论研究，以及旨在实现超导性的薄膜异质结构的设计。其中一种结构是 \(\text{LaAlO}_3\)/\(\text{LaNiO}_3\) 超晶格，最近被提议用于创建具有单占据能带的人工层状镍氧化物异质结构。此前相关实验中未观察到超导性，部分原因被认为与 \(e_g\) 轨道的不完全极化有关。
在此，我们报告了在与无限层铜氧化物等结构的无限层镍氧化物中观察到的超导性。通过软化学拓扑还原法，通过还原钙钛矿前驱相合成了 \(\text{NdNiO}_2\) 和 \(\text{Nd}_{0.8}\text{Sr}_{0.2}\text{NiO}_2\) 单晶薄膜。虽然 \(\text{NdNiO}_2\) 在低温下表现出电阻上升现象，但对 \(\text{Nd}_{0.8}\text{Sr}_{0.2}\text{NiO}_2\) 的电阻率、临界电流密度和磁场响应的测量表明其超导转变温度约为\(9\)到\(15\)开尔文。由于这种化合物属于一系列还原层状镍氧化物晶体结构，这些结果表明可能存在类似于铜氧化物和铁砷超导体的镍氧化物超导体家族。
The occurrence of superconductivity in proximity to various strongly correlated phases of matter has drawn extensive focus on their normal state properties, to develop an understanding of the state from which superconductivity emerges. The recent finding of superconductivity in layered nickelates raises similar interests. However, transport measurements of doped infinite-layer nickelate thin films have been hampered by materials limitations of these metastable compounds: in particular, a high density of extended defects. Here, by moving to a substrate (LaAlO3)0.3(Sr2TaAlO6)0.7 that better stabilizes the growth and reduction conditions, we can synthesize the doping series of Nd1–xSrxNiO2 essentially free from extended defects. In their absence, the normal state resistivity shows a low-temperature upturn in the underdoped regime, linear behaviour near optimal doping and quadratic temperature dependence for overdoping. This is phenomenologically similar to the copper oxides despite key distinctions—namely, the absence of an insulating parent compound, multiband electronic structure and a Mott–Hubbard orbital alignment rather than the charge-transfer insulator of the copper oxides. We further observe an enhancement of superconductivity, both in terms of transition temperature and range of doping. These results indicate a convergence in the electronic properties of both superconducting families as the scale of disorder in the nickelates is reduced.
超导性在多种强关联物质相附近的出现，引发了对其正常态性质的广泛关注，以期理解超导性所起源的状态。最近在层状镍氧化物中发现的超导性也引起了类似的兴趣。然而，由于这些亚稳态化合物的材料限制，特别是存在大量扩展缺陷，对掺杂无限层镍氧化物薄膜的输运测量受到了限制。在此，通过转向一种更好的稳定生长和还原条件的衬底（(LaAlO₃)₀.₃(Sr₂TaAlO₆)₀.₇），我们能够合成出基本不含扩展缺陷的Nd₁₋ₓSrₓNiO₂掺杂系列。在没有扩展缺陷的情况下，正常态电阻率在欠掺杂区域表现出低温上翘，在接近最佳掺杂时呈线性行为，而在过掺杂时则表现为二次温度依赖性。尽管存在关键区别——即不存在绝缘母体化合物、多带电子结构以及与铜氧化物的电荷转移绝缘体不同的莫特-哈伯德轨道对齐，但这些现象在表象上与铜氧化物相似。我们还观察到超导性在转变温度和掺杂范围方面都有所增强。这些结果表明，随着镍氧化物中无序尺度的减小，这两种超导家族的电子性质趋于一致。
Since the discovery of high-temperature superconductivity in copper oxide materials1, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials2. One prime materials platform has been the rare-earth nickelates and, indeed, superconductivity was recently discovered in the doped compound Nd0.8Sr0.2NiO2 (ref. 3). Undoped NdNiO2 belongs to a series of layered square-planar nickelates with chemical formula Ndn+1NinO2n+2 and is known as the ‘infinite-layer’ (n = ∞) nickelate. Here we report the synthesis of the quintuple-layer (n = 5) member of this series, Nd6Ni5O12, in which optimal cuprate-like electron filling (d8.8) is achieved without chemical doping. We observe a superconducting transition beginning at ~13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd6Ni5O12 interpolates between cuprate-like and infinite-layer nickelate-like behaviour. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors that can be tuned via both doping and dimensionality.
自铜氧化物材料中发现高温超导性以来，人们一直在努力理解这一相的起源，并寻找新的类似铜氧化物的超导材料。稀土镍氧化物一直是主要的研究平台之一，事实上，最近在掺杂化合物Nd₀.₈Sr₀.₂NiO₂中发现了超导性。未掺杂的NdNiO₂属于一系列层状四方平面镍氧化物，其化学式为Ndₙ₊₁NiₙO₂ₙ₊₂，被称为“无限层”（n = ∞）镍氧化物。在此，我们报告了这一系列中五层（n = 5）成员Nd₆Ni₅O₁₂的合成，该化合物在没有化学掺杂的情况下实现了最佳的类似铜氧化物的电子填充（d⁸.⁸）。我们观察到从约13 K开始的超导转变。电子结构计算与磁阻和光谱测量相结合表明，Nd₆Ni₅O₁₂介于类似铜氧化物和类似无限层镍氧化物的行为之间。通过设计一种独特的超导镍氧化物，我们确定四方平面镍氧化物是一个可以通过掺杂和维度调节的新超导体家族。
The newly discovered nickelate superconductors so far only exist in epitaxial thin films synthesized by a topotactic reaction with metal hydrides. This method changes the nickelates from the perovskite to an infinite-layer structure by deintercalation of apical oxygens. Such a chemical reaction may introduce hydrogen (H), influencing the physical properties of the end materials. Unfortunately, H is insensitive to most characterization techniques and is difficult to detect because of its light weight. Here, in optimally Sr doped Nd0.8Sr0.2NiO2H epitaxial films, secondary-ion mass spectroscopy shows abundant H existing in the form of Nd0.8Sr0.2NiO2Hx (x ≅ 0.2–0.5). Zero resistivity is found within a very narrow H-doping window of 0.22 ≤ x ≤ 0.28, showing unequivocally the critical role of H in superconductivity. Resonant inelastic X-ray scattering demonstrates the existence of itinerant interstitial s (IIS) orbitals originating from apical oxygen deintercalation. Density functional theory calculations show that electronegative H– occupies the apical oxygen sites annihilating IIS orbitals, reducing the IIS–Ni 3d orbital hybridization. This leads the electronic structure of H-doped Nd0.8Sr0.2NiO2Hx to be more two-dimensional-like, which might be relevant for the observed superconductivity. We highlight that H is an important ingredient for superconductivity in epitaxial infinite-layer nickelates.
迄今为止，新发现的镍氧化物超导体仅存在于通过金属氢化物的拓扑反应合成的外延薄膜中。这种方法通过脱插顶端氧将镍氧化物从钙钛矿结构转变为无限层结构。这种化学反应可能会引入氢（H），从而影响最终材料的物理性质。不幸的是，由于氢的质量很轻，它对大多数表征技术不敏感，很难被检测到。在此，对于最佳Sr掺杂的Nd₀.₈Sr₀.₂NiO₂H外延薄膜，二次离子质谱显示以Nd₀.₈Sr₀.₂NiO₂Hₓ（x ≅ 0.2–0.5）形式存在大量氢。在非常狭窄的氢掺杂窗口0.22 ≤ x ≤ 0.28内发现了零电阻，这明确显示了氢在超导性中的关键作用。共振非弹性X射线散射证明了来自顶端氧脱插的巡游间隙s（IIS）轨道的存在。密度泛函理论计算表明，电负性H⁻占据了顶端氧位点，消灭了IIS轨道，减少了IIS-Ni 3d轨道的杂化。这使得氢掺杂的Nd₀.₈Sr₀.₂NiO₂Hₓ的电子结构更具二维特性，这可能与观察到的超导性有关。我们强调，氢是外延无限层镍氧化物中超导性的一个重要成分。
The occurrence of unconventional superconductivity in cuprates has long motivated the search for manifestations in other layered transition metal oxides. Recently, superconductivity is found in infinite-layer nickelate (Nd,Sr)NiO2 and (Pr,Sr)NiO2 thin films, formed by topotactic reduction from the perovskite precursor phase. A topic of much current interest is whether rare-earth moments are essential for superconductivity in this system. In this study, it is found that with significant materials optimization, substantial portions of the La1−xSrxNiO2 phase diagram can enter the regime of coherent low-temperature transport (x = 0.14 - 0.20), with subsequent superconducting transitions and a maximum onset of ≈9 K at x = 0.20. Additionally, the unexpected indication of a superconducting ground state in undoped LaNiO2 is observed, which likely reflects the self-doped nature of the electronic structure. Combining the results of (La/Pr/Nd)1−xSrxNiO2 reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron-hole populations across the lanthanides.
铜氧化物中非常规超导性的出现长期以来一直激励着人们在其他层状过渡金属氧化物中寻找类似现象。最近，在无限层镍氧化物（Nd,Sr）NiO₂和（Pr,Sr）NiO₂薄膜中发现了超导性，这些薄膜是通过从钙钛矿前驱相的拓扑还原形成的。目前一个备受关注的问题是，稀土矩是否是该系统中超导性所必需的。在这项研究中，发现通过显著的材料优化，La₁₋ₓSrₓNiO₂相图的大部分区域可以进入相干低温输运区域（x = 0.14 - 0.20），随后发生超导转变，且在x = 0.20时最大起始温度约为9 K。此外，观察到未掺杂的LaNiO₂中意外出现了超导基态的迹象，这可能反映了电子结构的自掺杂特性。结合（La/Pr/Nd）₁₋ₓSrₓNiO₂的结果揭示了一个广义的超导穹顶，其特征是随着镧系元素的变化，单位晶胞体积和相对电子-空穴群体的系统性变化。
Charge and spin orders are intimately related to superconductivity in copper oxide superconductors. Elucidation of the competing orders in various nickel oxide compounds is crucial, given the fact that superconductivity has been discovered in Nd0.8Sr0.2NiO2 films. Herein, we report structural, electronic transport, magnetic, and thermodynamic characterizations of single crystals of La3Ni2O7 and La3Ni2O6. La3Ni2O7 is metallic with mixed Ni2+ and Ni3+ valent states. Resistivity measurements yield two transition-like kinks at ∼ 110 and 153 K. The kink at 153 K is further revealed from magnetization and specific heat measurements, indicative of the formation of charge and spin density waves. La3Ni2O6 single crystals obtained from the topochemical reduction of La3Ni2O7 are insulating and show an anomaly at ∼176 K on magnetic susceptibility. The transition-like behaviors of La3Ni2O7 and La3Ni2O6 are analogous to those observed in La4Ni3O10 and La4Ni3O8, suggesting that charge and spin density waves are a common feature in the ternary La−Ni−O system with mixed-valent states of nickel.
在铜氧化物超导体中，电荷序和自旋序与超导性密切相关。鉴于在Nd₀.₈Sr₀.₂NiO₂薄膜中发现了超导性，阐明各种镍氧化物化合物中的竞争序至关重要。在此，我们报告了La₃Ni₂O₇和La₃Ni₂O₆单晶的结构、电子输运、磁性和热力学特性。La₃Ni₂O₇具有金属特性，存在混合的Ni²⁺和Ni³⁺价态。电阻率测量显示出在约110 K和153 K处有两个类似转变的拐点。153 K处的拐点进一步通过磁化率和比热测量得到证实，表明电荷密度波和自旋密度波的形成。通过La₃Ni₂O₇的拓扑化学还原得到的La₃Ni₂O₆单晶具有绝缘特性，并在磁化率上显示出约176 K的异常。La₃Ni₂O₇和La₃Ni₂O₆的类似转变行为类似于La₄Ni₃O₁₀和La₄Ni₃O₈中观察到的现象，表明电荷密度波和自旋密度波是具有混合价态镍的三元La−Ni−O体系中的一个共同特征。
We propose the parent compound of the newly discovered superconducting nickelate Nd1−𝑥Sr𝑥NiO2 as a self-doped Mott insulator, in which the low-density Nd 5𝑑 conduction electrons couple to localized Ni 3𝑑𝑥2−𝑦2 electrons to form Kondo spin singlets at low temperatures. This proposal is motivated with our analyses of the reported resistivity and Hall coefficient data in the normal state, showing logarithmic temperature dependence at low temperatures. In the strong Kondo coupling limit, we derive a generalized 𝑡−𝐽 model with both Kondo singlets and nickel holons moving through the lattice of an otherwise nickel spin-1/2 background. The antiferromagnetic long-range order is therefore suppressed as observed in experiments. With Sr doping, the number of holons on the nickel sites increases, giving rise to the superconductivity and a strange metal phase analogous to those in superconducting copper oxides.
我们提出新发现的超导镍氧化物Nd₁₋ₓSrₓNiO₂的母体化合物是一种自掺杂的莫特绝缘体，在这种绝缘体中，低密度的Nd 5d导电电子与局域化的Ni 3dx²−y²电子耦合，在低温下形成Kondo自旋单态。这一提议是基于我们对正常态下报道的电阻率和霍尔系数数据的分析，这些数据显示出在低温下具有对数温度依赖性。在强Kondo耦合极限下，我们推导出一个广义的t-J模型，其中Kondo单态和镍空穴在镍自旋1/2背景的晶格中移动。因此，如实验中观察到的那样，反铁磁长程序被抑制。随着Sr掺杂，镍位点上的空穴数量增加，从而导致超导性和类似于超导铜氧化物中的奇异金属相的出现。
We report the phase diagram of Nd1−𝑥Sr𝑥NiO2 infinite layer thin films grown on SrTiO3. A superconducting dome spanning 0.125<x<0.25 is found, remarkably similar to cuprates, albeit over a narrower doping window. However, while cuprate superconductivity is bounded by an insulator for underdoping and a metal for overdoping, here we observe weakly insulating behavior on either side of the dome. Furthermore, the normal state Hall coefficient is always small and proximate to a continuous zero crossing in doping and in temperature, in contrast to the \(\sim 1/𝑥\) dependence observed for cuprates. This suggests the presence of both electronlike and holelike bands, consistent with band structure calculations.
我们报告了在SrTiO₃上生长的Nd₁₋ₓSrₓNiO₂无限层薄膜的相图。发现了一个跨越0.125<x<0.25的超导穹顶，尽管其掺杂窗口较窄，但与铜氧化物非常相似。然而，与铜氧化物超导性在欠掺杂时被绝缘体限制、过掺杂时被金属限制不同，我们在这里观察到在穹顶两侧都表现出弱绝缘行为。此外，正常态的霍尔系数始终很小，并且在掺杂和温度上接近连续的零交叉，这与铜氧化物观察到的 ∼1/x 依赖性形成对比。这表明存在电子型和空穴型能带，与能带结构计算一致。
We report the observation of superconductivity in infinite-layer Ca-doped LaNiO2 (La1−xCaxNiO2) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La1−xCaxNiO2 thin films are entirely insulating from 300 K down to 2 K. A superconducting dome is observed at 0.15 < x < 0.3 with weakly insulating behavior at the overdoped regime. Moreover, the sign of the Hall coefficient RH changes at low temperature for samples with a higher doping level. However, distinct from the Nd- and Pr-based nickelates, the RH-sign-change temperature remains at around 35 K as the doping increases, which begs further theoretical and experimental investigation to reveal the role of the 4f orbital to the (multi)band nature of the superconducting nickelates. Our results also emphasize a notable role of lattice correlation on the multiband structures of the infinite-layer nickelates.
我们报告了在无限层掺杂钙的LaNiO₂（La₁₋ₓCaₓNiO₂）薄膜中观察到的超导性，并构建了它们的相图。与Nd和Pr基镍氧化物中的金属-绝缘体转变不同，未掺杂和欠掺杂的La₁₋ₓCaₓNiO₂薄膜从300 K到2 K完全处于绝缘态。在0.15 < x < 0.3处观察到一个超导穹顶，过掺杂区域表现出弱绝缘行为。此外，对于掺杂水平较高的样品，霍尔系数RH的符号在低温下发生变化。然而，与Nd和Pr基镍氧化物不同，随着掺杂水平的增加，RH符号变化的温度保持在约35 K，这需要进一步的理论和实验研究，以揭示4f轨道对超导镍氧化物的（多）带特性的贡献。我们的结果还强调了晶格关联在无限层镍氧化物的多带结构中的显著作用。
Infinite-layer Nd1−𝑥Sr𝑥NiO2 thin films with Sr doping level 𝑥 from 0.08 to 0.3 are synthesized and investigated. We find a superconducting dome 𝑥 between 0.12 and 0.235 accompanied by a weakly insulating behavior in both under- and overdoped regimes. The dome is akin to that in the electron-doped 214-type and infinite-layer cuprate superconductors. For 𝑥 ≥0.18, the normal state Hall coefficient (𝑅𝐻) changes the sign from negative to positive as the temperature decreases. The temperature of the sign changes decreases monotonically with decreasing 𝑥 from the overdoped side and approaches the superconducting dome at the midpoint, suggesting a reconstruction of the Fermi surface with the dopant concentration across the dome.
合成了掺杂Sr水平x从0.08到0.3的无限层Nd₁₋ₓSrₓNiO₂薄膜，并对其进行了研究。我们发现在0.12 ≤ x ≤ 0.235之间存在一个超导穹顶，同时在欠掺杂和过掺杂区域都表现出弱绝缘行为。这个穹顶类似于电子掺杂的214型和无限层铜氧化物超导体中的穹顶。对于x ≥ 0.18，在温度降低时，正常态霍尔系数（𝑅𝐻）的符号从负变正。从过掺杂侧随着x的减小，符号变化的温度单调下降，并在穹顶的中点附近接近，表明费米面随着掺杂浓度在穹顶上的变化而发生重构。
Oxygen vacancies play crucial roles in determining the physical properties of metal oxides, representing important building blocks in many scientific and technological fields due to their unique chemical, physical, and electronic properties. However, oxygen vacancies are often invisible because of their dilute concentrations. Therefore, characterizing and quantifying their presence is of utmost importance for understanding and realizing functional metal oxide devices. This, however, is oftentimes a non-trivial task. In this Perspective paper, we discuss the relevant regimes of concentrations and associated phenomena arising from oxygen vacancies. We then focus on experimental techniques available for observing oxygen vacancies at widely different levels of concentrations. Finally, we discuss current challenges and opportunities for utilizing oxygen vacancies in metal oxides.
氧空位在决定金属氧化物的物理性质方面起着关键作用，由于其独特的化学、物理和电子特性，它们在许多科学和技术领域中是重要的基本单元。然而，由于氧空位的浓度通常很低，它们往往难以被检测到。因此，对氧空位的存在进行表征和定量分析对于理解和实现功能性金属氧化物器件至关重要。然而，这通常并非易事。在这篇展望性论文中，我们讨论了氧空位浓度的相关范围及其引起的关联现象。然后，我们专注于可用于在不同浓度水平上观察氧空位的实验技术。最后，我们讨论了在金属氧化物中利用氧空位的当前挑战和机遇。
The superconducting nickelates were first proposed as potential analogs to the cuprate unconventional superconductors in 1999, but it took twenty years before superconductivity was successfully stabilized in epitaxial thin films. Since then, a flurry of both experimental and theoretical efforts have sought to understand the similarities and differences between the two systems and how they manifest in the macroscopic superconducting and normal state properties. Although the nickelates and cuprates indeed share many commonalities within their respective phase diagrams, several notable differences have also emerged, especially regarding their parent compounds, electronic hybridization, and fermiology. Here, we provide a survey of the rapidly developing landscape of layered nickelate superconductors, including recent experimental progress to probe not just the superconducting but also normal state and other ordered phases stabilized in these compounds.
超导镍氧化物最初于1999年被提出作为铜氧化物非常规超导体的潜在类似物，但在外延薄膜中成功稳定超导性却花了二十年时间。此后，大量的实验和理论研究试图理解这两个体系之间的相似性和差异性，以及它们如何体现在宏观的超导和正常态性质中。尽管镍氧化物和铜氧化物在其各自的相图中确实有许多共同点，但也出现了一些显著的差异，特别是在它们的母体化合物、电子杂化和费米学方面。在此，我们对层状镍氧化物超导体这一迅速发展的领域进行了综述，包括最近的实验进展，这些进展不仅探测了超导态，还探测了这些化合物中稳定的正常态和其他有序相。
The search continues for nickel oxide-based materials with electronic properties similar to cuprate high-temperature superconductors. The recent discovery of superconductivity in the doped infinite-layer nickelate NdNiO2 has strengthened these efforts. Here, we use X-ray spectroscopy and density functional theory to show that the electronic structure of LaNiO2 and NdNiO2, while similar to the cuprates, includes significant distinctions. Unlike cuprates, the rare-earth spacer layer in the infinite-layer nickelate supports a weakly interacting three-dimensional 5d metallic state, which hybridizes with a quasi-two-dimensional, strongly correlated state with \(3d_{x^2-y^2}\) symmetry in the NiO2 layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare-earth intermetallics, which are well known for heavy fermion behaviour, where the NiO2 correlated layers play an analogous role to the \(4f\) states in rare-earth heavy fermion compounds. This Kondo- or Anderson-lattice-like ‘oxide-intermetallic’ replaces the Mott insulator as the reference state from which superconductivity emerges upon doping.
寻找具有类似于铜氧化物高温超导体电子性质的镍氧化物基材料的努力仍在继续。最近在掺杂的无限层镍氧化物NdNiO₂中发现的超导性加强了这些努力。在此，我们利用X射线光谱学和密度泛函理论表明，LaNiO₂和NdNiO₂的电子结构虽然与铜氧化物相似，但也存在显著的区别。与铜氧化物不同，无限层镍氧化物中的稀土间隔层支持一个弱相互作用的三维5d金属态，该态与NiO₂层中具有\(3d_{x^2-y^2}\)对称性的准二维强关联态杂化。因此，无限层镍氧化物可以被视为稀土金属间化合物的“兄弟”，这些化合物以重费米子行为而闻名，其中NiO₂关联层在稀土重费米子化合物中扮演类似于\(4f\)态的角色。这种类似Kondo或Anderson晶格的“氧化物-金属间化合物”取代了莫特绝缘体作为参考态，超导性正是在这种参考态的基础上通过掺杂而出现的。
Superconductivity with 𝑇𝑐 ≈15 K was recently found in doped NdNiO2. The Ni1+O2 layers are expected to be Mott insulators, so hole doping should produce Ni2+ with 𝑆 =1, incompatible with robust superconductivity. We show that the NiO2 layers fall inside a critical region where the large 𝑝𝑑 hybridization favors a singlet 1𝐴1 hole-doped state like in CuO2. However, we find that the superexchange is about one order smaller than in cuprates, thus a magnon “glue” is very unlikely and another mechanism needs to be found.
最近在掺杂的 NdNiO2 中发现了约 15 K 的超导性。NdNiO2 中的 Ni1+O2 层被认为是莫特绝缘体，因此空穴掺杂应该会产生自旋为 1 的 Ni2+，这与稳健的超导性不相容。研究表明，NiO2 层处于一个临界区域内，较大的 pd 杂化有利于形成类似于 CuO2 中的单重态 1A1 空穴掺杂态。然而，我们发现其超交换作用比铜氧化物小一个数量级，因此磁振子“胶水”非常不可能，需要寻找其他机制。
We report on superconductivity in Nd1−xEuxNiO2 using Eu as a 4f dopant of the parent NdNiO2 infinite-layer compound. We use an all–in situ molecular beam epitaxy reduction process to achieve the superconducting phase, providing an alternate method to the ex situ CaH2 reduction process to induce superconductivity in the infinite-layer nickelates. The Nd1−xEuxNiO2 samples exhibit a step-terrace structure on their surfaces, have a Tc onset of 21 K at x = 0.25, and have a large upper critical field that may be related to Eu 4f doping.
我们报告了在Nd₁₋ₓEuₓNiO₂中观察到的超导性，其中Eu作为母体化合物NdNiO₂无限层中的4f掺杂剂。我们采用全原位分子束外延还原工艺实现了超导相，为在无限层镍氧化物中诱导超导性提供了一种与体外CaH₂还原工艺不同的方法。Nd₁₋ₓEuₓNiO₂样品的表面呈现出台阶-平台结构，在x = 0.25时Tc起始温度为21 K，并且具有较大的上临界场，这可能与Eu的4f掺杂有关。
In materials science, much effort has been devoted to the reproduction of superconductivity in chemical compositions, analogous to cuprate superconductors since their discovery over 30 years ago. This approach was recently successful in realising superconductivity in infinite-layer nickelates. Although differing from cuprates in electronic and magnetic properties, strong Coulomb interactions suggest that infinite-layer nickelates have a propensity towards various symmetry-breaking orders that populate cuprates. Here we report the observation of charge density waves (CDWs) in infinite-layer NdNiO2 films using Ni L3 resonant X-ray scattering. Remarkably, CDWs form in Nd 5d and Ni 3d orbitals at the same commensurate wavevector (0.333, 0) reciprocal lattice units, with non-negligible out-of-plane dependence and an in-plane correlation length of up to ~60 Å. Spectroscopic studies reveal a strong connection between CDWs and Nd 5d–Ni 3d orbital hybridization. Upon entering the superconducting state at 20% Sr doping, the CDWs disappear. Our work demonstrates the existence of CDWs in infinite-layer nickelates with a multiorbital character distinct from cuprates, which establishes their low-energy physics.
在材料科学中，人们致力于在类似于铜氧化物超导体的化学组成中复现超导性，自30多年前发现铜氧化物超导体以来，这一努力从未停止。最近，这种方法在无限层镍氧化物中取得了成功。尽管在电子和磁性特性上与铜氧化物有所不同，但强烈的库仑相互作用表明，无限层镍氧化物倾向于表现出各种在铜氧化物中常见的对称性破缺序。在此，我们报告了在无限层NdNiO₂薄膜中观察到的电荷密度波（CDWs），这是通过Ni L₃共振X射线散射实现的。值得注意的是，CDWs在Nd 5d和Ni 3d轨道上形成，其波矢为（0.333,0）倒格点单位，具有不可忽视的面外依赖性，且面内相关长度可达约60 Å。光谱研究表明，CDWs与Nd 5d-Ni 3d轨道杂化之间存在强烈的联系。当进入20%Sr掺杂的超导态时，CDWs消失。我们的工作证明了无限层镍氧化物中存在具有多轨道特性的CDWs，这与铜氧化物明显不同，从而确立了其低能物理。
The article investigates the structural transition, electric transport, and electronic structures in the compressed trilayer nickelate La4Ni3O10. The study explores how the material's properties change under compression, focusing on its structural transformations, electrical conductivity, and underlying electronic mechanisms. The findings provide insights into the behavior of trilayer nickelates under pressure and their potential applications in condensed matter physics.
本文研究了在压缩条件下的三层镍酸盐La4Ni3O10的结构转变、电输运性质和电子结构。研究探讨了材料在压缩下性质的变化，重点关注其结构转变、电导率以及背后的电子机制。研究结果为理解三层镍酸盐在压力下的行为及其在凝聚态物理中的潜在应用提供了新的见解。
The discovery of superconductivity in infinite-layer nickelates brings us tantalizingly close to a material class that mirrors the cuprate superconductors. We measured the magnetic excitations in these nickelates using resonant inelastic x-ray scattering at the Ni L3-edge. Undoped NdNiO2 possesses a branch of dispersive excitations with a bandwidth of approximately 200 milli–electron volts, which is reminiscent of the spin wave of strongly coupled, antiferromagnetically aligned spins on a square lattice. The substantial damping of these modes indicates the importance of coupling to rare-earth itinerant electrons. Upon doping, the spectral weight and energy decrease slightly, whereas the modes become overdamped. Our results highlight the role of Mottness in infinite-layer nickelates.
无限层镍氧化物中超导性的发现让我们接近于找到一类类似于铜氧化物超导体的材料。我们通过在Ni L3边进行共振非弹性X射线散射，测量了这些镍氧化物的磁激发。未掺杂的NdNiO₂具有一支带宽约为200毫电子伏特的色散激发，这让人联想到在正方形晶格上强耦合、反铁磁排列的自旋的自旋波。这些模式的显著阻尼表明了与稀土巡游电子耦合的重要性。在掺杂后，光谱权重和能量略有下降，而模式变得过阻尼。我们的结果突出了无限层镍氧化物中Mottness（莫特特性）的作用。
Revealing the momentum-resolved electronic structure of infinite-layer nickelates is essential for understanding this class of unconventional superconductors but has been hindered by the formidable challenges in improving the sample quality. In this work, we report the angle-resolved photoemission spectroscopy of superconducting La0.8Sr0.2NiO2 films prepared by molecular beam epitaxy and in situ atomic-hydrogen reduction. The measured Fermi topology closely matches theoretical calculations, showing a large Ni \(d_{x^2-y^2}\)–derived Fermi sheet that evolves from hole-like to electron-like along kz and a three-dimensional (3D) electron pocket centered at the Brillouin zone corner. The Ni \(d_{x^2-y^2}\)–derived bands show a mass enhancement (m*/mDFT) of 2 to 3, while the 3D electron band shows negligible band renormalization. Moreover, the Ni \(d_{x^2-y^2}\)–derived states also display a band dispersion anomaly at higher binding energy, reminiscent of the waterfall feature and kinks observed in cuprates.
揭示无限层镍氧化物的动量分辨电子结构对于理解这一类非常规超导体至关重要，但此前由于样品质量提升的巨大挑战而受到阻碍。在本工作中，我们报告了通过分子束外延和原位原子氢还原制备的超导La0.8Sr0.2NiO2薄膜的角分辨光电子能谱。测量得到的费米拓扑结构与理论计算高度吻合，显示出一个由Ni \(d_{x^2-y^2}\)轨道导出的费米面，该费米面从空穴型向电子型演化，并且在布里渊区角中心处有一个三维（3D）电子口袋。Ni \(d_{x^2-y^2}\)导出的能带表现出2到3的质量增强（m*/mDFT），而三维电子带则显示出可忽略的能带重整化。此外，Ni \(d_{x^2-y^2}\)导出的状态还在更高的束缚能处显示出能带色散异常，类似于在铜氧化物中观察到的“瀑布”特征和能带扭结。
Since the discovery of high-Tc superconductivity in cuprates, numerous experimental and theoretical investigations have been carried out aiming at finding more superconductors with higher Tc and unveiling the mysteries mechanisms. In this work, the authors report the observation of a positive pressure effect on the superconducting transition temperature (Tc) in Pr0.82Sr0.18NiO2 thin films. The Tc increases monotonically from ~17 K at ambient pressure to ~31 K at 12.1 GPa without showing any sign of saturation. This result indicates that the Tc of these nickelate superconductors can be further enhanced by higher pressures. The study provides insights into the potential of using pressure to raise the superconducting transition temperature in this new class of superconductors.
自从铜氧化物高温超导体被发现以来，人们进行了大量的实验和理论研究，旨在寻找具有更高超导转变温度（Tc）的超导体，并揭示其神秘的机制。在本研究中，作者们报告了在Pr0.82Sr0.18NiO2薄膜中观察到的正压力效应，其超导转变温度（Tc）随压力增加而单调上升，从环境压力下的约17 K增加到12.1 GPa下的约31 K，并且没有显示出饱和迹象。这一结果表明，通过施加更高压力可以进一步提高这类镍氧化物超导体的Tc。该研究为利用压力提高这一新型超导体的超导转变温度提供了新的见解。
A hallmark of many unconventional superconductors is the presence of many-body interactions that give rise to broken-symmetry states intertwined with superconductivity. Recent resonant soft X-ray scattering experiments report commensurate 3a0 charge density wave order in infinite-layer nickelates, which has important implications regarding the universal interplay between charge order and superconductivity in both cuprates and nickelates. Here we present X-ray scattering and spectroscopy measurements on a series of NdNiO2+x samples, which reveal that the signatures of charge density wave order are absent in fully reduced, single-phase NdNiO2. The 3a0 superlattice peak instead originates from a partially reduced impurity phase where excess apical oxygens form ordered rows with three-unit-cell periodicity. The absence of any observable charge density wave order in NdNiO2 highlights a crucial difference between the phase diagrams of cuprate and nickelate superconductors.
许多非常规超导体的一个重要特征是存在多体相互作用，这些相互作用导致了与超导性交织在一起的破缺对称性态。最近的共振软X射线散射实验报告称，在无限层镍氧化物中观察到了3a₀电荷密度波序，这对于理解铜氧化物和镍氧化物中超导性与电荷序之间的普遍相互作用具有重要意义。在此，我们对一系列NdNiO₂+x样品进行了X射线散射和光谱测量，结果显示，在完全还原的单相NdNiO₂中不存在电荷密度波序的迹象。相反，3a₀超晶格峰实际上来源于部分还原的杂质相，其中多余的顶氧原子形成了具有三个单元周期的有序行。在NdNiO₂中未观察到任何电荷密度波序，这突显了铜氧化物和镍氧化物超导体相图之间的一个关键差异。
The discovery of superconductivity in infinite-layer nickelates has reignited interest in understanding the mechanisms underlying high-temperature superconductivity. Recent studies have reported various signatures of unconventional superconductivity in these materials, including magnetic excitations and potential charge density wave orders. This article presents evidence for d-wave superconductivity in infinite-layer nickelates based on low-energy electrodynamics measurements. The findings suggest that the superconducting order parameter in these materials has a nodal structure similar to that observed in cuprate superconductors, providing important insights into the universal mechanisms of high-temperature superconductivity.
在无限层镍氧化物中发现的超导性重新点燃了对高温超导机制的理解兴趣。最近的研究报告称，这些材料中存在各种非常规超导性的迹象，包括磁激发和潜在的电荷密度波序。本文基于低能电动力学测量，提供了无限层镍氧化物中d波超导性的证据。研究结果表明，这些材料中的超导序参数具有类似于铜氧化物超导体中观察到的节点结构，为理解高温超导的普适机制提供了重要见解。
Superconductivity in infinite-layer nickelates holds exciting analogies with that of cuprates, with similar structures and 3𝑑-electron count. Using resonant inelastic x-ray scattering, we studied electronic and magnetic excitations and charge density correlations in Nd1−𝑥Sr𝑥NiO2 thin films with and without an SrTiO3 capping layer. We observe dispersing magnons only in the capped samples, progressively dampened at higher doping. We detect an elastic resonant scattering peak in the uncapped 𝑥 =0 compound at wave vector (∼⅓,0), reminiscent of the charge order signal in hole-doped cuprates. The peak weakens at 𝑥 =0.05 and disappears in the superconducting 𝑥 =0.20 film. The role of the capping on the electronic reconstruction far from the interface remains to be understood.
无限层镍氧化物中的超导性与铜氧化物中的超导性具有许多相似之处，包括类似的结构和3𝑑电子数量。通过共振非弹性X射线散射，我们研究了有无SrTiO₃盖层的Nd₁₋ₓSrₓNiO₂薄膜中的电子激发、磁激发以及电荷密度关联。我们仅在有盖层的样品中观察到色散的磁子，且随着掺杂浓度的增加，这些磁子逐渐被抑制。在未盖层的𝑥 = 0化合物中，我们在波矢（∼⅓,0）处检测到一个弹性共振散射峰，这让人联想到空穴掺杂铜氧化物中的电荷序信号。该峰在𝑥 = 0.05时减弱，并在超导的𝑥 = 0.20薄膜中消失。盖层在远离界面处对电子重构的作用仍有待进一步理解。
The discovery of superconductivity in infinite-layer nickelates has sparked significant interest in understanding the underlying mechanisms of high-temperature superconductivity. This study investigates the electronic and magnetic properties of infinite-layer nickelates using resonant inelastic X-ray scattering (RIXS) and reports the observation of a broken translational symmetry state. The results reveal a strong interlayer magnetic exchange coupling and the presence of charge order, which are crucial for understanding the superconducting mechanism in these materials. The findings provide important insights into the complex interplay between charge, spin, and lattice degrees of freedom in infinite-layer nickelates.
在无限层镍氧化物中发现的超导性激发了对高温超导机制的深入研究。本研究利用共振非弹性X射线散射（RIXS）研究了无限层镍氧化物的电子和磁性质，并报告了观察到的破缺平移对称性态。研究结果揭示了强的层间磁交换耦合和电荷序的存在，这对于理解这些材料中的超导机制至关重要。这些发现为理解无限层镍氧化物中电荷、自旋和晶格自由度之间的复杂相互作用提供了重要见解。
The discovery of superconductivity in Nd0.8Sr0.2NiO2 introduced a new family of layered nickelate superconductors that has now been extended to include a range of strontium doping, praseodymium or lanthanum in place of neodymium, and the five-layer compound Nd6Ni5O12. A number of studies have indicated that electron correlations are strong in these materials, a feature that often leads to the emergence of magnetism. Here we report muon spin rotation/relaxation studies of a series of superconducting infinite-layer nickelates. Regardless of the rare earth ion or doping, we observe an intrinsic magnetic ground state arising from local moments on the nickel sublattice. The coexistence of magnetism—which is likely to be antiferromagnetic and short-range ordered—with superconductivity is reminiscent of some iron pnictides and heavy fermion compounds, and qualitatively distinct from the doped cuprates.
在Nd0.8Sr0.2NiO2中发现的超导性引入了一类新的层状镍氧化物超导体，这一家族现在已经扩展到包括一系列锶掺杂、用镨或镧替代钕，以及五层化合物Nd6Ni5O12。多项研究表明，这些材料中的电子关联性很强，这一特性通常会导致磁性的出现。在此，我们报告了一系列超导无限层镍氧化物的μ子自旋旋转/弛豫研究。无论稀土离子或掺杂情况如何，我们观察到一个由镍亚晶格上的局域矩引起的内在磁性基态。这种磁性——很可能是反铁磁性和短程有序的——与超导性的共存让人联想到某些铁基超导体和重费米子化合物，与掺杂的铜氧化物有明显的区别。
The recent discovery of the superconductivity in the doped infinite layer nickelates RNiO2 (R = La, Pr, Nd) is of great interest since the nickelates are isostructural to doped (Ca, Sr)CuO2 having superconducting transition temperature (Tc) of about 110 K. Verifying the commonalities and differences between these oxides will certainly give a new insight into the mechanism of high Tc superconductivity in correlated electron systems. In this paper, we review experimental and theoretical works on this new superconductor and discuss the future perspectives for the 'nickel age' of superconductivity.
最近在掺杂的无限层镍氧化物RNiO₂（R = La, Pr, Nd）中发现的超导性引起了广泛关注，因为这些镍氧化物与掺杂的(Ca, Sr)CuO₂具有相同的晶体结构，而后者的超导转变温度（Tc）约为110 K。验证这些氧化物之间的相似性和差异性，将为理解强关联电子系统中的高温超导机制提供新的见解。本文综述了关于这一新型超导体的实验和理论研究，并讨论了镍氧化物超导性研究的未来方向。
The recent discovery of pressure-induced superconductivity in the bilayer La3Ni2O7 (LNO) has opened a new platform for the study of unconventional superconductors. In this publication, we investigate theoretically the whole family of bilayer 327-type nickelates 𝑅3Ni2O7 (𝑅 = rare-earth elements) under pressure. From La to Lu, the lattice constants and volume decrease, leading to enhanced in-plane and out-of-plane hoppings, resulting in an effectively reduced electronic correlation 𝑈/𝑊. Furthermore, the Ni's 𝑡2𝑔 states shift away from the 𝑒𝑔 states, while the crystal-field splitting between 𝑑3𝑧2−𝑟2 and 𝑑𝑥2−𝑦2 is almost unchanged. In addition, six candidates were found to become stable in the 𝐹𝑚𝑚𝑚 phase, with increasing values of critical pressure as the atomic number increases. Similar to the case of LNO, the 𝑠±-wave pairing tendency dominates in all candidates, due to the nesting between the M=(𝜋,𝜋) and the X=(𝜋,0) and Y=(0,𝜋) points in the Brillouin zone. Then, 𝑇𝑐 is expected to decrease as the radius of rare-earth (RE) ions decreases. Our results suggest that LNO is already the “optimal” candidate, with Ce a close competitor, among the whole of the RE bilayer nickelates, and to increase 𝑇𝑐 we suggest growing on special substrates with larger in-plane lattice spacings.
最近在双层La₃Ni₂O₇（LNO）中发现的由压力诱导的超导性为研究非常规超导体开辟了新的平台。在本文中，我们从理论上研究了在压力下双层327型镍氧化物𝑅₃Ni₂O₇（𝑅 = 稀土元素）的整个家族。从La到Lu，晶格常数和体积减小，导致平面内和平面外跃迁增强，从而有效地降低了电子关联𝑈/𝑊。此外，Ni的𝑡₂𝑔态从𝑒𝑔态移开，而𝑑₃z²⁻r²和𝑑x²⁻y²之间的晶体场劈裂几乎保持不变。此外，我们发现有六种候选化合物在𝐹mmm相中变得稳定，随着原子序数的增加，临界压力值也增加。与LNO的情况类似，由于布里渊区中M=(𝜋,𝜋)点与X=(𝜋,0)和Y=(0,𝜋)点之间的嵌套，所有候选化合物中都以𝑠±波配对倾向为主。因此，随着稀土（RE）离子半径的减小，Tc预计将降低。我们的结果表明，在所有稀土双层镍氧化物中，LNO已经是“最佳”候选化合物，而Ce是一个接近的竞争者。为了提高Tc，我们建议在具有更大平面内晶格间距的特殊衬底上生长。
Using ab initio band structure and DFT+dynamical mean-field theory methods we examine the effects of electron-electron interactions on the normal state electronic structure, Fermi surface, and magnetic correlations of the recently discovered double-layer perovskite superconductor La3Ni2O7 under pressure. Our results suggest the formation of a negative charge transfer mixed-valence state with the Ni valence close to 1.75 +. We find a remarkable orbital-selective renormalization of the Ni 3𝑑 bands, with 𝑚*/𝑚∼3 and 2.3 for the Ni 3𝑧2−𝑟2 and 𝑥2−𝑦2 orbitals, respectively, in agreement with experimental estimates. Our results for the 𝑘-dependent spectral functions and Fermi surfaces show significant incoherence of the Ni 3𝑧2−𝑟2 states, implying the proximity of the Ni 3𝑑 states to orbital-dependent localization. Based on our analysis of the static magnetic susceptibility, we propose the possible formation of the spin and charge (or bond) density wave stripe states in high-pressure La3Ni2O7.
利用从头算能带结构和DFT+动力学平均场理论方法，我们研究了电子-电子相互作用对最近发现的双层钙钛矿超导体La₃Ni₂O₇在压力下的正常态电子结构、费米面和磁性关联的影响。我们的结果表明，形成了一个负电荷转移的混合价态，Ni的价态接近1.75+。我们发现Ni的3d带存在显著的轨道选择性重整化，Ni的3dz²−r²和3dx²−y²轨道的有效质量增强因子分别为m*/m ≈ 3和2.3，与实验估计一致。我们对k依赖的光谱函数和费米面的结果显示，Ni的3dz²−r²态存在显著的非相干性，暗示Ni的3d态接近轨道依赖的局域化。基于我们对静态磁化率的分析，我们提出在高压La₃Ni₂O₇中可能形成自旋和电荷（或键）密度波条纹态。
Although high-transition-temperature (high-\(T_{\text{c}}\)) superconductivity in cuprates has been known for more than three decades, the underlying mechanism remains unknown. Cuprates are the only unconventional superconductors that exhibit bulk superconductivity with \(T_{\text{c}}\) above the liquid-nitrogen boiling temperature of \(77\,\text{K}\). Here we observe that high-pressure resistance and mutual inductive magnetic susceptibility measurements showed signatures of superconductivity in single crystals of \(\text{La}_{3}\text{Ni}_{2}\text{O}_{7}\) with maximum \(T_{\text{c}}\) of \(80\,\text{K}\) at pressures between \(14.0\,\text{GPa}\) and \(43.5\,\text{GPa}\). The superconducting phase under high pressure has an orthorhombic structure of \(Fmmm\) space group with the \(3d_{x^2-y^2}\) and \(3d_{z^2}\) orbitals of \(\text{Ni}\) cations strongly mixing with oxygen \(2p\) orbitals. Our density functional theory calculations indicate that the superconductivity emerges coincidently with the metallization of the \(\sigma\)-bonding bands under the Fermi level, consisting of the \(3d_{z^2}\) orbitals with the apical oxygen ions connecting the \(\text{Ni–O}\) bilayers. Thus, our discoveries provide not only important clues for the high-\(T_{\text{c}}\) superconductivity in this Ruddlesden–Popper double-layered perovskite nickelates but also a previously unknown family of compounds to investigate the high-\(T_{\text{c}}\) superconductivity mechanism.
尽管铜氧化物高温超导性（高温超导）已为人所知超过三十年，但其背后的机制仍然未知。铜氧化物是唯一表现出体超导性的非常规超导体，其超导转变温度（\(T_{\text{c}}\)）高于液氮沸点77 K。在此，我们观察到，在14.0 GPa至43.5 GPa的压力下，La₃Ni₂O₇单晶的高压电阻和相互感应磁化率测量显示出超导性的迹象，最大\(T_{\text{c}}\)为80 K。高压下的超导相具有正交晶结构，空间群为Fmmm，其中Ni阳离子的\(3d_{x^2-y^2}\)和\(3d_{z^2}\)轨道与氧\(2p\)轨道强烈混合。我们的密度泛函理论计算表明，超导性与\(\sigma\)键合带在费米能级下的金属性同时出现，这些\(\sigma\)键合带由连接\(\text{Ni–O}\)双层的顶端氧离子的\(3d_{z^2}\)轨道组成。因此，我们的发现不仅为这种Ruddlesden–Popper双层钙钛矿镍氧化物中的高温超导性提供了重要线索，而且还提供了一个以前未知的化合物家族，以研究高温超导机制。
Motivated by the recent discovery of superconductivity in La3Ni2O7 under pressure, we discuss the basic ingredients of a model that captures its microscopic physics under pressure tuning. We anchor our description in terms of the spectroscopic evidence of strong correlations in this system. In a bilayer Hubbard model including the Ni 3𝑑 𝑥2−𝑦2 and 𝑧2 orbitals, we show the ground state of the model crosses over from a low-spin 𝑆=1/2 state to a high-spin 𝑆=3/2 state. In the high-spin state, the two 𝑥2−𝑦2 and the bonding 𝑧2 orbitals are all close to half-filling, which promotes a strong orbital selectivity in a broad crossover regime of the phase diagram pertinent to the system. Based on these results, we construct an effective multiorbital 𝑡−𝐽 model to describe the superconductivity of the system, and find the leading pairing channel to be an intraorbital spin singlet with a competition between the extended 𝑠-wave and 𝑑𝑥2−𝑦2 symmetries. Our results highlight the role of strong multiorbital correlation effects in driving the superconductivity of La3Ni2O7.
受La₃Ni₂O₇在压力下表现出超导性的启发，我们讨论了一个模型的基本要素，该模型能够捕捉其在压力调控下的微观物理特性。我们将描述建立在该体系中强关联的光谱学证据基础上。在一个包括Ni 3dx²−y²和z²轨道的双层Hubbard模型中，我们展示了模型的基态从低自旋S=1/2态转变为高自旋S=3/2态。在高自旋态中，两个x²−y²轨道和成键的z²轨道都接近半填充，这在与系统相关的相图的广泛交叉区域中促进了强烈的轨道选择性。基于这些结果，我们构建了一个有效的多轨道t-J模型来描述系统的超导性，并发现主要的配对通道是轨道内的自旋单态，其中扩展的s波和dx²−y²对称性之间存在竞争。我们的结果强调了强多轨道关联效应在驱动La₃Ni₂O₇超导性中的作用。
The newly discovered Ruddlesden-Popper bilayer La3Ni2O7 reaches a remarkable superconducting transition temperature 𝑇𝑐≈80 K under a pressure of above 14 GPa. Here we propose a minimal bilayer two-orbital model of the high-pressure phase of La3Ni2O7. Our model is constructed with the Ni-3𝑑𝑥2−𝑦2, 3𝑑3𝑧2−𝑟2 orbitals by using Wannier downfolding of the density functional theory calculations, which captures the key ingredients of the material, such as band structure and Fermi surface topology. There are two electron pockets, 𝛼, 𝛽, and one hole pocket, 𝛾, on the Fermi surface, in which the 𝛼, 𝛽 pockets show mixing of two orbitals, while the 𝛾 pocket is associated with Ni−𝑑3𝑧2−𝑟2 orbital. The random phase approximation spin susceptibility reveals a magnetic enhancement associated with the 𝑑3𝑧2−𝑟2 state. A higher energy model with O−𝑝 orbitals is also provided for further study.
最近发现的Ruddlesden-Popper双层La₃Ni₂O₇在超过14 GPa的压力下达到了约80 K的超导转变温度（Tc）。在此，我们提出了一个La₃Ni₂O₇高压相的最小双层两轨道模型。我们的模型通过Wannier折叠密度泛函理论计算构建，使用了Ni-3dx²−y²和3d3z²−r²轨道，捕捉了材料的关键要素，如能带结构和费米面拓扑。在费米面上有两个电子口袋α、β和一个空穴口袋γ，其中α、β口袋显示出两个轨道的混合，而γ口袋与Ni−d3z²−r²轨道相关。随机相位近似自旋易感性揭示了与d3z²−r²态相关的磁性增强。还提供了一个包含O−p轨道的更高能级模型以供进一步研究。
Recently, the bilayer perovskite nickelate La3Ni2O7 has been reported to show evidence of high-temperature superconductivity (SC) under a moderate pressure of about 14 GPa. To investigate the superconducting mechanism, pairing symmetry, and the role of apical-oxygen deficiencies in this material, we perform a random-phase approximation based study on a bilayer model consisting of the 𝑑𝑥2−𝑦2 and 𝑑3𝑧2−𝑟2 orbitals of Ni atoms in both the pristine crystal and the crystal with apical-oxygen deficiencies. Our analysis reveals an 𝑠±-wave pairing symmetry driven by spin fluctuations. The crucial role of pressure lies in that it induces the emergence of the 𝛾 pocket, which is involved in the strongest Fermi-surface nesting. We further found the emergence of local moments in the vicinity of apical-oxygen deficiencies, which significantly suppresses the 𝑇𝑐. Therefore, it is possible to significantly enhance the 𝑇𝑐 by eliminating oxygen deficiencies during the synthesis of the samples.
最近，双层钙钛矿镍氧化物La₃Ni₂O₇在约14 GPa的中等压力下显示出高温超导性（SC）的证据。为了研究其超导机制、配对对称性以及顶端氧缺陷的作用，我们基于一个双层模型进行了随机相位近似研究。该模型由 pristine 晶体和顶端氧缺陷晶体中的Ni原子的dx²−y²和d3z²−r²轨道组成。我们的分析揭示了由自旋涨落驱动的s±波配对对称性。压力的关键作用在于其诱导了γ口袋的出现，该口袋参与了最强的费米面嵌套。我们还发现，在顶端氧缺陷附近出现了局域磁矩，这显著抑制了Tc。因此，在样品合成过程中消除氧缺陷有可能显著提高Tc。
Recently, the bulk nickelate La3Ni2O7 is reported to show a signature of high-temperature superconductivity under high pressure above 14 GPa. We analyze the pairing mechanism and pairing symmetry in a bilayer Hubbard model with two orbitals in the 𝐸𝑔 multiplet. In the weak to moderate interaction regime, our functional renormalization group (FRG) calculations yield 𝑠±-wave Cooper pairing triggered by leading spin fluctuations. The gap function changes sign across the Fermi pockets, and in real space the pairing is dominated by intra-unit-cell intraorbital components with antiphase between the on-site ones. In the strong-coupling limit, we develop a low-energy effective theory in terms of atomic one- and two-electron states in the 𝐸𝑔 multiplet. The variational treatment of the effective theory produces results consistent with the FRG ones, suggesting the robustness of such a pairing function. The driving force for superconductivity in the strong-coupling limit can be attributed to the local pair-hopping term and the spin exchange on vertical bonds. We also discuss a possible scenario for the weak insulating behavior under low pressures in terms of the tendency toward the formation of charge order in the strong-coupling limit.
最近，块体镍氧化物La₃Ni₂O₇被报道在超过14 GPa的高压下显示出高温超导性的迹象。我们分析了双层Hubbard模型中两个\(E_g\)多重态轨道的配对机制和配对对称性。在弱到中等相互作用区域，我们的泛函重正化群（FRG）计算得出由主要自旋涨落触发的s±波库珀配对。能隙函数在费米口袋之间改变符号，在实空间中，配对主要由单位晶胞内同一轨道的组分主导，现场组分之间存在反相位。在强耦合极限下，我们基于\(E_g\)多重态中的原子单电子和双电子态开发了一个低能有效理论。有效理论的变分处理产生了与FRG一致的结果，表明这种配对函数的稳健性。在强耦合极限下，超导性的驱动力可以归因于局部对跃迁项和垂直键上的自旋交换。我们还讨论了在强耦合极限下，低压下弱绝缘行为的一种可能情景，即倾向于形成电荷序。
The newly discovered high-temperature superconductivity in La3Ni2O7 under pressure has attracted a great deal of attention. The essential ingredient characterizing the electronic properties is the bilayer NiO2 planes coupled by the interlayer bonding of 3𝑑𝑧2 orbitals through the intermediate oxygen atoms. In the strong coupling limit, the low-energy physics is described by an intralayer antiferromagnetic spin-exchange interaction 𝐽∥ between 3𝑑𝑥2−𝑦2 orbitals and an interlayer one 𝐽⊥ between 3𝑑𝑧2 orbitals. Taking into account Hund’s rule on each site and integrating out the 3𝑑𝑧2 spin degree of freedom, the system reduces to a single-orbital bilayer 𝑡−𝐽 model based on the 3𝑑𝑥2−𝑦2 orbital. By employing the slave-boson approach, the self-consistent equations for the bonding and pairing order parameters are solved. Near the physically relevant 1/4-filling regime (doping 𝛿 =0.3 ∼0.5), the interlayer coupling 𝐽⊥ tunes the conventional single-layer 𝑑-wave superconducting state to the 𝑠-wave one. A strong 𝐽⊥ could enhance the interlayer superconducting order, leading to a dramatically increased 𝑇𝑐. Interestingly, there could exist a finite regime in which an 𝑠 +𝑖𝑑 state emerges.
最近发现的La₃Ni₂O₇在压力下的高温超导性引起了广泛关注。其电子特性的关键因素是通过中间氧原子的层间键合耦合的双层NiO₂平面。在强耦合极限下，低能物理由层内反铁磁自旋交换相互作用J∥（3dx²−y²轨道之间）和层间相互作用J⊥（3dz²轨道之间）描述。考虑每个位点上的Hund规则并积分掉3dz²自旋自由度后，系统简化为基于3dx²−y²轨道的单轨道双层t-J模型。通过使用奴玻色子方法，求解了键合和配对序参量的自洽方程。在接近物理相关的1/4填充区域（掺杂δ=0.3~0.5），层间耦合J⊥将常规的单层d波超导态调谐为s波态。强烈的J⊥能增强层间超导序，从而显著提高Tc。有趣的是，可能存在一个有限区域，在其中会出现s+id态。
Recently, superconductivity with a 𝑇𝑐 up to 78 K has been reported in bulk samples of the bilayer nickelate La3Ni2O7 at pressures above 14 GPa. Important theoretical tasks are the formulation of relevant low-energy models and the clarification of the normal state properties. Here, we study the correlated electronic structure of the high-pressure phase in a four-orbital low-energy subspace using different many-body approaches: 𝐺𝑊, dynamical mean field theory (DMFT), extended DMFT (EDMFT) and 𝐺𝑊+EDMFT, with realistic frequency-dependent interaction parameters. The nonlocal correlation and screening effects captured by 𝐺𝑊+EDMFT result in an instability toward the formation of charge stripes, with the 3𝑑𝑧2 as the main active orbital. We also comment on the potential relevance of the rare-earth self-doping pocket, since hole doping suppresses the ordering tendency.
最近，双层镍氧化物La₃Ni₂O₇在超过14 GPa的压力下展现出高达78 K的超导性。重要的理论任务是制定相关的低能模型并阐明正常态特性。在此，我们使用不同的多体方法研究高压相在四轨道低能子空间中的相关电子结构：GW、动态平均场理论（DMFT）、扩展DMFT（EDMFT）和GW+EDMFT，采用现实的频率依赖相互作用参数。GW+EDMFT捕获的非局域相关和屏蔽效应导致电荷条纹形成的不稳定性，其中3dz²为主要活性轨道。我们还讨论了稀土自掺杂口袋的潜在相关性，因为空穴掺杂抑制了有序趋势。
The recent discovery of high-𝑇𝑐 superconductivity in bilayer nickelate La3Ni2O7 under high pressure has stimulated great interest concerning its pairing mechanism. We argue that the weak coupling model from the almost fully filled 𝑑𝑧2 bonding band cannot give rise to its high 𝑇𝑐, and thus propose a strong coupling model based on local interlayer spin singlets of Ni-𝑑𝑧2 electrons due to their strong on-site Coulomb repulsion. This leads to a minimal effective model that contains local pairing of 𝑑𝑧2 electrons and a considerable hybridization with near quarter-filled itinerant 𝑑𝑥2−𝑦2 electrons on nearest-neighbor sites. Their strong coupling provides a unique two-component scenario to achieve high-𝑇𝑐 superconductivity. Our theory highlights the importance of the bilayer structure of superconducting La3Ni2O7 and points out a potential route for the exploration of more high-𝑇𝑐 superconductors.
最近在双层镍氧化物La₃Ni₂O₇中发现其在高压下具有高温超导性，这激发了对其配对机制的极大兴趣。我们认为，基于几乎完全填满的dz²成键带的弱耦合模型无法解释其高温超导性，因此提出了一种基于Ni-dz²电子的局域层间自旋单态的强耦合模型，这是由于它们具有强烈的现场库仑排斥。这导致了一个最小有效模型，该模型包含dz²电子的局域配对以及与最近邻位点上接近四分之一填充的巡游dx²−y²电子的显著杂化。它们的强耦合为实现高温超导性提供了一种独特的双组分场景。我们的理论强调了超导La₃Ni₂O₇的双层结构的重要性，并指出了探索更多高温超导体的潜在途径。
Motivated by the report of superconductivity in bilayer La3Ni2O7 at high pressure, we examine the interacting electrons in this system. First-principles many-body theory is utilized to study the normal-state electronic properties. Below 100 K, a multiorbital non-Fermi-liquid state resulting from a loss of Ni-ligand coherence within a flat-band-dominated low-energy landscape is uncovered. The incoherent low-temperature Fermi surface displays strong mixing between Ni−𝑑𝑧2 and Ni−𝑑𝑥2−𝑦2 orbital character. In a model Hamiltonian picture, spin fluctuations originating mostly from the Ni−𝑑𝑧2 orbital give rise to strong tendencies towards a superconducting instability with a 𝐵1𝑔 or 𝐵2𝑔 order parameter. The dramatic enhancement of 𝑇c in pressurized La3Ni2O7 is due to stronger Ni−𝑑𝑧2 correlations compared to those in the infinite-layer nickelates.
受双层La₃Ni₂O₇在高压下表现出超导性的报道启发，我们研究了该体系中的相互作用电子。利用基于第一性原理的多体理论研究了其正常态的电子性质。在100 K以下，发现了一个多轨道非费米液体态，这是由于在以平带为主的低能景观中，Ni-配体相干性丧失所致。不相干的低温费米面显示出Ni−dz²和Ni−dx²−y²轨道特征之间的强烈混合。在模型哈密顿量图像中，主要来自Ni−dz²轨道的自旋涨落引发了强烈的超导不稳定性倾向，其序参数为B₁g或B₂g。在加压的La₃Ni₂O₇中Tc的显著增强是由于Ni−dz²相关性比无限层镍氧化物中的更强。
The recent report of pressure-induced structural transition and signature of superconductivity with Tc ≈ 80 K above 14 GPa in La3Ni2O7 crystals has garnered considerable attention. To further elaborate this discovery, we carried out comprehensive resistance measurements on La3Ni2O7 crystals grown in an optical-image floating zone furnace under oxygen pressure (15 bar) using a diamond anvil cell (DAC) and cubic anvil cell (CAC), which employ a solid (KBr) and liquid (glycerol) pressure-transmitting medium, respectively. Sample 1 measured in the DAC exhibits a semiconducting-like behavior with large resistance at low pressures and gradually becomes metallic upon compression. At pressures P ⩾ 13.7 GPa we observed the appearance of a resistance drop of as much as ∼ 50% around 70 K, which evolves into a kink-like anomaly at pressures above 40 GPa and shifts to lower temperatures gradually with increasing magnetic field. These observations are consistent with the recent report mentioned above. On the other hand, sample 2 measured in the CAC retains metallic behavior in the investigated pressure range up to 15 GPa. The hump-like anomaly in resistance around ∼ 130 K at ambient pressure disappears at P ⩾ 2 GPa. In the pressure range of 11–15 GPa we observed the gradual development of a shoulder-like anomaly in resistance at low temperatures, which evolves into a pronounced drop of resistance of 98% below 62 K at 15 GPa, reaching a temperature-independent resistance of 20 μΩ below 20 K. Similarly, this resistance anomaly can be progressively shifted to lower temperatures by applying external magnetic fields, resembling a typical superconducting transition. Measurements on sample 3 in the CAC reproduce the resistance drop at pressures above 10 GPa and realize zero resistance below 10 K at 15 GPa even though an unusual semiconducting-like behavior is retained in the normal state. Based on these results, we constructed a dome-shaped superconducting phase diagram and discuss some issues regarding the sample-dependent behaviors on pressure-induced high-temperature superconductivity in the La3Ni2O7 crystals.
最近关于La₃Ni₂O₇晶体在超过14 GPa的压力下发生结构转变以及表现出Tc ≈ 80 K的超导性迹象的报道引起了广泛关注。为了进一步阐述这一发现，我们对在光学图像浮区炉中生长的La₃Ni₂O₇晶体（生长氧压为15 bar）进行了全面的电阻测量。测量使用了金刚石压砧（DAC）和立方压砧（CAC），分别采用固体（KBr）和液体（甘油）作为传压介质。在DAC中测量的样品1表现出类似半导体的行为，在低压力下电阻较大，随着压缩逐渐变为金属态。在压力P ⩾ 13.7 GPa时，我们观察到在约70 K处电阻下降了约50%，这一现象在超过40 GPa的压力下演变为类似拐点的异常，并且随着外加磁场的增加逐渐向低温移动。这些观测结果与上述最近的报道一致。另一方面，在CAC中测量的样品2在研究的压力范围内（高达15 GPa）保持金属行为。在常压下约130 K处的电阻峰状异常在P ⩾ 2 GPa时消失。在11–15 GPa的压力范围内，我们观察到低温下电阻逐渐出现肩状异常，这一现象在15 GPa时演变为在62 K以下电阻显著下降98%，在20 K以下电阻达到20 μΩ且不再随温度变化。类似地，这种电阻异常可以通过施加外部磁场逐渐向低温移动，类似于典型的超导转变。在CAC中对样品3的测量重现了在超过10 GPa的压力下电阻下降的现象，并且在15 GPa时即使在正常态保留了不寻常的类似半导体的行为，也能在10 K以下实现零电阻。基于这些结果，我们构建了一个穹顶状的超导相图，并讨论了关于La₃Ni₂O₇晶体中压力诱导的高温超导性的一些样品依赖性问题。
Recently, an 80 K superconductor was discovered in La3Ni2O7 under high pressure. Density function theory calculations identify 𝑑𝑥2−𝑦2,𝑑𝑧2 as the active orbitals on the bilayer square lattice with a 𝑑8−𝑥 configuration of Ni per site. Here, 𝑥 is the hole doping level. One naive expectation is to describe this system in terms of a two-orbital 𝑡−𝐽 model. However, we emphasize the importance of Hund's coupling 𝐽𝐻 and the 𝑥=0 limit should be viewed as a spin-one Mott insulator. Especially, the significant Hund's coupling shares the interlayer superexchange 𝐽⊥ of the 𝑑𝑧2 orbital to the 𝑑𝑥2−𝑦2 orbital, an effect that cannot be captured by conventional perturbation or mean-field approaches. This study first explores the limit where the 𝑑𝑧2 orbital is Mott localized, dealing with a one-orbital bilayer 𝑡−𝐽 model focused on the 𝑑𝑥2−𝑦2 orbital. Notably, we find that strong interlayer pairing survives up to 𝑥=0.5 hole doping driven by the transmitted 𝐽⊥, which explains the existence of a high Tc superconductor in the experiment at this doping level. Next, we uncover the more realistic situation where the 𝑑𝑧2 orbital is slightly hole-doped and cannot be simply integrated out. We take the 𝐽𝐻→+∞ limit and propose a type II 𝑡−𝐽 model with four spin-half singlon (𝑑7) states and three spin-one doublon (𝑑8) states. Employing a parton mean-field approach, we recover similar results as in the one-orbital 𝑡−𝐽 model, but now with the effect of the 𝐽⊥ automatically generated.
最近，在高压下的La₃Ni₂O₇中发现了一种80 K的超导体。密度泛函理论计算确定了dx²−y²和dz²是在双层正方晶格上具有Ni的d⁸⁻ˣ构型的活性轨道。这里的x表示空穴掺杂水平。一个简单的期望是用一个双轨道t-J模型来描述这个系统。然而，我们强调Hund耦合J_H的重要性，x=0的极限应该被视为一个自旋为一的莫特绝缘体。特别是，显著的Hund耦合将dz²轨道的层间超交换J⊥共享给dx²−y²轨道，这种效应无法通过传统的微扰或平均场方法捕捉到。这项研究首先探索了dz²轨道莫特局域化的极限，处理了一个专注于dx²−y²轨道的单轨道双层t-J模型。值得注意的是，我们发现在x=0.5空穴掺杂水平下，强烈的层间配对得以存活，这是由传递的J⊥驱动的，这解释了在这个掺杂水平下实验中存在高温超导体的原因。接下来，我们揭示了更现实的情况，即dz²轨道有轻微的空穴掺杂，不能简单地被积分掉。我们取J_H→+∞极限，并提出了一个II型t-J模型，包含四个自旋为1/2的单重态（d⁷）和三个自旋为一的双重态（d⁸）。采用部分子平均场方法，我们得到了与单轨道t-J模型相似的结果，但现在J⊥的效应是自动生成的。
High-Tc superconductivity with possible Tc ≈ 80 K has been reported in the single crystal of La3Ni2O7 under high pressure. Based on the electronic structure given by the density functional theory calculations, we propose an effective bi-layer model Hamiltonian including both 3dz2 and 3dx2–y2 orbital electrons of the nickel cations. The main feature of the model is that the 3dz2 electrons form inter-layer σ-bonding and anti-bonding bands via the apical oxygen anions between the two layers, while the 3dx2–y2 electrons hybridize with the 3dz2 electrons within each NiO2 plane. The chemical potential difference of these two orbital electrons ensures that the 3dz2 orbitals are close to half-filling and the 3dx2–y2 orbitals are near quarter-filling. The strong on-site Hubbard repulsion of the 3dz2 orbital electrons gives rise to an effective inter-layer antiferromagnetic spin super-exchange J. Applying pressure can self dope holes on the 3dz2 orbitals with the same amount of electrons doped on the 3dx2–y2 orbitals. By performing numerical density-matrix renormalization group calculations on a minimum setup and focusing on the limit of large J and small doping of 3dz2 orbitals, we find the superconducting instability on both the 3dz2 and 3dx2–y2 orbitals by calculating the equal-time spin singlet pair–pair correlation function. Our numerical results may provide useful insights in the high-Tc superconductivity in single crystal La3Ni2O7 under high pressure.
在高压下La₃Ni₂O₇单晶中报告了可能具有Tc ≈ 80 K的高温超导性。基于密度泛函理论计算给出的电子结构，我们提出了一个有效的双层模型哈密顿量，包括镍阳离子的3dz²和3dx²−y²轨道电子。该模型的主要特点是，3dz²电子通过两层之间的顶端氧阴离子形成层间σ键合和反键合带，而3dx²−y²电子在每个NiO₂平面内与3dz²电子杂化。这两种轨道电子的化学势差确保了3dz²轨道接近半填充，而3dx²−y²轨道接近四分之一填充。3dz²轨道电子的强现场Hubbard排斥导致了有效的层间反铁磁自旋超交换J。施加压力可以在3dz²轨道上自掺杂空穴，同时在3dx²−y²轨道上掺杂相同数量的电子。通过在最小设置上进行数值密度矩阵重整化群计算，并专注于大J和3dz²轨道小掺杂的极限，我们通过计算等时自旋单态对-对关联函数，在3dz²和3dx²−y²轨道上都发现了超导不稳定性。我们的数值结果可能为理解高压下单晶La₃Ni₂O₇中的高温超导性提供了有用的见解。
We perform a theoretical examination of the local electronic structure in the recently discovered bilayer high-temperature superconductor La3Ni2O7. Our method begins with a bilayer two-orbital tight-binding model, incorporating various pairing interaction channels. We determine superconducting order parameters by self-consistently solving the real-space Bogoliubov–de Gennes (BdG) equations, revealing a robust and stable extended 𝑠-wave pairing symmetry. We investigate the single impurity effect using both self-consistent BdG equations and non-self-consistent 𝑇-matrix methods, uncovering low-energy in-gap states that can be explained with the 𝑇-matrix approach. Additionally, we analyze magnetic vortex states using a self-consistent BdG technique, which shows a peak-hump structure in the local density of states at the vortex center. Our results provide identifiable features that can be used to determine the pairing symmetry of the superconducting La3Ni2O7 material.
我们对最近发现的双层高温超导体La₃Ni₂O₇中的局域电子结构进行了理论研究。我们的方法从一个双层双轨道紧束缚模型开始，并引入了各种配对相互作用通道。我们通过自洽求解实空间的Bogoliubov-de Gennes（BdG）方程来确定超导序参量，揭示了一种稳健且稳定的扩展s波配对对称性。我们使用自洽BdG方程和非自洽T矩阵方法研究了单个杂质的影响，发现了可以用T矩阵方法解释的低能带内态。此外，我们还利用自洽BdG技术分析了磁通涡旋态，发现在涡旋中心的局域态密度中存在峰-驼峰结构。我们的结果提供了可识别的特征，可用于确定La₃Ni₂O₇超导材料的配对对称性。
The recently discovered high-Tc superconductor La3Ni2O7 has sparked renewed interest in unconventional superconductivity. Here we study superconductivity in pressurized La3Ni2O7 based on a bilayer two-orbital t−J model, using the renormalized mean-field theory. Our results reveal a robust s±-wave pairing driven by the inter-layer dz2 magnetic coupling, which exhibits a transition temperature within the same order of magnitude as the experimentally observed Tc ~ 80 K. We establish a comprehensive superconducting phase diagram in the doping plane. Notably, the La3Ni2O7 under pressure is found to be situated roughly in the optimal doping regime of the phase diagram. When the dx2-y2 orbital becomes close to half-filling, d-wave and d + is pairing can emerge from the system. We discuss the interplay between Fermi surface topology and different pairing symmetries. The stability of the s±-wave pairing against Hund’s coupling and other magnetic exchange couplings is discussed.
最近发现的高温超导体 La3Ni2O7 重新引发了对非常规超导性的关注。本文基于双层两轨道 t-J 模型，使用重整化平均场理论研究了加压 La3Ni2O7 中的超导性。结果表明，由层间 dz2 轨道磁耦合驱动的 s± 波配对非常稳健，其转变温度与实验观测到的 Tc 约 80 K 同量级。我们在掺杂平面上建立了一个全面的超导相图。值得注意的是，加压下的 La3Ni2O7 大致位于相图的最优掺杂区域。当 dx2-y2 轨道接近半填充时，系统中可以出现 d 波和 d+is 波配对。我们讨论了费米面拓扑结构与不同配对对称性之间的相互作用，并探讨了 s± 波配对在面对 Hund 耦合和其他磁交换耦合时的稳定性。
By means of density functional theory plus dynamical mean-field theory, we investigate the correlated electronic structures of La3Ni2O7 under high pressure. Our calculations show that La3Ni2O7 is a multiorbital Hund metal. Both the 3𝑑𝑧² and 3𝑑𝑥²−𝑦² orbitals of Ni are close to being half filled and contribute the bands crossing the Fermi level. Band renormalization and orbital selective electronic correlation are observed. Through imaginary-time correlation functions, the discovery of high-spin configuration, spin-frozen phase, and spin-orbital separation shows that the system is in a frozen moment phase at high temperatures above 290 K and is a Fermi liquid at low temperatures, which is further confirmed by the calculated spin, orbital, and charge susceptibilities under high temperatures. Our paper uncovers Hundness in La3Ni2O7 under high pressure.
通过密度泛函理论（DFT）结合动态平均场理论（DMFT），我们研究了La₃Ni₂O₇在高压下的相关电子结构。我们的计算表明，La₃Ni₂O₇是一种多轨道Hund金属。Ni的3𝑑𝑧²和3𝑑𝑥²−𝑦²轨道接近半填充，并且这些轨道的能带穿过费米能级。我们观察到了能带重整化和轨道选择性电子关联。通过虚时相关函数，我们发现了高自旋构型、自旋冻结相以及自旋-轨道分离，这表明该系统在高于290 K的高温下处于冻结自旋相，而在低温下是费米液体。这一结论进一步通过计算高温下的自旋、轨道和电荷的涨落得到了验证。我们的研究揭示了在高压下La₃Ni₂O₇中的Hund特性。
The recent discovery of superconductivity in the Ruddlesden–Popper bilayer nickelate, specifically La3Ni2O7, has generated significant interest in the exploration of high-temperature superconductivity within this material family. In this study, we present the crystallographic and electrical resistivity properties of two distinct Ruddlesden–Popper nickelates: the bilayer La3Ni2O7 (referred to as 2222-phase) and a previously uncharacterized phase, La3Ni2O7 (1313-phase). The 2222-phase is characterized by a pseudo F-centered orthorhombic lattice, featuring bilayer perovskite [LaNiO3] layers interspaced by rock salt [LaO] layers, forming a repeated ...2222... sequence. Intriguingly, the 1313-phase, which displays semiconducting properties, crystallizes in the Cmmm space group and exhibits a pronounced predilection for a C-centered orthorhombic lattice. Within this structure, the perovskite [LaNiO3] layers exhibit a distinctive long-range ordered arrangement, alternating between single- and trilayer configurations, resulting in a ...1313... sequence. This report contributes to novel insights into the crystallography and the structure–property relationship of Ruddlesden–Popper nickelates, paving the way for further investigations into their unique physical properties.
最近在Ruddlesden-Popper双层镍氧化物中发现的超导性，特别是La₃Ni₂O₇，激发了在这一材料家族中探索高温超导性的浓厚兴趣。在这项研究中，我们展示了两种不同的Ruddlesden-Popper镍氧化物的晶体学和电阻率特性：双层La₃Ni₂O₇（称为2222相）和一个之前未被表征的相，La₃Ni₂O₇（1313相）。2222相具有一个伪F心正交晶格，其特征是双层钙钛矿[LaNiO₃]层被岩盐[LaO]层间隔开，形成一个重复的...2222...序列。有趣的是，表现出半导体特性的1313相结晶在Cmmm空间群中，并显示出对C心正交晶格的显著偏好。在这个结构中，钙钛矿[LaNiO₃]层呈现出独特的长程有序排列，单层和三层配置交替出现，形成一个...1313...序列。这份报告为Ruddlesden-Popper镍氧化物的晶体学以及结构-性能关系提供了新的见解，为深入研究其独特的物理性质铺平了道路。
Recently, a bulk nickelate superconductor La3Ni2O7 is discovered at pressures with a remarkable high transition temperature Tc ∼ 80 K. Here, we study a Hubbard model with tight-binding parameters derived from ab initio calculations of La3Ni2O7, by employing large scale determinant quantum Monte Carlo and cellular dynamical mean-field theory. Our result suggests that the superexchange couplings in this system are comparable to that of cuprates. The system is a charge transfer insulator as the hole concentration becomes four per site at large Hubbard U. Upon hole doping, two low-energy spin-singlet bands emerge in the system exhibiting distinct correlation properties: while the one composed of the out-of-plane \(\text{Ni-}d_{3z^{2}-r^{2}}\) and O-pz orbitals demonstrates strong antiferromagnetic correlations and narrow effective bandwidth, the in-plane singlet band consisting of the \(\text{Ni-}d_{x^{2}-y^{2}}\) and O-px/py orbitals is in general more itinerant. Over a broad range of hole doping, the doped holes occupy primarily the \(d_{x^{2}-y^{2}}\) and px/py orbitals, whereas the \(d_{3z^{2}-r^{2}}\) and pz orbitals retain underdoped. We propose an effective t-J model to capture the relevant physics and discuss the implications of our result for comprehending the La3Ni2O7 superconductivity.
最近，发现了一种块体镍氧化物超导体La₃Ni₂O₇，其在高压下具有显著的高温超导转变温度Tc ≈ 80 K。在此，我们通过采用大规模行列式量子蒙特卡洛和单元格动力学平均场理论，研究了基于La₃Ni₂O₇的从头算结果导出的紧束缚参数的Hubbard模型。我们的结果表明，该体系中的超交换作用与铜氧化物相当。当Hubbard U较大时，体系是一个电荷转移绝缘体，因为每个位点的空穴浓度达到4。在空穴掺杂下，体系中出现了两个低能自旋单态带，它们表现出不同的关联特性：由垂直于平面的\(\text{Ni-}d_{3z^{2}-r^{2}}\)和O-pz轨道组成的带表现出强烈的反铁磁关联和较窄的有效带宽，而由\(\text{Ni-}d_{x^{2}-y^{2}}\)和O-px/py轨道组成的平面内单态带则更为巡游。在广泛的空穴掺杂范围内，掺杂的空穴主要占据\(d_{x^{2}-y^{2}}\)和px/py轨道，而\(d_{3z^{2}-r^{2}}\)和pz轨道则保持欠掺杂。我们提出了一个有效的t-J模型来捕捉相关物理特性，并讨论了我们的结果对理解La₃Ni₂O₇超导性的影响。
The discovery of high-temperature superconductivity in La3Ni2O7 at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO6 octahedral bilayers stacked along the 𝑐-axis direction was consistently posited in initial studies on La3Ni2O7. Here, we reassess this structure in optical floating zone-grown La3Ni2O7 single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO6 octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La3Ni2O7, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity.
在超过14 GPa的压力下，La₃Ni₂O₇中发现的高温超导性激发了广泛的研究努力。然而，超导相的基本特性，包括其可能具有的丝状特性，目前仍然是有争议的话题。相反，在对La₃Ni₂O₇的最初研究中，一致提出了一个沿c轴方向堆叠的NiO₆八面体双层的晶体结构。在此，我们重新评估了在光学浮区法生长的La₃Ni₂O₇单晶中的这种结构，这些单晶表现出丝状超导性的迹象。通过使用扫描透射电子显微镜和高压下单晶X射线衍射，我们在这些晶体中观察到多种晶体学相，其中主要相显示出NiO₆八面体的单层和三层交替排列，这与之前建议的双层结构存在显著偏差。利用密度泛函理论，我们剖析了单层和三层结构单元对La₃Ni₂O₇电子能带结构的各自贡献，为先进的理论建模和未来对单层-三层结构在超导性方面的潜力评估提供了坚实基础。
The recent discovery of superconductivity in La3Ni2O7−δ under high pressure with a transition temperature around 80 K (ref. 1) has sparked extensive experimental2,3,4,5,6 and theoretical efforts7,8,9,10,11,12. Several key questions regarding the pairing mechanism remain to be answered, such as the most relevant atomic orbitals and the role of atomic deficiencies. Here we develop a new, energy-filtered, multislice electron ptychography technique, assisted by electron energy-loss spectroscopy, to address these critical issues. Oxygen vacancies are directly visualized and are found to primarily occupy the inner apical sites, which have been proposed to be crucial to superconductivity13,14. We precisely determine the nanoscale stoichiometry and its correlation to the oxygen K-edge spectra, which reveals a significant inhomogeneity in the oxygen content and electronic structure within the sample. The spectroscopic results also reveal that stoichiometric La3Ni2O7 has strong charge-transfer characteristics, with holes that are self-doped from Ni sites into O sites. The ligand holes mainly reside on the inner apical O and the planar O, whereas the density on the outer apical O is negligible. As the concentration of O vacancies increases, ligand holes on both sites are simultaneously annihilated. These observations will assist in further development and understanding of superconducting nickelate materials. Our imaging technique for quantifying atomic deficiencies can also be widely applied in materials science and condensed-matter physics.
最近在La₃Ni₂O₇−δ中发现其在高压下具有约80 K的超导转变温度，这一发现引发了广泛的实验和理论研究。关于配对机制的几个关键问题仍待解答，例如最相关的原子轨道以及原子缺陷的作用。在此，我们开发了一种新的能量过滤多切片电子叠层成像技术，并结合电子能量损失谱来解决这些关键问题。氧空位被直接可视化，并发现主要占据内顶端位点，这些位点被认为对超导性至关重要。我们精确测定了纳米尺度的化学计量比及其与氧K边谱的相关性，揭示了样品中氧含量和电子结构的显著不均匀性。光谱结果还表明，化学计量的La₃Ni₂O₇具有强烈的电荷转移特性，空穴从Ni位点自掺杂到O位点。配体空穴主要存在于内顶端O和面内O上，而外顶端O上的密度可以忽略不计。随着氧空位浓度的增加，这两个位点上的配体空穴同时被湮灭。这些观察结果将有助于进一步开发和理解超导镍氧化物材料。我们用于量化原子缺陷的成像技术也可以广泛应用于材料科学和凝聚态物理领域。
Recent experimental observations have showed some signatures of superconductivity close to 80 K in La3Ni2O7 under pressure and have raised the hope of achieving high-temperature superconductivity in bulk nickelates. However, a zero-resistance state—a key characteristic of a superconductor—was not observed. Here we show that the zero-resistance state does exist in single crystals of La3Ni2O7−δ using a liquid pressure medium at up to 30 GPa. We also find that the system remains metallic under applied pressures, suggesting the absence of a metal–insulator transition proximate to the superconductivity. Moreover, analysis of the normal state T-linear resistance reveals a link between this strange-metal behaviour and superconductivity. The association between strange-metal behaviour and high-temperature superconductivity is very much in line with other classes of unconventional superconductors, including the cuprates and Fe-based superconductors. Further investigations exploring the interplay of strange-metal behaviour and superconductivity, as well as possible competing electronic or structural phases, are essential to understand the mechanism of superconductivity in this system.
近期的实验观察显示，在压力下La₃Ni₂O₇接近80 K时表现出一些超导迹象，这激发了在块体镍氧化物中实现高温超导的希望。然而，零电阻态——超导体的一个关键特征——尚未被观察到。在此，我们利用液体压力介质在高达30 GPa的压力下证明了La₃Ni₂O₇−δ单晶中存在零电阻态。我们还发现，该系统在施加压力下保持金属态，表明在超导性附近不存在金属-绝缘体转变。此外，对正常态T线性电阻的分析揭示了这种奇异金属行为与超导性之间的联系。奇异金属行为与高温超导性的关联非常符合其他非常规超导体类别，包括铜氧化物和铁基超导体。进一步探讨奇异金属行为与超导性之间的相互作用，以及可能的竞争电子或结构相，对于理解该系统的超导机制至关重要。
The latest discovery of high temperature superconductivity near 80 K in La3Ni2O7 under high pressure has attracted much attention. Many proposals are put forth to understand the origin of superconductivity. The determination of electronic structures is a prerequisite to establish theories to understand superconductivity in nickelates but is still lacking. Here we report our direct measurement of the electronic structures of La3Ni2O7 by high-resolution angle-resolved photoemission spectroscopy. The Fermi surface and band structures of La3Ni2O7 are observed and compared with the band structure calculations. Strong electron correlations are revealed which are orbital- and momentum-dependent. A flat band is formed from the Ni-3d orbitals around the zone corner which is ~ 50 meV below the Fermi level and exhibits the strongest electron correlation. In many theoretical proposals, this band is expected to play the dominant role in generating superconductivity in La3Ni2O7. Our observations provide key experimental information to understand the electronic structure and origin of high temperature superconductivity in La3Ni2O7.
最近在La₃Ni₂O₇中发现其在高压下具有约80 K的高温超导性，这引起了广泛关注。为了理解超导的起源，提出了许多理论。确定电子结构是建立理解镍氧化物超导性理论的先决条件，但目前仍缺乏相关研究。在此，我们通过高分辨率角分辨光电子能谱直接测量了La₃Ni₂O₇的电子结构。观察到La₃Ni₂O₇的费米面和能带结构，并与能带结构计算进行了比较。揭示了强烈的电子关联，这种关联具有轨道和动量依赖性。在费米能级以下约50 meV的布里渊区角落处，由Ni-3d轨道形成的平带表现出最强的电子关联。在许多理论提案中，这个带被认为是产生La₃Ni₂O₇中超导性的主要因素。我们的观测结果为理解La₃Ni₂O₇的电子结构和高温超导性的起源提供了关键的实验信息。
We synthesized polycrystalline La3Ni2O7−𝛿 (𝛿≈0.07) samples by using the sol-gel method without postannealing under high oxygen pressure, and then measured temperature-dependent resistivity under various hydrostatic pressures up to 18 GPa by using the cubic anvil and two-stage multianvil apparatus. We find that the density-wave-like anomaly in resistivity is progressively suppressed with increasing pressure and the resistivity drop corresponding to the onset of superconductivity emerges at pressure as low as ∼6 GPa. Zero resistivity is achieved at 9 GPa below \(𝑇^{\text{zero}}_{c}\approx 6.6\,\text{ K}\), which increases quickly with pressure to 41 K at 18 GPa. However, the diamagnetic response was not detected in the ac magnetic susceptibility measurements up to 15 GPa, indicating a filamentary nature of the observed superconductivity in the studied pressure range. The constructed 𝑇−𝑃 phase diagram reveals an intimate relationship between superconductivity, density-wave-like order, and the strange-metal-like behaviors. The observation of zero-resistance state in the polycrystalline La3Ni2O7−𝛿 samples under high pressures not only corroborates the recent report of superconductivity in the pressurized La3Ni2O7 crystals but also facilitates further studies on this emerging family of nickelate high-𝑇c superconductors.
我们通过溶胶-凝胶法合成了La₃Ni₂O₇−𝛿（𝛿≈0.07）多晶样品，未进行高压氧后退火处理，然后使用立方压砧和两级多面压砧装置在高达18 GPa的各种静水压力下测量了温度依赖电阻率。我们发现，随着压力的增加，电阻率中的类密度波异常逐渐被抑制，并且在低至约6 GPa的压力下出现了对应于超导性起始的电阻率下降。在9 GPa下实现了零电阻，其零电阻温度\(𝑇^{\text{zero}}_{c}\approx 6.6\,\text{ K}\)，随着压力的增加迅速上升至18 GPa下的41 K。然而，在交流磁化率测量中，直到15 GPa的压力下仍未检测到抗磁响应，这表明在研究的压力范围内观察到的超导性具有丝状性质。构建的T-P相图揭示了超导性、类密度波序和类奇异金属行为之间的密切关系。在高压下多晶La₃Ni₂O₇−𝛿样品中零电阻态的观测，不仅证实了最近关于在加压La₃Ni₂O₇晶体中超导性的报道，而且促进了对这一新兴镍氧化物高温超导体家族的进一步研究。
Inspired by a recent experiment showing that La3Ni2O7 exhibits high 𝑇𝑐 superconductivity under high pressure, we theoretically revisit the possibility of superconductivity in this material. We find that superconductivity can take place, which is somewhat similar to that of the bilayer Hubbard model consisting of the Ni 3𝑑3𝑧2−𝑟2 orbitals. Although the coupling with the 3𝑑𝑥2−𝑦2 orbitals degrades superconductivity, 𝑇𝑐 can still be high enough to understand the experiment thanks to the very high 𝑇𝑐 reached in the bilayer Hubbard model.
受最近一项实验的启发，该实验显示La₃Ni₂O₇在高压下表现出高温超导性，我们从理论上重新审视了这种材料的超导可能性。我们发现，超导性确实可以发生，其机制在某种程度上类似于由Ni的3d3z²−r²轨道组成的双层Hubbard模型。尽管与3dx²−y²轨道的耦合会降低超导性，但由于双层Hubbard模型本身能够达到非常高的Tc，因此Tc仍然足够高，足以解释实验观察到的现象。
Motivated by the recently discovered high-Tc superconductor La3Ni2O7, we comprehensively study this system using density functional theory and random phase approximation calculations. At low pressures, the Amam phase is stable, containing the Y2− mode distortion from the Fmmm phase, while the Fmmm phase is unstable. Because of small differences in enthalpy and a considerable Y2− mode amplitude, the two phases may coexist in the range between 10.6 and 14 GPa, beyond which the Fmmm phase dominates. In addition, the magnetic stripe-type spin order with wavevector (π, 0) was stable at the intermediate region. Pairing is induced in the s±-wave channel due to partial nesting between the M = (π, π) centered pockets and portions of the Fermi surface centered at the X = (π, 0) and Y = (0, π) points. This resembles results for iron-based superconductors but has a fundamental difference with iron pnictides and selenides. Moreover, our present efforts also suggest La3Ni2O7 is qualitatively different from infinite-layer nickelates and cuprate superconductors.
受最近发现的高温超导体La₃Ni₂O₇的启发，我们利用密度泛函理论和随机相位近似计算对该体系进行了全面研究。在低压下，Amam相是稳定的，包含从Fmmm相的Y₂⁻模式畸变，而Fmmm相是不稳定的。由于焓值差异较小且Y₂⁻模式振幅较大，这两种相可能在10.6到14 GPa的压力范围内共存，超过这一范围后，Fmmm相占主导地位。此外，在中间区域，具有波矢（π, 0）的磁性条纹型自旋序是稳定的。由于以M = (π, π)为中心的口袋与以X = (π, 0)和Y = (0, π)为中心的费米面部分之间存在部分嵌套，因此在s±波通道中诱导了配对。这与铁基超导体的结果相似，但与铁砷化物和硒化物存在根本性差异。此外，我们的研究还表明，La₃Ni₂O₇在本质上与无限层镍氧化物和铜氧化物超导体不同。
The pursuit of discovering new high-temperature superconductors that diverge from the copper-based model has profound implications for explaining mechanisms behind superconductivity and may also enable new applications. Here our investigation shows that the application of pressure effectively suppresses the spin–charge order in trilayer nickelate La4Ni3O10−δ single crystals, leading to the emergence of superconductivity with a maximum critical temperature (Tc) of around 30 K at 69.0 GPa. The d.c. susceptibility measurements confirm a substantial diamagnetic response below Tc, indicating the presence of bulk superconductivity with a volume fraction exceeding 80%. In the normal state, we observe a strange metal behaviour, characterized by a linear temperature-dependent resistance extending up to 300 K. Furthermore, the layer-dependent superconductivity observed hints at a unique interlayer coupling mechanism specific to nickelates, setting them apart from cuprates in this regard. Our findings provide crucial insights into the fundamental mechanisms underpinning superconductivity, while also introducing a new material platform to explore the intricate interplay between the spin–charge order, flat band structures, interlayer coupling, strange metal behaviour and high-temperature superconductivity.
寻找与铜基模型不同的新型高温超导体，对于解释超导性背后的机制具有深远意义，并且也可能推动新的应用发展。在此，我们的研究表明，对三层镍氧化物La₄Ni₃O₁₀₋𝛿单晶施加压力能够有效抑制其自旋-电荷序，从而诱导出超导性，其最大临界温度（Tc）约为30 K，出现在69.0 GPa的压力下。直流磁化率测量确认在Tc以下存在显著的抗磁响应，表明体超导性的存在，其体积分数超过80%。在正常态，我们观察到一种奇异金属行为，其特征是电阻随温度线性变化，这种关系可延伸至300 K。此外，我们观察到的层依赖性超导性暗示了镍氧化物具有独特的层间耦合机制，这使它们与铜氧化物在这方面区分开来。我们的发现为理解超导性背后的基本机制提供了关键见解，同时也引入了一个新的材料平台，用于探索自旋-电荷序、平带结构、层间耦合、奇异金属行为和高温超导性之间的复杂相互作用。
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-\(d_{3z^2-r^2}\)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.
在La₃Ni₂O₇单晶中发现其在压力下具有约80 K的超导临界温度，这一发现受到了广泛关注。La₃Ni₂O₇在常压下并不表现出超导性，但在大约115 K时会发生一个相变（T* ≃ 115 K）。理解电子关联和电荷动力学是揭示超导性及其他不稳定性起源的重要一步。在此，我们的光学研究表明，La₃Ni₂O₇具有强烈的电子关联，这种关联显著降低了电子的动能，并使该体系接近莫特（Mott）相。低频光学电导率揭示了来自费米能级附近多个能带的两个Drude组分。T*处的相变消除了表现出非费米液体行为的Drude组分，而具有费米液体行为的组分几乎不受影响。结合理论结果，这些观测表明，由于T*处的相变，以Ni-\(d_{3z^2-r^2}\)轨道为主的费米面被移除。我们的实验结果为理解T*处的相变以及La₃Ni₂O₇中的超导性提供了关键信息。
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.
最近，有报道称在镍氧化物（La₃Ni₂O₇）中，在约15–40 GPa的条件下发现了一种Tc = 80 K的新型超导体，这是继铜氧化物之后第二种Tc高于液氮温度的非常规超导体。然而，该报告中绘制的相图主要基于在低温和高压条件下的输运测量，而假设对应的X射线衍射（XRD）结果是在室温下进行的。这促使我们开展原位高压和低温同步辐射XRD实验，以确定哪种相负责高Tc态。除了从正交晶Amam结构到正交晶Fmmm结构的相变外，在La₃Ni₂O₇表现出超导性的19 GPa和40 K条件下，还发现了一个具有I4/mmm空间群的四方相。基于这种四方结构的计算表明，接近费米能级的电子态主要由Ni原子的eg轨道（3dz²和3dx²−y²）主导，这些轨道位于氧八面体晶体场中。分析了Tc与这种结构演变之间的相关性，特别是Ni–O八面体的规则性和平面内Ni–O–Ni键角。这项工作为识别双层镍氧化物中超导性最有可能的相提供了新的见解。
The discovery of high-temperature superconductivity near 80 K in bilayer nickelate La3Ni2O7 under high pressures has renewed the exploration of superconducting nickelate in bulk materials. The extension of superconductivity in other nickelates in a broader family is also essential. Here, we report the experimental observation of superconducting signature in trilayer nickelate La4Ni3O10 under high pressures. By using a modified sol-gel method and post-annealing treatment under high oxygen pressure, we successfully obtained polycrystalline La4Ni3O10 samples with different transport behaviors at ambient pressure. Then we performed high-pressure electrical resistance measurements on these samples in a diamond-anvil-cell apparatus. Surprisingly, the signature of possible superconducting transition with a maximum transition temperature (Tc) of about 20 K under high pressures is observed, as evidenced by a clear drop of resistance and the suppression of resistance drops under magnetic fields. Although the resistance drop is sample-dependent and relatively small, it appears in all of our measured samples. We argue that the observed superconducting signal is most likely to originate from the main phase of La4Ni3O10. Our findings will motivate the exploration of superconductivity in a broader family of nickelates and shed light on the understanding of the underlying mechanisms of high-Tc superconductivity in nickelates.
在双层镍氧化物La₃Ni₂O₇中发现其在高压下接近80 K的高温超导性，重新激发了对块体材料中超导镍氧化物的探索。在更广泛的镍氧化物家族中扩展超导性也是至关重要的。在此，我们报告了在高压下对三层镍氧化物La₄Ni₃O₁₀中观察到的超导迹象的实验结果。通过改进的溶胶-凝胶法和在高压氧下的后退火处理，我们成功获得了在常压下具有不同输运行为的多晶La₄Ni₃O₁₀样品。随后，我们在金刚石压砧装置中对这些样品进行了高压电阻测量。令人惊讶的是，我们观察到了可能的超导转变迹象，其最大转变温度（Tc）约为20 K，表现为电阻的明显下降以及在磁场下电阻下降的抑制。尽管电阻下降是样品依赖的且相对较小，但它出现在我们测量的所有样品中。我们认为，观察到的超导信号最有可能来自La₄Ni₃O₁₀的主要相。我们的发现将激励在更广泛的镍氧化物家族中探索超导性，并为理解镍氧化物中高温超导性的机制提供启示。
Recently, the discovery of superconductivity with a critical temperature Tc up to 80 K in Ruddlesden–Popper phases Lan+1NinO3n+1 (n = 2) under pressure has garnered considerable attention. Up to now, the superconductivity was only observed in La3Ni2O7 single crystal grown with the optical-image floating zone furnace under oxygen pressure. It remains to be understood the effect of chemical doping on superconducting La3Ni2O7 as well as other Ruddlesden–Popper phases. Here, we systematically investigate the effect of external pressure and chemical doping on polycrystalline Ruddlesden–Popper phases. Our results demonstrate that the application of pressure and doping effectively tunes the transport properties of Ruddlesden–Popper phases. We find pressure-induced superconductivity up to 86 K in La3Ni2O7 polycrystalline sample, while no signatures of superconductivity are observed in La2NiO4 and La4Ni3O10 polycrystalline samples under high pressure up to 50 GPa. Our study sheds light on the exploration of high-Tc superconductivity in nickelates.
最近，在受压的Ruddlesden-Popper相Lan+1NinO3n+1（n = 2）中发现的超导性引起了广泛关注，其临界温度Tc可达80 K。迄今为止，超导性仅在通过光学图像浮区炉在氧压下生长的La₃Ni₂O₇单晶中被观察到。化学掺杂对超导La₃Ni₂O₇以及其他Ruddlesden-Popper相的影响仍有待理解。在此，我们系统地研究了外部压力和化学掺杂对多晶Ruddlesden-Popper相的影响。我们的结果表明，施加压力和掺杂有效地调节了Ruddlesden-Popper相的输运性质。我们在La₃Ni₂O₇多晶样品中发现了高达86 K的压力诱导超导性，而在高达50 GPa的高压下，La₂NiO₄和La₄Ni₃O₁₀多晶样品中均未观察到超导迹象。我们的研究为探索镍氧化物中的高温超导性提供了新的见解。
High-𝑇𝑐 superconductivity (SC) has been found experimentally in the bilayer material La3Ni2O7 under high pressure recently, in which the Ni-3𝑑3𝑧2−𝑟2 and 3𝑑𝑥2−𝑦2 orbitals are expected to play a key role in the electronic structure and the SC. Here we study the two-orbital electron correlations and the nature of the SC in the framework of the dynamical mean-field theory using the bilayer two-orbital Hubbard model downfolded from the band structure of La3Ni2O7. We find that each of the two orbitals forms 𝑠±-wave SC pairing. Because of the interorbital hoppings, the two-orbital SCs are concomitant, and furthermore they transition to Mott insulating states simultaneously when tuning the system to half filling. The Hund's coupling induced local interorbital spin coupling enhances the electron correlations pronouncedly and is crucial to the SC.
最近在双层材料La₃Ni₂O₇中，在高压下实验性地发现了高温超导性（SC），其中Ni的3d3z²−r²和3dx²−y²轨道被认为在电子结构和超导性中起关键作用。在此，我们使用从La₃Ni₂O₇的能带结构折叠而来的双层双轨道Hubbard模型，在动力学平均场理论框架内研究这两个轨道的电子关联和超导性的本质。我们发现，这两个轨道各自形成了s±波超导配对。由于轨道间跃迁的存在，这两个轨道的超导性是同时发生的，并且当将系统调节到半填充时，它们同时转变为莫特绝缘态。Hund耦合引起的局域轨道间自旋耦合显著增强了电子关联，并且对超导性至关重要。
Recent studies of La3Ni2O7 have identified a bilayer (2222) structure and an unexpected alternating monolayer-trilayer (1313) structure, both of which feature signatures of superconductivity near 80 K under high pressures. Using angle-resolved photoemission spectroscopy, we measure the electronic structure of 1313 samples. In contrast to the previously studied 2222 structure, we find that the 1313 structure hosts a flat band with a markedly different binding energy, as well as an additional electron pocket and band splittings. By comparison to local-density approximation calculations, we find renormalizations of the Ni−𝑑𝑧2 and Ni−𝑑𝑥2−𝑦2 derived bands to be about 5 to 7 and about 4, respectively, suggesting strong correlation effects. These results reveal important differences in the electronic structure brought about by the distinct structural motifs with the same stoichiometry. Such differences may be relevant to the putative high temperature superconductivity.
最近对 La3Ni2O7 的研究发现了一种双层（2222）结构和一种意外的交替单层-三层（1313）结构，这两种结构在高压下都表现出接近 80 K 的超导性特征。通过角分辨光电子能谱，我们测量了 1313 样品的电子结构。与之前研究的 2222 结构相比，我们发现 1313 结构中存在一个具有显著不同束缚能的平带，以及一个额外的电子口袋和带分裂。通过与局域密度近似计算的比较，我们发现 Ni−𝑑𝑧2 和 Ni−𝑑𝑥2−𝑦2 导出的能带重整化分别约为 5 到 7 和 4，这表明存在强关联效应。这些结果揭示了由于具有相同化学计量比的不同结构单元而带来的电子结构的重要差异。这些差异可能与假设的高温超导性有关。
Recently, the bilayer nickelate La3Ni2O7 has been discovered as a new superconductor with transition temperature Tc near 80 K under high pressure. Despite extensive theoretical and experimental work to understand the nature of its superconductivity, the requirement of extreme pressure restricts the use of many experimental probes and limits its application potential. Here we present signatures of superconductivity in La3Ni2O7 thin films at ambient pressure, facilitated by the application of epitaxial compressive strain. The onset Tc varies roughly from 26 to 42 K, with higher Tc values correlating with smaller in-plane lattice constants. We observed the co-existence of other Ruddlesden–Popper phases within the films and dependence of transport behaviour with ozone annealing, suggesting that the observed low zero resistance Tc of around 2 K can be attributed to stacking defects, grain boundaries and oxygen stoichiometry. This finding initiates numerous opportunities to stabilize and study superconductivity in bilayer nickelates at ambient pressure, and to facilitate the broad understanding of the ever-growing number of high temperature and unconventional superconductors in the transition metal oxides.
最近，双层镍氧化物La₃Ni₂O₇被发现是一种新的超导体，在高压下其转变温度Tc接近80 K。尽管已有大量理论和实验研究致力于理解其超导性，但极端压力的要求限制了许多实验探测手段的使用，并限制了其应用潜力。在此，我们报告了在常压下La₃Ni₂O₇薄膜中观察到的超导迹象，这一现象得益于外延压缩应变的应用。Tc的起始温度大致在26到42 K之间，较高的Tc值与较小的平面内晶格常数相关。我们观察到薄膜中存在其他Ruddlesden-Popper相，并且发现输运行为依赖于臭氧退火，这表明观察到的约2 K的低零电阻Tc可以归因于堆叠缺陷、晶界和氧含量。这一发现为在常压下稳定和研究双层镍氧化物中的超导性开辟了众多机会，并有助于广泛理解过渡金属氧化物中日益增多的高温和非常规超导体。
Beyond 14 GPa of pressure, bilayered La3Ni2O7 was recently found to develop strong superconductivity above the liquid nitrogen boiling temperature. An immediate essential question is the pressure-induced qualitative change of electronic structure that enables the exciting high-temperature superconductivity. We investigate this timely question via a numerical multiscale derivation of effective many-body physics. At the atomic scale, we first clarify that the system has a strong charge transfer nature with itinerant carriers residing mainly in the in-plane oxygen between spin-1 Ni2+ ions. We then elucidate in electron-volt scale and sub-electron-volt scale the key physical effect of the applied pressure: it induces a cupratelike electronic structure via fractionalizing the Ni ionic spin from 1 to 1/2. This suggests a high-temperature superconductivity in La3Ni2O7 with microscopic mechanism and (𝑑-wave) symmetry similar to that in the cuprates.
在超过14 GPa的压力下，最近发现双层La₃Ni₂O₇表现出强烈的超导性，其超导转变温度高于液氮沸点。一个迫切需要回答的问题是：压力诱导的电子结构的定性变化是如何实现令人兴奋的高温超导性的。我们通过数值多尺度推导有效多体物理来研究这一及时的问题。在原子尺度上，我们首先澄清该体系具有强烈的电荷转移特性，巡游载流子主要位于自旋为1的Ni²⁺离子之间的平面内氧原子上。然后，我们在电子伏特尺度和亚电子伏特尺度上阐明了施加压力的关键物理效应：它通过将Ni离子的自旋从1分裂为1/2，诱导出类似铜氧化物的电子结构。这表明La₃Ni₂O₇中的高温超导性具有与铜氧化物类似的微观机制和（d波）对称性。
Motivated by the recent discovery of high-temperature superconductivity in bilayer La3Ni2O7 under pressure, we study its electronic properties and superconductivity due to strong electron correlation. Using the inversion symmetry, we decouple the low-energy electronic structure into block-diagonal symmetric and antisymmetric sectors. It is found that the antisymmetric sector can be reduced to a one-band system near half filling, while the symmetric bands occupied by about two electrons are heavily overdoped individually. Using the strong coupling mean field theory, we obtain strong superconducting pairing with B1g symmetry in the antisymmetric sector. We propose that due to the spin-orbital exchange coupling between the two sectors, B1g pairing is induced in the symmetric bands, which in turn boosts the pairing gap in the antisymmetric band and enhances the high-temperature superconductivity with a congruent d-wave symmetry in pressurized La3Ni2O7.
受双层La₃Ni₂O₇在压力下表现出高温超导性的启发，我们研究了其电子性质和由于强电子关联导致的超导性。利用反演对称性，我们将低能电子结构分解为块对角化的对称和反对称部分。我们发现，反对称部分可以简化为一个接近半填充的单带系统，而对称带中每个带占据约两个电子，严重过掺杂。利用强耦合平均场理论，我们在反对称部分获得了具有B₁g对称性的强超导配对。我们提出，由于两个部分之间的自旋轨道交换耦合，B₁g配对在对称带中被诱导，这反过来又增强了反对称带中的配对能隙，并在加压的La₃Ni₂O₇中增强了具有一致d波对称性的高温超导性。
The discovery of superconductivity in Sr-doped infinite-layer NdNiO2 films introduced a new class of formally Ni1+ (3d9) cuprate-like superconductors. Recent work has shown a pressure-driven increase of the superconducting transition temperature Tc in Sr-doped infinite-layer PrNiO2. Very recently, high Tc ~ 80 K was reported in the bilayer nickelate La3Ni2O7 under external pressure. This study explores the structural and electronic properties of A3Ni2O7 rare-earth nickelates (A = La-Lu, Y, Sc) under high pressure (0–150 GPa) using first principles simulations. The authors compile a structural phase diagram, highlighting the orthorhombic distortion, octahedral anisotropy, and octahedral rotations. They find that chemical and external pressure are distinct and counteracting control parameters, limiting the perspectives of chemical precompression. The study identifies unexpected correlations between Tc and the in-plane Ni-O-Ni bond angles and suggests Tb3Ni2O7 as an interesting candidate for superconductivity at ambient pressure.
在Sr掺杂的无限层NdNiO₂薄膜中发现的超导性引入了一类新的形式上为Ni¹⁺（3d⁹）的类似铜氧化物的超导体。最近的研究表明，在Sr掺杂的无限层PrNiO₂中，超导转变温度Tc随压力增加。最近，有研究在双层镍氧化物La₃Ni₂O₇中报告了高达80 K的Tc。本研究利用第一性原理模拟，探索了在高压（0–150 GPa）下A₃Ni₂O₇稀土镍氧化物（A = La-Lu, Y, Sc）的结构和电子性质。作者们编制了一个结构相图，重点突出正交晶畸变、八面体各向异性和八面体旋转。他们发现化学压力和外部压力是两种不同且相互抵消的控制参数，限制了化学预压缩的前景。该研究发现了Tc与平面内Ni-O-Ni键角之间的意外相关性，并建议Tb₃Ni₂O₇是环境压力下超导性的一个有趣候选材料。
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 La3Ni2O7 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 La3Ni2O7−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 in the inelastic scattering spectra at 45 meV. 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 La3Ni2O7.
尽管对高温超导的研究已有数十年，但其机制仍未有令人信服的理论；然而，自旋涨落被广泛认为在形成超导库珀对中起着关键作用。最近，在双层镍氧化物La3Ni2O7中发现的80 K附近的高温超导性为阐明高温超导的起源提供了新的平台。我们对La3Ni2O7−δ的多晶样品在环境压力下进行了弹性及非弹性中子散射研究。在10 K以下未发现磁性序。中子衍射测量中未发现长程磁性序，可能归因于磁矩较小。然而，我们在45 meV处观察到一个弱的平坦自旋涨落信号。观察到的自旋激发可被解释为条纹型反铁磁序的强层间和弱层内磁耦合的结果。我们的结果提供了关于自旋动力学的重要信息，因此对于理解La3Ni2O7中的超导性至关重要。
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