该研究基于两分量模型提出了一个统一理论，用以解释双层镍酸盐超导体在加压块材与薄膜中的一系列核心实验现象。理论以轨道镍原子的强关联局域电子与巡游电子间的层间超交换耦合和杂化为核心，预言超导转变温度随掺杂变化呈现两类行为：当层间超交换耦合较强时，电子或空穴掺杂分别形成两个超导穹顶，半填充附近出现非超导的层间价键态；当耦合较弱或适中时，两个穹顶合并为一个跨越半填充的单一穹顶，但最高温度较低。掺杂增加会使正常态从费米液体转变为非费米液体或弱绝缘态，并在最优掺杂附近出现准线性电阻散射率。氧空位或化学替代可破坏层间价键，同时抑制超导并诱发巡游电子的局域近藤散射，解释了非超导样品中观测到的电阻对数温度依赖及负磁阻。该框架统一说明了块材和薄膜中超导转变与正常态的差异、孔穴掺杂及氧计量对穹顶状的影响，以及超导与近藤效应的竞争关系。基于理论，作者提出通过掺杂或降低层间磁耦合可在常压下实现块材超导，并预言电子掺杂将获得更高的转变温度。
该论文基于先前提出的有效d_{x^2-y^2}轨道双层t-J∥-J⊥模型，结合第一性原理计算输入的模型参数，通过奴隶玻色子平均场和密度矩阵重正化群方法，统一解释了通过氧化学计量、元素替代、压力或应变调控La₃Ni₂O₇超导转变温度（Tc）的一系列实验。模型发现，由于d_{x^2-y^2}轨道接近四分之一填充，其Tc调控行为与空穴掺杂的过掺杂铜氧化物类似。在掺杂依赖性上，系统呈现粒子-空穴不对称性：空穴掺杂使系统更过掺杂而抑制Tc，电子掺杂则相反，这解释了氧过量或Ca/Sr替代La导致的Tc抑制以及氧化学计量调控中的“半穹顶”行为。在相互作用依赖性上，Tc随层间反铁磁超交换相互作用J⊥变化，这解释了Sm/Nd替代La增强体材料Tc、体材料Tc-压力呈“直角三角形”关系以及薄膜中压应变增强Tc的实验现象。与弱耦合理论（Tc主要依赖于态密度）和d_{z^2}轨道主导的配对机制（Tc与d_{z^2}空穴密度成比例）相比，该模型提供了更自然统一的解释。论文进一步提出，通过不引入无序的电子掺杂（如用更高价元素替代La）可提高Tc。
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Absence of Ni₂/Ni₃ charge disproportionation and possible roles of O₂ p holes in La₃Ni₂O₇−δ revealed by hard x-ray photoemission spectroscopy
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Recently, the Ruddlesden-Popper bilayer nickelate $La_3Ni_2O_7$ has emerged as a superconductor with a transition temperature ($T_c$) of ~ 80 K above 14 GPa$^{[1-4]}$. Efforts to search for nickelate superconductors with higher $T_c$$^{[5,6]}$, to grow reproducible high-quality single crystals$^{[2,7-10]}$, and to eliminate reliance on demanding high gas pressure synthesis conditions$^{[11]}$, remain significant challenges. Here we report superconductivity up to 92 K under high pressure in single crystals of bilayer nickelates synthesized at ambient pressure using flux methods. High quality $La_2SmNi_2O_{7-δ}$ single crystals with dimensions up to 220 μm on edge were successfully grown. At ~ 15 GPa, these crystals exhibit superconductivity with an onset transition temperature ($T_c^{onset}$) of 68 K and zero-resistance temperature ($T_c^{zero}$) of 47 K. Increasing pressure further enhances both transition temperatures, reaching record values for nickelates: $T_{c,max}^{onset}$ = 92 K and $T_{c,max}^{zero}$ = 73 K @ 21 GPa. Notably, higher $T_c$ correlates with larger in-plane lattice distortion at ambient conditions for bilayer nickelates. Furthermore, we observed a structural transition from monoclinic $P2_1/a$ to tetragonal $I4/mmm$ at ~ 18 GPa, indicating that tetragonal structure is not a prerequisite for superconductivity to appear in this bilayer nickelate. This study provides an easy-to-access method for growing reproducible high-quality bilayer nickelate single crystals and offers new insights into achieving higher Tc superconductivity.
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