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压力下双层镍氧超导体展现出高达80 K的超导临界温度, 使Ruddlesden-Popper (RP)相层状镍氧化物成为研究非常规高温超导机制的新平台. 本文从弱耦合理论计算角度出发, 系统回顾了近期在RP相层状镍酸盐中非常规超导配对机制的理论研究进展, 内容涵盖配对对称性、主导轨道成分及其自旋涨落特征等方面, 涉及加压条件下的La3Ni2O7, La4Ni3O10, La5Ni3O11块材以及常压条件下的La3Ni2O7薄膜等多个体系. 这些材料普遍表现出以Ni-3\mathrmd_z^2与3\mathrmd_x^2-y^2轨道为主导的低能电子自由度. 在RP块材中, 无规相近似、泛函重整化群和涨落交换近似等弱耦合方法普遍支持一种由自旋涨落介导、以层间\mathrmd_z^2轨道为主导的\mathrms^\pm波配对机制. 其中, La3Ni2O7块材的超导可能与费米面上γ口袋的出现密切相关, 该口袋源于\mathrmd_z^2轨道成键态的金属化过程. 另一方面, La4Ni3O10的超导特性主要取决于洪特耦合强度和掺杂浓度, 而非能带细节; 而La5Ni3O11则因层间约瑟夫森效应, 呈现出穹顶型的压力-超导相图. 对于La3Ni2O7薄膜, 理论研究表明其可能存在\mathrms^\pm波与\mathrmd_xy波竞争的配对特征. 此外, 常压下的自旋密度波序与超导存在紧密联系. 整体而言, 弱耦合理论不仅解释了实验现象, 还为在常压下实现高温超导提供了理论思路.The discovery of superconductivity in Ruddlesden-Popper (RP) phase layered nickelates under high pressure has opened a new avenue for exploring nontraditional pairing mechanisms beyond cuprates and iron-based superconductors. In particular, La3Ni2O7 exhibits a superconducting transition temperature ( T_\rm c ) as high as 80 K at ~15 GPa, making it the second class of oxides that achieve liquid-nitrogen temperature superconductivity. Subsequent experiments have extended superconductivity to related compounds such as La4Ni3O10 and La5Ni3O11, as well as epitaxially grown thin films at ambient pressure. These findings have motivated extensive theoretical efforts to elucidate the microscopic pairing mechanism.This review summarizes recent progress from the perspective of weak-coupling theories, including random phase approximation (RPA), functional renormalization group (FRG), and fluctuation-exchange (FLEX) approaches. Density functional theory (DFT) calculations reveal that the low-energy degrees of freedom are dominated by Ni 3\rm d_z^2 and 3\rm d_x^2-y^2 orbitals. In La3Ni2O7, pressure-induced metallization of the bonding 3\rm d_z^2 band produces the γ pocket, enhancing spin fluctuations, and stabilizing superconductivity. These fluctuations support superconductivity through interlayer 3\rm d_z^2 pairing characterized by an \rm s^\pm gap. Hole doping or substitution may restore the γ pocket and achieve bulk superconductivity at ambient pressure.For La4Ni3O10, theoretical calculations indicate predominantly \rm s^\pm pairing from interlayer 3\rm d_z^2 orbitals, with weaker strength than La3Ni2O7, explaining its lower T_\rm c and showing little sensitivity to band structure. In La5Ni3O11, composed of alternating single-layer and bilayer units, superconductivity mainly arises from the bilayer subsystem, again dominated by 3\rm d_z^2 orbitals. Interestingly, the interplay between inter-bilayer Josephson coupling and the suppression of density of states leads to a dome-shaped T_\rm c -pressure phase diagram, differing from the monotonic behavior of La3Ni2O7.Epitaxial (La, Pr)3Ni2O7 thin films display superconductivity above 40 K at ambient pressure. Theory predicts doping-dependent pairing: \rm s^\pm symmetry is favored at low doping levels, while dxy pairing emerges at higher doping, in agreement with experimental indications of both nodeless and nodal gap behaviors.In addition to superconductivity, the experiments reveal the spin-density-wave (SDW) sequence in bulk La3Ni2O7 and La4Ni3O10 at ambient pressure. Weak-coupling calculations confirm that these SDWs are driven by Fermi surface nesting and that their suppression under pressure gives rise to strong spin fluctuations which act as the glue for Cooper pairing. This highlights the intimate connection between the density-wave parent states and high-pressure superconductivity in nickelates.In summary, weak-coupling theories provide a unified framework for RP nickelates, highlighting the key roles of 3\rm d_z^2 orbitals, interlayer pairing, and spin fluctuations. They suggest that pressure, doping, substitution, and epitaxial strain can optimize superconductivity and potentially achieve high- T_\rm c phases at ambient pressure. Key challenges remain in clarifying orbital competition, the SDW-superconductivity interplay, and strong-correlation effects, requiring close collaboration between advanced experiments and multi-orbital many-body theory.
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Keywords:
- unconventional superconductivity /
- pairing symmetry /
- nickelate superconductors /
- weak-coupling theory
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中国物理学会期刊
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