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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本文利用极化分辨电子拉曼散射测量了高质量La₃Ni₂O₇单晶，在150 K以下的密度波转变中观察到显著的、对称性依赖的谱权重重新分布：B₁g通道呈现不对称峰，B₂g通道呈现对称宽峰，分别对应布里渊区X/Y点附近和对角线方向的电子激发。定量分析提取出两组SDW能隙值，B₁g通道能隙约为37.5–40.4 meV（2Δ/kBTc≈5.5–5.9），B₂g通道能隙约为23.0 meV（2Δ/kBTc≈3.4），表明前者属中等至强耦合，后者属弱耦合。这种动量选择的各向异性耦合强度无法用简单的弱耦合嵌套理论解释，揭示了非常规SDW源于各向异性电子关联。能隙的温度依赖性明显弱于平均场预期，且B₂g通道各向同性与B₁g通道弱各向异性也支持两种不同耦合机制的共存。该工作确立了La₃Ni₂O₇中SDW的电子特性，为理解压力下镍酸盐高温超导的涌现提供了微观基础。
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本研究利用第一性原理密度泛函理论（GGA及GGA+U）探讨了具有双层NiO2无限层结构的La3Ni2O5F的电子与磁性。La(O/F)La阻挡层实现了NiO2双层的严格隔离，形成纯粹二维电子与磁系统。计算发现一个由间隙区域电子密度构成的E单带，该带不与任何原子轨道关联，沿M-A方向下穿费米能级，提供0.09个穴/Ni的自掺杂，使Ni实际价态变为+1.09。该E带费米面呈柱状，占布里渊区9%面积。dpσ带近乎半满但受自掺杂偏移至范霍夫奇点附近。磁响应显示出不同于以往镍酸盐的反常特性，磁化率在较大磁场下趋于消失。无磁相变的现象可归因于强烈的二维自旋涨落及偏离半填充的自掺杂效应。这些结果揭示了Ni^{1+}离子在该体系中独特的行为，以及界面阻挡层对间隙带形态的关键影响。