High-temperature cuprate superconductivity remains a defining problem in condensed-matter physics. Among myriad approaches to addressing this problem has been the study of alternative transition metal oxides with similar structures and 3d electron count that are suggested as proxies for cuprate physics. None of these analogues has been superconducting, and few are even metallic. Here, we report that the low-valent, quasi-two-dimensional trilayer compound Pr₄Ni₃O₈ avoids a charge-stripe-ordered phase previously reported for La₄Ni₃O₈, leading to a metallic ground state. X-ray absorption spectroscopy shows that metallic Pr₄Ni₃O₈ exhibits a low-spin configuration with significant orbital polarization and pronounced d_x²−y² character in the unoccupied states above the Fermi energy, a hallmark of the cuprate superconductors. Density functional theory calculations corroborate this finding, and reveal that the d_x²−y² orbital dominates the near-E_f occupied states as well. Belonging to a regime of 3d electron count found for hole-doped cuprates, Pr₄Ni₃O₈ thus represents one of the closest analogues to cuprates yet reported and a singularly promising candidate for high-T_c superconductivity if electron doping could be achieved.

中文翻译：

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金属方平面镍酸盐中的大轨道极化

高温铜酸盐超导性仍然是凝聚态物理中的一个主要问题。解决这一问题的众多方法之一是研究具有相似结构和3d电子数量的替代过渡金属氧化物，这些氧化物被建议作为铜酸盐物理学的代理。这些类似物都不是超导的，甚至很少是金属的。在这里，我们报告说，低价的准二维三层化合物Pr ₄ Ni ₃ O ₈避免了先前报道的La ₄ Ni ₃ O ₈的电荷条带化相，从而导致了金属基态。X射线吸收光谱显示金属Pr ₄ Ni₃ O ₈呈现出低自旋构型，具有明显的轨道极化，并且在费米能以上（铜质超导体的特征）上方的未占据状态下具有明显的d _{x ² - y ²}特征。密度泛函理论计算证实了这一发现，并揭示了d _{x ² - y ²}轨道也主宰了近E _f占据状态。属于空穴掺杂的铜酸盐，Pr ₄ Ni ₃ O ₈，具有3 d电子计数的形式因此，它代表了与已报道的铜酸盐最接近的类似物之一，并且如果可以实现电子掺杂，则有望成为高T _c超导性的唯一候选者。