[论文解读] Searching for Superconductivity in High Entropy Oxide Ruddlesden-Popper Cuprate Films

Alessandro R. Mazza, Xingyao Gao+11 more|arXiv (Cornell University)|Nov 18, 2021

Magnetic and transport properties of perovskites and related materials参考文献 58被引用 31

一句话总结

本研究通过Ce+4（电子）和Sr+2（空穴）共掺杂，探究了高熵Ruddlesden-Popper铜氧化物薄膜（La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4中的超导性，成功制备出单相、外延生长的薄膜。尽管通过退火调节氧化学计量比以降低电阻率并诱导金属性，但因A位阳离子尺寸差异导致Cu-O平面出现显著畸变（σ² > 0.015 Ų），超导性仍未出现，该结果经EXAFS和输运测量证实。

ABSTRACT

In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and Sr+2 at concentrations known to induce superconductivity in the simple parent compounds, Nd2CuO4 and La2CuO4. Electron doped (La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped (La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be single crystal, epitaxially strained, and highly uniform. Transport measurements demonstrate that all as-grown films are insulating regardless of doping. Annealing studies show that resistivity can be tuned by modifying oxygen stoichiometry and inducing metallicity but without superconductivity. These results in turn are connected to extended x-ray absorption fine structure (EXAFS) results indicating that the lack of superconductivity in the high entropy cuprates likely originates from a large distortion within the Cu-O plane ({\sigma}2>0.015 {\AA}2) due to A-site cation size variance, which drives localization of charge carriers. These findings describe new opportunities for controlling charge- and orbital-mediated functional responses in Ruddlesden-Popper crystal structures, driven by balancing of cation size and charge variances that may be exploited for functionally important behaviors such as superconductivity, antiferromagnetism, and metal-insulator transitions, while opening less understood phase spaces hosting doped Mott insulators, strange metals, quantum criticality, pseudogaps, and ordered charge density waves.

研究动机与目标

• 探究具有五种不同A位阳离子的高熵氧化物Ruddlesden-Popper铜氧化物中的超导性。
• 研究电子（Ce）和空穴（Sr）掺杂对电子输运和结构稳定性的影响。
• 确定氧空位工程是否能在这些熵稳定体系中诱导金属性或超导性。
• 将Cu-O平面的结构畸变与超导行为的抑制联系起来。

提出的方法

• 在720 °C下采用脉冲激光沉积（PLD）外延生长单晶态（La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4薄膜。
• 通过Ce+4（n型）和Sr+2（p型）的可控掺杂，探测电子相演化过程。
• 原位X射线衍射（XRD）监测退火过程中结构稳定性和氧空位的形成。
• 采用扩展X射线吸收精细结构（EXAFS）量化Cu-O平面畸变和配位环境变化。
• 通过输运测量评估电阻率、金属-绝缘体转变及超导转变温度。
• 利用XANES确认掺杂后Cu价态的预期变化。

实验结果

研究问题

• RQ1 能否通过电子或空穴掺杂在高熵Ruddlesden-Popper铜氧化物中诱导出超导性？
• RQ2 A位阳离子尺寸差异在多大程度上影响Cu-O平面的结构畸变？
• RQ3 氧空位工程在多大程度上可调控电阻率并诱导这些薄膜的金属性？
• RQ4 为何即使实现了金属性行为，超导性仍未能出现？
• RQ5 Cu-O平面的弯曲和无序在多大程度上抑制了超导配对？

主要发现

• 所有生长态薄膜——未掺杂、Ce共掺杂及Sr共掺杂——在所有测量温度下均表现为绝缘性。
• 在还原性（H2）或氧化性（臭氧）气氛中退火可降低电阻率，并在Ce共掺杂薄膜中诱导金属-绝缘体转变。
• EXAFS显示所有样品中Cu-O平面均存在显著畸变（σ² > 0.015 Ų），归因于A位阳离子尺寸差异。
• 氧空位的形成可减小Cu-O平面畸变，且与电阻率降低相关，表明结构均质化是关键机制。
• 在任何薄膜中均未观察到超导转变，即使对氧化学计量比进行了广泛调控。
• 超导性的主要障碍被确定为Cu-O平面的结构无序，而非电子掺杂或氧含量本身。