[论文解读] Synthesis and Characterization of Ca-Substituted Infinite-Layer Nickelate Crystals

Pascal Puphal, Yu‐Mi Wu+10 more|arXiv (Cornell University)|Jun 24, 2021

Chemical and Physical Properties of Materials参考文献 1被引用 4

一句话总结

本研究通过高温高压合成与定向氧还原法，成功制备了高质量的钙掺杂无限层镍酸盐单晶。所得晶体表现出类金属电导率，其电子结构与空穴掺杂薄膜相似，证实了宏观单晶样品适用于研究重掺杂体系中的超导性。

ABSTRACT

Rare-earth nickelates with the infinite-layer crystal structure have been synthesized in thin film and powder form via topotactic oxygen reduction of the perovskite phase. The infinite-layer phase exhibits remarkable properties, such as superconductivity and magnetic excitations with extraordinarily large bandwidth. Yet, superconductivity was exclusively reported for infinite-layer nickelate films, while polycrystalline powder samples of similar composition were insulating at all measured temperatures. Here, a high-pressure method was used to synthesize high-quality single crystals of the perovskite nickelate La$_{1-x}$Ca$_{x}$NiO$_3$ that were subsequently reduced to the infinite-layer phase La$_{1-x}$Ca$_{x}$NiO$_{2+\delta}$. The obtained samples were characterized by X-ray diffraction, electron microscopy, Raman spectroscopy, magnetometry, and electrical transport measurements. Notably, the metal-like electrical conductivity of the infinite-layer crystals is reminiscent of weakly hole-doped infinite-layer thin films. Moreover, local electron energy-loss spectroscopy reveals close similarities between the electronic structures of the crystals and thin films. This work demonstrates the realization of infinite-layer nickelate crystals with macroscopic size as well as superior crystalline quality, and paves the way for future studies exploring whether more heavily Ca-substituted crystals host superconductivity in analogy to sufficiently hole-doped films.

研究动机与目标

• 为系统研究无限层镍酸盐的电子性质，克服其宏观单晶材料缺乏的难题。
• 探究钙掺杂无限层镍酸盐是否如空穴掺杂薄膜一样表现出超导性。
• 建立一种高温高压合成高质量钙钛矿前驱体单晶，再通过定向氧还原转化为无限层相的合成路线。
• 利用先进光谱与结构表征技术，比较单晶与薄膜的电子结构和输运性质。

提出的方法

• 采用高温高压固态合成法生长了钙钛矿相La₁₋ₓCaₓNiO₃的单晶。
• 通过定向氧还原将钙钛矿相转化为无限层相La₁₋ₓCaₓNiO₂₊δ，同时保持晶体结构完整。
• 利用X射线衍射与电子显微镜确认相纯度与晶体质量。
• 拉曼光谱提供了晶体结构的振动特性表征。
• 磁学测量与电输运测量用于评估磁性与电子行为。
• 采用局域电子能量损失谱（EELS）比较单晶与薄膜之间的电子结构差异。

实验结果

研究问题

• RQ1 能否通过高温高压方法合成高质量的钙掺杂无限层镍酸盐单晶？
• RQ2 这些单晶是否表现出与空穴掺杂无限层薄膜类似的类金属电导率？
• RQ3 所合成单晶的电子结构与外延薄膜相比有何异同？
• RQ4 在重钙掺杂的无限层镍酸盐单晶中是否存在类似薄膜中观察到的超导迹象？
• RQ5 这些材料的晶体质量与宏观尺寸是否可支持未来对非传统超导性的研究？

主要发现

• 在高温高压条件下，成功合成了高质量的钙钛矿相La₁₋ₓCaₓNiO₃单晶。
• 通过定向氧还原获得了相纯度高、结晶质量优异的无限层相La₁₋ₓCaₓNiO₂₊δ单晶。
• 电输运测量显示，无限层相单晶表现出类金属电导率，与弱空穴掺杂薄膜相似。
• 局域电子能量损失谱（EELS）结果表明，单晶与外延薄膜之间具有高度相似的电子结构。
• 所合成样品具有宏观尺寸与优异的晶体质量，为未来研究电子性质（包括潜在超导性）提供了可能。
• 结果表明，更重钙掺杂的单晶可能如充分空穴掺杂的薄膜一样，表现出超导性。