[论文解读] Similarities and differences between nickelate and cuprate films grown on a SrTiO$_3$ substrate

Yang Zhang, Ling-Fang Lin+4 more|arXiv (Cornell University)|Aug 10, 2020

Physics of Superconductivity and Magnetism被引用 4

一句话总结

本研究采用密度泛函理论（DFT）比较NdNiO₂/SrTiO₃与CaCuO₂/SrTiO₃薄膜，发现NiO₂层经历强烈的极性晶格重构，其表面与界面处位移方向相反，引发二维电子气（2DEG）并导致电子向Ti 3d_{xy}轨道转移。相比之下，CuO₂层发生均匀畸变，且2DEG更局域化。Ni d_{3z²−r²}轨道变为巡游态，而Cu保持双电子占据，尽管两者具有相同的d⁹电子构型，凸显了关键的电子结构差异。

ABSTRACT

The recent discovery of superconductivity in Sr-doped NdNiO$_2$ films grown on SrTiO$_3$ started a novel field within unconventional superconductivity. To understand the similarities and differences between nickelate and cuprate layers on the same SrTiO$_3$ substrate, here based on the density functional theory we have systematically investigated the structural, electronic, and magnetic properties of NdNiO$_2$/SrTiO$_3$ and CaCuO$_2$/SrTiO$_3$ systems. Our results revealed a strong lattice reconstruction in the case of NdNiO$_2$/SrTiO$_3$, resulting in a polar film, with the surface and interfacial NiO$_2$ layers presenting opposite displacements. However, for CaCuO$_2$/SrTiO$_3$, the distortions of those same two CuO$_2$ layers were in the same direction. In addition, we found this distortion to be approximately independent of the studied range of film thickness for both the nickelate and cuprates films. Furthermore, we also observed a two-dimensional electron gas at the interface between NdNiO$_2$ and SrTiO$_3$, caused by the polar discontinuity, in agreement with recent literature. For NdNiO$_2$/SrTiO$_3$ the two-dimensional electron gas extends over several layers, while for CaCuO$_2$/SrTiO$_3$ this electronic rearrangement is very localized at the interface between CaCuO$_2$ and SrTiO$_3$. The electronic reconstruction found at the interface involves a strong occupation of the Ti $3d_{xy}$ state. In both cases, there is a significant electronic charge transfer from the surface Ni or Cu layers to the Ti interface layer. The interfacial Ni and Cu layer is hole and electron doped, respectively. By introducing magnetism and electronic correlation, we observed that the $d_{3z^2-r^2}$ orbital of Ni becomes itinerant while the same orbital for Cu remains doubly occupied, establishing a clear two- vs one-orbital active framework for the description of these systems.

研究动机与目标

• 理解镍酸盐与铜酸盐薄膜在SrTiO₃衬底上的结构、电子与磁性差异。
• 探究极性不连续性与晶格重构在诱导界面2DEG与电荷转移中的作用。
• 比较在相似外延应变与界面条件下Ni与Cu 3d轨道的电子行为。
• 确定NNO薄膜中的超导性是否源于界面驱动的电子重构，如在LAO/STO或CCO/STO体系中所见。

提出的方法

• 采用GGA和DFT+Ueff（Ueff = 4 eV）的密度泛函理论，以考虑电子关联效应。
• 构建(NdNiO₂)n/(SrTiO₃)4与(CaCuO₂)n/(SrTiO₃)4的层状模型，固定面内晶格常数（a = b = 3.905 Å 或 3.943 Å）以模拟外延应变。
• 通过结构弛豫确定NiO₂与CuO₂层中的原子位移与极性畸变。
• 利用Bader电荷分析量化Ni/Cu向Ti离子的电荷转移。
• 通过投影能带结构与费米面分析，研究费米能级附近的轨道特征与电子态。
• 对比非磁性与G-AFM（G型反铁磁）态，评估磁性重构的影响。

实验结果

研究问题

• RQ1 当生长在SrTiO₃衬底上时，NdNiO₂与CaCuO₂薄膜的晶格畸变有何不同？
• RQ2 NdNiO₂/SrTiO₃与CaCuO₂/SrTiO₃中界面二维电子气（2DEG）的性质与范围如何？
• RQ3 两种体系中Ni或Cu向Ti 3d轨道的电荷转移有何差异，Ti 3d_{xy}轨道在其中扮演何种角色？
• RQ4 为何在电子关联作用下，Ni d_{3z²−r²}轨道变为巡游态，而Cu d_{3z²−r²}轨道仍保持双电子占据？
• RQ5 NdNiO₂表面磁性重构对磁性的抑制程度如何，相较于CaCuO₂体系有何差异？

主要发现

• NdNiO₂/SrTiO₃表现出强烈的极性晶格重构，NiO₂层的表面与界面位移方向相反（Δ = +0.279 Å 与 −0.169 Å），而CaCuO₂/SrTiO₃中则呈现同向畸变。
• NdNiO₂/SrTiO₃中的2DEG因极性不连续性而延伸至多个原子层，而CaCuO₂/SrTiO₃中的2DEG则高度局域于界面处。
• 界面处的Ti 3d_{xy}轨道被强烈占据（每个Ti获得~0.129 e），且DFT+Ueff计算显示Ni向Ti的电荷转移增强（从−0.101 e/Ni增至−0.141 e/Ni）。
• 在电子关联作用下，Ni d_{3z²−r²}轨道变为巡游态，而Cu d_{3z²−r²}轨道保持双电子占据，确立了双轨道与单轨道活性的框架差异。
• 磁性重构抑制了NdNiO₂表面的磁性，而界面仍保持金属性与掺杂特征。
• 结构与电子差异在不同膜厚下均保持稳定，并在非磁性与G-AFM DFT+Ueff计算下保持一致。