[论文解读] Observation of perfect diamagnetism and interfacial effect on the electronic structures in Nd0.8Sr0.2NiO2 superconducting infinite layers

Shengwei Zeng, Xinmao Yin+14 more|arXiv (Cornell University)|Apr 29, 2021

Magnetic and transport properties of perovskites and related materials参考文献 21被引用 6

一句话总结

本研究在高质量 Nd0.8Sr0.2NiO2 薄膜中证实了完美的抗磁性，确认了体相超导性，同时揭示了超薄薄膜（<5.5 nm）由于界面效应和应变效应而表现出厚度驱动的电子型能带结构。X射线吸收谱和能带结构计算表明，较薄的薄膜中 Ni-O 杂化增强，解释了负霍尔系数的持续存在以及界面处超导性的出现。

ABSTRACT

Nickel-based complex oxides have served as a playground for decades in the quest for a copper-oxide analog of the high-temperature superconductivity. They may provide clues towards understanding the mechanism and an alternative route for high-temperature superconductors. The recent discovery of superconductivity in the infinite-layer nickelate thin films has fulfilled this pursuit. However, material synthesis remains challenging, direct demonstration of perfect diamagnetism is still missing, and understanding of the role of the interface and bulk to the superconducting properties is still lacking. Here, we show high-quality Nd0.8Sr0.2NiO2 thin films with different thicknesses and demonstrate the interface and strain effects on the electrical, magnetic and optical properties. Perfect diamagnetism is achieved, confirming the occurrence of superconductivity in the films. Unlike the thick films in which the normal-state Hall-coefficient changes signs as the temperature decreases, the Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a thickness-driven band structure modification. Moreover, X-ray absorption spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer nickelates, and the hybridization is enhanced as the thickness decreases. Consistent with band structure calculations on the nickelate/SrTiO3 heterostructure, the interface and strain effect induce a dominating electron-like band in the ultrathin film, thus causing the sign-change of the Hall-coefficient.

研究动机与目标

• 通过直接观测完美抗磁性，确认 Nd0.8Sr0.2NiO2 无限层薄膜中存在超导性。
• 研究薄膜厚度和界面效应在电子和输运性质中的作用。
• 理解厚膜中霍尔系数符号变化与超薄膜中其持续存在的原因。
• 利用X射线吸收谱探测Ni-O杂化的电子结构演化。

提出的方法

• 采用脉冲激光沉积法在 SrTiO3 基底上生长具有不同厚度的高质量 Nd0.8Sr0.2NiO2 薄膜。
• 通过 SQUID 磁力仪测量磁响应，以检测完美抗磁性。
• 原位 X 射线吸收谱分析 Ni 2p 和 O K 边谱，提取 Ni-O 杂化特性。
• 通过霍尔效应测量跟踪载流子类型和浓度随温度和薄膜厚度的变化。
• 对 NiO/SrTiO3 异质结构进行密度泛函理论计算，以模拟界面能带结构和应变效应。
• 将实验数据与理论模型相关联，解释超薄薄膜中电子型能带占主导的原因。

实验结果

研究问题

• RQ1 Nd0.8Sr0.2NiO2 是否表现出完美抗磁性，从而证实其本征超导性？
• RQ2 薄膜厚度如何影响 Nd0.8Sr0.2NiO2 的霍尔系数和载流子类型？
• RQ3 界面应变和 Ni-O 杂化在调控超薄镍酸盐薄膜电子结构中的作用是什么？
• RQ4 为何在厚度小于 5.5 nm 的薄膜中霍尔系数保持负值，而厚膜中则发生变化？
• RQ5 界面效应在多大程度上主导了无限层镍酸盐的电子性质？

主要发现

• 在 Nd0.8Sr0.2NiO2 薄膜中观测到完美抗磁性，证实了超导性的出现。
• 在厚度小于 5.5 nm 的薄膜中，霍尔系数保持负值，表明载流子具有持续的电子型特征。
• X射线吸收谱显示，较薄薄膜中 Ni-O 杂化增强，与共价性增加相关。
• 能带结构计算表明，界面应变诱导了超薄薄膜中主导的电子型能带。
• 厚度驱动的能带结构调制解释了超薄薄膜中霍尔系数符号未发生反转的原因。
• 界面效应和应变效应在稳定电子型特征并促进超薄 Nd0.8Sr0.2NiO2 中超导性的形成中起关键作用。