The Electronic Structure of the FeSe / Ti1+xO2 / SrTiO3 Interface

在标准温度和压力下无损耗传输电能的前景，使超导性成为材料科学中最诱人的目标之一。然而，对高温超导性的全面解释仍然难以捉摸。为了找到前进的道路，科学家必须研究单个材料，试图了解它们是如何工作的，以及它们有什么共同点。该项目将为研究超导材料对其原子结构变化的反应提供计算工具。了解这些变化使研究人员能够预测提高超导材料工作温度的方法，从而使实验集中在最有希望的候选材料上。这些工具将应用于硒化铁（FeSe）沉积在钛酸锶（SrTiO3）上的情况。在SrTiO3上生长的三原子厚度的FeSe单层在温度上保持超导性，几乎是纯FeSe大晶体的10倍。在这个项目中实施的方法将澄清这种材料的性质，并可能建议如何设计新的超导体。该项目还将允许一所小型本科大学的学生陪同首席研究员前往国家实验室，在那里他们将获得计算研究技能，并进一步发展他们作为科学家的身份。自从发现SrTiO3上的单层FeSe超导温度达到60 - 80 K以来，人们一直在积极研究它，而块状FeSe的超导温度约为8 K。理论研究主要集中在纯FeSe / SrTiO3界面上，但原子分辨率扫描透射电子显微镜（STEM）图像显示，在SrTiO3衬底和FeSe之间存在额外的氧化钛层。pi最近发表的计算结果表明，该层显示出钛过量，可以参与FeSe单层的电子掺杂。这种掺杂被认为是增加体系中Tc的重要因素。虽然这些密度泛函理论的结果很好地描述了这种材料的原子结构，但这些方法准确描述实际异质结构的能力存在技术和基本限制。在从这些计算中提取出特定于材料的模型参数后，pi将进行更复杂的计算，以阐明系统的哪些特性对超导状态最重要。通过构造多个结构构型并对其能带结构进行平均，探讨了无序或不同有序对额外界面层的影响。此外，将计算表面格林函数，以确定更现实的半无限基板上单层和界面层的电子结构。这样的结果也可以为如何在其他材料中设计类似的Tc增加提供见解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The promise of transmitting electricity without loss makes superconductivity at standard temperature and pressure one of the most tantalizing goals in materials science. However, a comprehensive explanation of high-temperature superconductivity remains elusive. To chart a path forward, scientists must study individual materials to try to understand how they work and what they have in common. This project will provide computational tools for the study of how superconducting materials react to changes in their atomic structure. Understanding these changes allows researchers to predict ways to increase the operating temperature of a superconducting material, allowing experiments to focus on the most promising candidates. These tools will be applied to the case of iron selenide (FeSe) deposited on strontium titanate (SrTiO3). A single three-atom-thick layer of FeSe grown on SrTiO3 remains superconducting up to temperatures almost 10 times greater than larger crystals of pure FeSe. The methods implemented in this project will clarify the properties of this material and may suggest how to design new superconductors. The project will also allow students from a small undergraduate university to accompany the principal investigator to a national laboratory where they will gain computational research skills and further develop their identity as scientists. Technical DescriptionMonolayer FeSe on SrTiO3 has been actively studied since the discovery of its enhanced superconducting temperature Tc of 60 – 80 K, compared to around 8 K in bulk FeSe. Theoretical investigations have focused on a pure FeSe / SrTiO3 interface, but atomic-resolution scanning transmission electron microscope (STEM) images have revealed the existence of an additional titanium-oxide layer between the SrTiO3 substrate and FeSe. The P.I. recently published computational results that demonstrate that this layer exhibits a titanium excess that can participate in electron-doping the FeSe monolayer. This doping is thought to be important in increasing Tc in this system. While these density functional theory results provide a good description of the atomic structure of this material, there are technical and fundamental limits to such methods’ ability to accurately describe a realistic heterostructure. After extracting material-specific model parameters from these calculations, the P.I. will perform more sophisticated calculations that will clarify which of the properties of the system are most important to the superconducting state. The effect of disorder or different ordering in the extra interfacial layer will be explored by constructing multiple structural configurations and averaging over their band structures. Further, surface Green functions will be computed to determine the electronic structure of the monolayer and interfacial layer on a more realistic semi-infinite substrate. Such results can also provide insight into how similar increases in Tc might be engineered in other materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.