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

无损耗传输电力的承诺使标准温度和压力下的超导成为材料科学中最诱人的目标之一。然而，对高温超导电性的全面解释仍然难以捉摸。为了绘制出一条前进的道路，科学家们必须研究个别材料，试图了解它们是如何工作的，以及它们有什么共同之处。该项目将为研究超导材料如何对其原子结构的变化做出反应提供计算工具。通过了解这些变化，研究人员可以预测提高超导材料工作温度的方法，从而使实验能够专注于最有希望的候选材料。这些工具将应用于沉积在钛酸锶(SrTiO3)上的硒化铁(FeSe)的情况。在钛酸锶上生长的一层三原子厚的FeSe仍然保持超导性，温度几乎是更大的纯FeSe晶体的10倍。这个项目中实施的方法将阐明这种材料的性质，并可能为如何设计新的超导体提供建议。该项目还将允许来自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。自从发现单层FeSe的超导温度提高到60-80K，而体相FeSe的超导温度约为8K以来，人们一直在积极地研究SrTiO3上的FeSe。理论研究主要集中在纯FeSe/SrTiO_3界面上，但原子分辨扫描电子显微镜(STEM)图像显示，在SrTiO_3衬底和FeSe之间存在额外的钛氧化层。P.I.最近公布的计算结果表明，这一层显示出过量的钛，可以参与FeSe单分子膜的电子掺杂。这种掺杂被认为是提高该体系的T_c的重要因素。虽然这些密度泛函理论结果很好地描述了这种材料的原子结构，但这种方法准确描述现实异质结构的能力存在技术和基本限制。从这些计算中提取特定于材料的模型参数后，P.I.将执行更复杂的计算，以澄清系统的哪些属性对超导状态最重要。通过构造多个结构构型和平均它们的能带结构来探索无序或不同有序性对额外界面层的影响。此外，将计算表面格林函数以确定在更真实的半无限衬底上的单分子层和界面层的电子结构。这样的结果也可以让我们深入了解如何在其他材料中设计类似的TC增加。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.