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

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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.