Green Function Approach for Calculating Surface Electronic Structure of Iron Selenide on Strontium Titanate*

Electronic band structure has proven to be a useful way of observing the electronic properties of materials. Due to the cumbersome nature through which electronic band plots are generated, there is value in developing computational methods to calculate them in more complicated systems, like multilayered structures. We examine the crystalline superconductor Iron Selenide (FeSe), which exhibits a distinct shift in its surface band structure when isolated versus monolayer FeSe grown on bulk Strontium Titanate (STO) expressed through a change in superconducting temperature from the order of 10K to 70K. This case is simplified by exploiting the periodic nature of the molecular structure in crystalline materials, allowing for unit cells to be defined in different layers. For this, we turn to the use of DFT calculations to provide the basis for our model Hamiltonians, which describe the interactions between layers. Using these calculations, we modify previously written methods for calculating surface electronic structure for homogeneous materials to fit our inhomogeneous case and investigate changes to the Fermi surface in FeSe.