ORALS

Iron-based unconventional superconductors with quasi-two-dimensional crystal structure have attracted intense interest after the critical temperature of FeSe was enhanced by more than one order of magnitude in the thin layer deposited on top of an insulating oxide substrate. In heterostructures comprising interfaces of FeSe with topological insulators, additional interesting physical phenomena are predicted to arise e.g. in the form of topological superconductivity [1]. Importantly, the tetragonal FeSe phase relevant for superconductivity is stabilized by excess Fe, leading to non-stoichiometric Fe(1+δ)Se compounds [2]. However, the number of first-principles computational studies considering excess Fe is limited. We have studied Fe(1+δ)Se employing the coherent potential approximation and the tight-binding linear muffin-tin orbital method, which are well suited for disordered systems and can treat systems with even a very small off-stoichiometry without the need for a large supercell. It also allows us to explicitly address the impact of chalcogen vacancies. Furthermore, we have examined the effect of chalcogen height for both FeSe and Fe(1+δ)Se. This parameter has been determined with only a limited accuracy so far, and it appears to affect the band structure significantly here. At an interface such geometrical properties can be strongly modified, as compared to the bulk case. Calculated band structures are compared to experimental ARPES data [3].

References:
[1] Liu, X., et al. Electronic structure and superconductivity of {FeSe}-related superconductors. J. Phys.: Condens. Matter 27 (2015) 183201
[2] Medvedev S et al, Nat. Mater. 8 (2009) 630
[3] Fikáček J. , et al, Step-edge assisted large scale FeSe monolayer growth on epitaxial Bi Se thin films. New J. Phys. 22 (2020) 073050