Self-doping effect and possible antiferromagnetism at titanium-layers in the iron-based superconductor Ba$_2$Ti$_2$Fe$_2$As$_4$O

TL;DR

This study uses first-principles calculations to explore the electronic structure of Ba$_2$Ti$_2$Fe$_2$As$_4$O, revealing self-doping effects, suppression of Fe antiferromagnetism, and potential antiferromagnetic order at Ti layers related to a 125 K anomaly.

Contribution

It provides a detailed theoretical analysis of the electronic and magnetic properties of Ba$_2$Ti$_2$Fe$_2$As$_4$O, highlighting the self-doping mechanism and magnetic instabilities in this superconductor.

Findings

Significant electron transfer from Ti to Fe suppresses stripe-like antiferromagnetism.

Presence of multiple Fermi surface sheets from Ti-3d and Fe-3d states.

Potential Neel-type antiferromagnetic instability at Ti sites explains the 125 K anomaly.

Abstract

The electronic structure of Ba$_2$Ti$_2$Fe$_2$As$_4$O, a newly discovered superconductor, is investigated using first-principles calculations based on local density approximations. Multiple Fermi surface sheets originating from Ti-3$d$ and Fe-3$d$ states are present corresponding to the conducting Ti$_2$As$_2$O and Fe$_2$As$_2$ layers respectively. Compared with BaFe$_2$As$_2$, sizeable changes in the related Fermi surface sheets indicate significant electron transfer (about 0.12$e$) from Ti to Fe, which suppresses the stripe-like antiferromagnetism at the Fe sites and simultaneously induces superconductivity. Our calculations also suggest that an additional N\'{e}el-type antiferromagnetic instability at the Ti sites is relatively robust against the electron transfer, which accounts for the anomaly at 125 K in the superconducting Ba$_2$Ti$_2$Fe$_2$As$_4$O.