First-principles calculation of transition-metal impurities in LaFeAsO

TL;DR

This study uses first-principles density-functional calculations to analyze how various transition-metal impurities affect the electronic structure of LaFeAsO, providing insights into impurity levels and their impact on superconducting properties.

Contribution

It systematically characterizes impurity effects in LaFeAsO using ab initio methods and constructs effective Hamiltonians for different transition-metal impurities.

Findings

Impurity levels vary significantly among Mn, Co, Ni, Zn, and Ru.

Impurities cause a rigid shift in the band structure near the Fermi level.

Ru impurities increase transfer integrals due to orbital spread.

Abstract

We present a systematic ab initio study based on density-functional calculations to understand impurity effects in iron-based superconductors. Effective tight-binding Hamiltonians for the d-bands of LaFeAsO with various transition-metal impurities such as Mn, Co, Ni, Zn, and Ru are constructed using maximally-localized Wannier orbitals. Local electronic structures around the impurity are quantitatively characterized by their onsite potential and transfer hoppings to neighboring sites. We found that the impurities are classified into three groups according to the derived parameters: For Mn, Co, and Ni, their impurity-3d levels measured from the Fe-3d level are nearly 0.3 eV, -0.3 eV, and -0.8 eV, respectively, while, for the Zn case, the d level is considerably deep as -8 eV. For the Ru case, although the onsite-level difference is much smaller as O(0.1) eV, the transfer integrals around the impurity site are larger than those of the pure system by 20% \sim 30%, due to the large spatial spread of the Ru-4d orbitals. We also show that, while excess carriers are tightly trapped around the impurity site (due to the Friedel sum rule), there is a rigid shift of band structure near the Fermi level, which has the same effect as carrier doping.