Where Are the Extra d Electrons in Transition-Metal Substituted Fe Pnictides?

TL;DR

This study uses density functional calculations to show that Co and Ni substitutions in Fe pnictides are isovalent and act as scatterers rather than doping agents, challenging traditional interpretations.

Contribution

It reveals that Co and Ni do not donate extra electrons but are isovalent, acting as scattering centers instead of carrier dopants in Fe pnictides.

Findings

Co and Ni are isovalent to Fe in substitution

Extra d electrons are localized at the substitution site

Substitutions act as scattering centers, not dopants

Abstract

Transition-metal substitution in Fe pnictides leading to superconductivity is usually interpreted in terms of carrier doping to the system. We report on a density functional calculation of the local substitute electron density and demonstrate that substitutions like Co and Ni for Fe do not carrier dope but rather are isovalent to Fe. We find that the extra d electrons for Co and Ni are almost totally located within the muffin-tin sphere of the substituted site. We suggest that Co and Ni act more like random scatterers scrambling momentum space and washing out parts of the Fermi surface.