Observation of charge accumulation and onsite Coulomb repulsion at transition metal impurities in the iron pnictides

TL;DR

This study investigates how transition metal impurities in iron pnictides affect electronic structure and Coulomb interactions, revealing charge accumulation and increased onsite Coulomb repulsion with different impurities, which impacts superconductivity.

Contribution

It provides direct experimental evidence of charge accumulation and Coulomb interaction variations at transition metal impurities in iron pnictides.

Findings

Transition metal states shift to higher binding energies with increasing atomic number.

Charge accumulates at Ni and Cu impurities as shown by Auger spectroscopy.

Onsite Coulomb interaction increases from Co to Cu impurities.

Abstract

We report a combined valence band photoemission and Auger spectroscopy study of single crystalline Ca(Fe,Co)2As2 and Ba(Fe,TM)2As2 with TM=Ni or Cu. The valence band photoemission data show directly that the TM-states move to higher binding energies with increasing atomic number, contributing less and less to the states close to the Fermi level. Furthermore, the 3d8 final state of the LVV Auger decay, which is observed for Ni and Cu, unambiguously reveals the accumulation of charge at these impurities. We also show that the onsite Coulomb interaction on the impurity strongly increases when moving from Co over Ni to Cu. Our results quantify the impurity potentials and imply that the superconducting state is robust against impurity scattering.