Doping-induced metallicity and coexistence of magnetic subsystems in {K$_{2}$Fe$_{4+x}$Se$_{5}$}

TL;DR

This study uses electronic structure calculations to explore how doping in K2Fe4+xSe5 induces metallicity and allows magnetic subsystems to coexist, providing insights into superconductivity and antiferromagnetism.

Contribution

It reveals the coexistence of impurity and host magnetic subsystems due to doping, offering a new perspective on their role in superconductivity and magnetic properties.

Findings

Doping introduces impurity bands that influence electronic states.

Weak magnetic interactions lead to prevalent spin fluctuations.

Coexistence of magnetic subsystems suggests a link to Fe-pnictide superconductors.

Abstract

Electronic structure calculations are used to analyze the electronic and magnetic properties in {K$_{2}$Fe$_{4+x}$Se$_{5}$}. Fe atoms can be divided into two distinct groups. The $x{=}0$ (parent) compound forms an insulating, collinear, local moment phase with high N{\'{e}}el temperature. We show that large biquadratic exchange coupling and exchange-elastic interactions stabilize the magnetic order. For $x{>}0$ the additional Fe atoms fill vacancy sites. They form impurity bands for small $x$, which broaden as $x$ increases. They determine the states at the Fermi level and may be viewed as a magnetic subsystem separate from the host. Spin fluctuations are prevalent because magnetic interactions between the `defect' and the `parent' atoms are relatively weak, while chemical fluctuations are prevalent for low $x$. Fluctuations of either type leads to the formation of a weakly metallic state. The unusual coexistence of the two magnetic subsystems offers a new perspective as to how superconductivity and strong antiferromagnetism can coexist. We argue that spin fluctuations of the impurity subsystem share common features with the Fe-pnictide superconductors.