# Evolution from $B_{2g}$ nematics to $B_{1g}$ nematics in heavily hole-doped iron-based superconductors

## TL;DR

This paper explains the transition from $B_{2g}$ to $B_{1g}$ nematic order in heavily hole-doped iron-based superconductors by analyzing magnetic ground state evolution and fluctuations using density functional theory.

## Contribution

It reveals that the change in magnetic ground state from single to double stripes causes the nematic symmetry shift, highlighting the role of magnetic fluctuations in this process.

## Key findings

- Nematic order shifts from $B_{2g}$ to $B_{1g}$ with hole doping.
- Magnetic ground state evolves from single to double stripe configurations.
- Magnetic fluctuations are crucial for stabilizing the double-stripe state.

## Abstract

Recent experiments reported an unusual nematic behavior of heavily hole-doped pnictides $A$Fe$_{2}$As$_{2}$, with alkali $A$ = Rb, Cs. In contrast to the $B_{2g}$ nematic order of the parent $Ae$Fe$_{2}$As$_{2}$ compounds (with alkaline earth $Ae$ = Sr, Ba), characterized by unequal nearest-neighbor Fe-Fe bonds, in the hole-doped systems nematic order is observed in the $B_{1g}$ channel, characterized by unequal next-nearest-neighbor Fe-Fe (diagonal Fe-As-Fe) bonds. In this work, using density functional theory, we attribute this behavior to the evolution of the magnetic ground state along the series $Ae_{1-x}A_{x}$Fe$_{2}$As$_{2}$, from single stripes for small $x$ to double stripes for large $x$. Our simulations using the reduced Stoner theory show that fluctuations of Fe moments are essential for the stability of the double-stripe configuration. We propose that the change in the nature of the magnetic ground state is responsible for the change in the symmetry of the vestigial nematic order that it supports.