# Role of Fermi surface for the pressure-tuned nematic transition in the BaFe$_2$As$_2$ family

## TL;DR

This study investigates how changes in the Fermi surface influence the pressure-induced nematic transition in BaFe$_2$As$_2$, revealing two regimes with different behaviors of structural and magnetic transition temperatures.

## Contribution

It provides experimental evidence linking Fermi surface topology changes to the pressure-dependent nematic transition in BaFe$_2$As$_2$, highlighting the electronic origin of structural order.

## Key findings

- Identification of two regimes with increasing or decreasing $ ext{T}_s - ext{T}_N$ under pressure
- Evidence that Fermi surface topology changes correlate with transition behavior
- Support for the electronic origin of nematic order in iron-based superconductors

## Abstract

The tetragonal-to-orthorhombic phase transition at $T_s$, which precedes the antiferromagnetic phase transition at $T_N$ in many iron-based superconductors, is considered as one of the manifestations of electronic \textit{nematic} order. By constructing temperature-pressure phase diagrams of pure and Co-doped BaFe$_2$As$_2$, we study the relation of $T_s$ and $T_N$ under pressure $p$. Our data disclose two qualitatively different regimes in which $\Delta T\,=\,T_s-T_N$ either increases or decreases with $p$. We provide experimental evidence that the transition between the two regimes may be associated with sudden changes of the Fermi surface topology. Therefore, our results not only support the electronic origin of the structural order, but also emphasize the importance of details of the Fermi surface for the evolution of nematic order under pressure.