# Orbital mismatch boosting nematic instability in iron-based superconductors

## TL;DR

This paper develops an effective theory for spin modes in iron-based superconductors, showing how orbital mismatches influence magnetic and nematic orders, explaining variations in nematic behavior among different compounds.

## Contribution

It introduces a model linking orbital nesting to nematic instability, highlighting the role of orbital mismatch in enhancing or suppressing nematic order.

## Key findings

- Orbital mismatch boosts spin-nematic order in FeSe.
- Pressure-induced changes in orbital structure suppress nematic order.
- The model explains differences between FeSe and 122 compounds.

## Abstract

We derive the effective action for the collective spin modes in iron-based superconductors. We show that, due to the orbital-selective nature of spin fluctuations, the magnetic and nematic instabilities are controlled by the degrees of orbital nesting between electron and hole pockets. Within a prototypical three-pockets model the hole-electron orbital mismatch is found to boost spin-nematic order. This explains the enhancement of the nematic order in FeSe as compared to 122 compounds, and its suppression under pressure, where the emergence of the second hole pocket compensates the orbital mismatch of the three-pockets configuration.