# Coexistence of ferromagnetic and stripe antiferromagnetic spin fluctuations in SrCo$_2$As$_2$

## TL;DR

This study reveals coexistence of ferromagnetic and antiferromagnetic spin fluctuations in SrCo$_2$As$_2$, linked to orbital changes caused by Co substitution, impacting superconductivity.

## Contribution

It demonstrates the coexistence of FM and AF spin fluctuations in SrCo$_2$As$_2$ and connects these to orbital switching from $t_{2g}$ to $e_g$ orbitals due to Co substitution.

## Key findings

- Coexistence of AF and FM spin fluctuations at specific wave vectors.
- Both types of fluctuations are associated with a flat $e_g$ band near the Fermi level.
- FM fluctuations are detrimental to superconductivity due to orbital switching.

## Abstract

We use inelastic neutron scattering to study energy and wave vector dependence of spin fluctuations in SrCo$_2$As$_2$, derived from SrFe$_{2-x}$Co$_x$As$_2$ iron pnictide superconductors. Our data reveals the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) spin fluctuations at wave vectors $\textbf{Q}_{\rm AF}$=(1,0) and $\textbf{Q}_{\rm FM}$=(0,0)/(2,0), respectively. By comparing neutron scattering results with those of dynamic mean field theory calculation and angle-resolved photoemission spectroscopy experiments, we conclude that both AF and FM spin fluctuations in SrCo$_2$As$_2$ are closely associated with a flat band of the $e_g$ orbitals near the Fermi level, different from the $t_{2g}$ orbitals in superconducting SrFe$_{2-x}$Co$_x$As$_2$. Therefore, Co-substitution in SrFe$_{2-x}$Co$_x$As$_2$ induces a $t_{2g}$ to $e_g$ orbital switching, and is responsible for FM spin fluctuations detrimental to the singlet pairing superconductivity.