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

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.