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 ${\text{Q}}_{\mathrm{AF}}$=(1,0) and ${\text{Q}}_{\mathrm{FM}}$=(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.

6 pages, 4 figures, supplemental material available upon request, to be published in PRL

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