Weaker nematic phase connected to the first order antiferromagnetic phase transition in SrFe2As2 compared to BaFe2As2

Understanding the nature of the electronic nematic phase in iron pnictide superconductors is important for elucidating its impact on high-temperature superconductivity. Here we use transport and inelastic neutron scattering to study spin excitations and in-plane resistivity anisotropy in uniaxial pressure detwinned BaFe2As2 and SrFe2As2, the parent compounds of iron pnictide superconductors. While BaFe2As2 exhibits weakly first-order tetragonal-to-orthorhombic structural and antiferromagnetic (AF) phase transitions below Ts>TN≈138K, SrFe2As2 has strongly coupled first-order structural and AF transitions below Ts=TN≈210K. We find that the direct signatures of the nematic phase persist to lower temperatures above the phase transition in the case of SrFe2As2 compared to BaFe2As2. Our findings support the conclusion that the strongly first-order nature of the magnetic transition in SrFe2As2 weakens the nematic phase and resistivity anisotropy in the system.