Modified magnetism within the coherence volume of superconducting Fe_1+δ Se_x Te_1-x

Neutron scattering is used to probe magnetic interactions as superconductivity develops in optimally doped Fe_1+δSe_xTe_1-x. Applying the first moment sum rule to comprehensive neutron scattering data, we extract the change in magnetic exchange energy Δ[ J_R-R′⟨S_R·S_R′⟩] in the superconducting state referenced to the normal state. Oscillatory changes are observed for Fe-Fe displacements |ΔR| < ξ, where ξ = 1.3(1) nm is the superconducting coherence length. Dominated by a large reduction in the second nearest neighbor exchange energy [-1.2(2) meV/Fe], the overall reduction in magnetic interaction energy is Δ⟨H_mag⟩=-0.31(9) meV/Fe. Comparison to the superconducting condensation energy ΔE_SC=-0.013(1) meV/Fe, which we extract from specific heat data, suggests the modified magnetism we probe drives superconductivity in Fe_1+δSe_xTe_1-x.