Abstract
Neutron Scattering is used to probe magnetic interactions as superconductivity develops in opti- mally doped Fe_(1+δ)Se_xTe_(1−x). Applying the first moment sum-rule to comprehensive neutron scatter- ing data, we extract the change in magnetic exchange energy ∆[J_(R-R′) ⟨S_R ·S_R′ ⟩] in the superconducting state referenced to the normal state. Oscillatory changes are observed for Fe-Fe displacements |∆R| < 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 ∆⟨Hmag⟩ = −0.31(9) meV/Fe. Comparison to the superconducting condensation energy ∆E_sc = −0.013(1) meV/Fe, which we extract from specific heat data, suggests the modified magnetism we probe drives superconductivity in Fe_(1+δ)Se_xTe_(1−x)
