Abstract
Neutron Scattering is used to probe magnetic interactions as superconductivity develops in opti- mally doped Fe_(1+未)Se_xTe_(1鈭抶). Applying the first moment sum-rule to comprehensive neutron scatter- ing data, we extract the change in magnetic exchange energy 鈭哰J_(R-R鈥�) 鉄⊿_R 路S_R鈥� 鉄 in the superconducting state referenced to the normal state. Oscillatory changes are observed for Fe-Fe displacements |鈭哛| < 尉, where 尉 = 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 鈭嗏煥Hmag鉄� = 鈭�0.31(9) meV/Fe. Comparison to the superconducting condensation energy 鈭咵_sc = 鈭�0.013(1) meV/Fe, which we extract from specific heat data, suggests the modified magnetism we probe drives superconductivity in Fe_(1+未)Se_xTe_(1鈭抶)
