From (π,0) magnetic order to superconductivity with (π,π) magnetic resonance in Fe_1.02Te_1-xSe_x

The iron chalcogenide Fe_1+y (Te_1-xSe_x ) is structurally the simplest of the Fe-based superconductors^1–3. Although the Fermi surface is similar to iron pnictides^4,5, the parent compound Fe_1+yTe exhibits antiferromagnetic order with an in-plane magnetic wave vector (π,0) (ref. 6). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (π,π) that connects hole and electron parts of the Fermi surface^7,8. Despite these differences, both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (π,π) (refs 9-11). A central question in this burgeoning field is therefore how (π,π) superconductivity can emerge from a (π,0) magnetic instability¹². Here, we report that the magnetic soft mode evolving from the (π,0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs as magnetic correlations at (π,0) are suppressed and the mode at (π, π) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.