Common origin of the two types of magnetic fluctuations in iron chalcogenides

We use inelastic neutron scattering to study the low-energy spin excitations in moderately doped nonsuperconducting Fe _1.01Te _0.72Se _0.28. The spin excitations in this system contain components near (0.5,0,0) and (0.5,0.5,0) in a-b plane reciprocal lattice units using tetragonal unit cell notation (a=b=3.772 and c=6.061). At low energies the scattering is centered around (0.5,0,0). With increasing energy, the spectral weight of low-energy spin excitations centered around (0.5,0,0) abruptly shifts around 3 meV to the incommensurate spin excitations centered around (0.5,0.5,0). However both types of spin fluctuations exhibit the identical temperature dependence. These results indicate that the (0.5,0,0)-type spin excitations and the incommensurate excitations around the (0.5,0.5,0) position have a common origin and both must be taken into account to understand the nature of magnetism and superconducting pairing in the iron chalcogenides.