Anisotropy of the (π, π) dynamic susceptibility in magnetically ordered (x=0.05) and superconducting (x=0.40) Fe_1.02Te_1-xSe_x

Polarized inelastic neutron scattering on magnetically ordered Fe_1.02Te_0.95Se_0.05 and superconducting Fe_1.02Te_0.60Se_0.40 has been used to elucidate the anisotropy of the magnetic dynamical susceptibility at or near the (π, π) momentum space position. By investigating Fe_1.02Te_0.60Se _0.40 in its superconducting state at 2 K, where a spin resonance at about 6 meV has been reported, we show that the susceptibility in the direction perpendicular to the c axis is by about 35% larger than for the direction along the c axis suggesting dominant singlet pairing of s ^± type. At higher temperatures the same type of anisotropy is present in the spin dynamics of the normal state. On the other hand, for the Fe_1.02Te_0.95Se_0.05 sample we find an almost isotropic response in the paramagnetic state. When the compound is magnetically ordered at (π, 0), the response near (π, π) persists, but with reduced intensity. The c-axis component is only slightly reduced whereas the in-plane component appears to be strongly reduced, leading to reverted anisotropy with respect to the superconducting compound.