Spin anisotropy in FeSe

Understanding the magnetism of a high-temperature superconductor is fundamental for elucidating its pairing mechanism. FeSe being structurally the simplest iron-based superconductor has attracted extensive research interest because of its unusual magnetism and intriguing superconducting properties. Here, we use polarized neutron scattering to study the low energy spin excitations in FeSe single crystals. In the superconducting state, two anisotropic resonance modes with different polarizations are observed. The spin anisotropy persists in the normal state, and becomes isotropic above the nematic ordering temperature. The observation of double anisotropic resonance modes in FeSe cannot be readily explained by a simple spin triplet exciton model. Various scenarios for the observed spin anisotropy and its implications for the pairing mechanism will be discussed.