Normal-State Hourglass Dispersion of the Spin Excitations in FeSexTe1-x

We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe0.4Te0.6 and essentially nonsuperconducting (NSC) FeSe0.45Te0.55. In contrast with BaFe2-x(Co, Ni)(x)As-2, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe0.4Te0.6 are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe0.45Te0.55, we argue that the observed incommensurate spin excitations in FeSe1-xTex are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling.