Coupling of spin and orbital excitations in the iron-based superconductor FeSe_0.5 Te_0.5

We present a combined analysis of neutron scattering and photoemission measurements on superconducting FeSe_0.5 Te_0.5. The low-energy magnetic excitations disperse only in the direction transverse to the characteristic wave vector (1 2, 0,0) whereas the electronic Fermi surface near (1 2, 0,0) appears to consist of four incommensurate pockets. While the spin resonance occurs at an incommensurate wave vector compatible with nesting, neither spin-wave nor Fermi-surface-nesting models can describe the magnetic dispersion. We propose that a coupling of spin and orbital correlations is key to explaining this behavior. If correct, it follows that these nematic fluctuations are involved in the resonance and could be relevant to the pairing mechanism.