Spin order and fluctuations in FeSe*

FeSe is the simplest Fe-based system. Depending on the environment, the superconducting transition temperature ranges between 9 and approximately 100 K. As opposed to the Fe pnictides, long range magnetic order is not found in FeSe. Yet, the charge and spin dynamics of FeSe may hold key information on the physics of high temperature superconductors in general. We report results of light scattering experiments as a function of polarization and temperature and simulate the spectra using exact diagonalization. With the parameters of a frustrated spin 1 Heisenberg model the experiment can be reproduced for all Raman active symmetries. We find a low energy peak in B_1g symmetry and assign this feature to excitations of nearly localized spins. Comparable agreement between theory and experiment can be achieved for neutron scattering data. The study furnishes evidence for nearly frustrated spin order and critical fluctuations of spin stripes with an ordering vector close to or at (π,0).