Orbital selective magnetism, nematicity, and fluctuations in FeSe

Iron pnictides and chalcogenides have rich phase diagrams displaying superconductivity, nematic and spin density wave order, fluctuations and short-range magnetism. Yet, the magnetism observed in FeSe, for instance, is not necessarily of the same type as that in the pnictides since the typical nesting conditions of the Fermi surfaces are much less robust in the chalcogenides and the question as to strong versus weak coupling magnetism arises. Raman scattering experiments afford a window into the type of ordering and allow one to distinguish between itinerant and localized magnetism. We show how the response from a weakly coupled itinerant system can be distinguished from that of a Heisenberg-type localized magnet. We present results of light scattering experiments as a function of polarization and temperature. In the pnictides the Raman spectra display all features of a spin density wave while the spectra of FeSe the are similar to those of systems with localized spins such as the cuprates. Our numerical simulations using exact diagonalization of a 4x4 cluster reproduce the experiments semi-quantitatively in the limit of a nearly frustrated spin-1 Heisenberg model (localized spins), in particular the low energy peak in B_1g symmetry. The results indicate that the electrons in some of the orbitals are more localized in FeSe than in the pnictides and reopen the discussion on the type of nematic fluctuations observed recently.