Magnetic properties of bulk and monolayer FeSe : A DFT+DMFT study

FeSe is unique among other iron-based superconductors, which shows no magnetic ordered state and becomes superconducting below 8 K in the undoped bulk system while the superconducting transition temperature soars by an order of magnitude for a monolayer FeSe on SrTiO₃ substrate. Using a DFT+DMFT method, we perform a comparative study on the magnetic properties of FeSe systems and LaFeAsO, another representative iron-based superconducting material. Calculated magnetic moment in the typical stripe-type magnetic ordering pattern is finite for LaFeAsO while negligible for bulk FeSe, consistently with experiments, and is also finite for monolayer FeSe suggesting the magnetic order is restored in monolayer FeSe. We suggest a mechanism explaining why the systems with the similar magnitudes of fluctuating spin (<S²>) have very different magnitude of ordered moment (<S_z>) focusing on the different aspects of local charge fluctuation. Our work provides a comprehensive understanding of magnetism in iron-based superconducting materials, and also emphasizes on the potential importance of magnetism in the high superconducting transition temperature of monolayer FeSe on SrTiO₃ substrate.