Spin and orbital degrees of freedom in FeSe (Part-II): field-theory perspective

Motivated by our recent ab-initio results we propose an effective field-theory model for coupled spin and orbital degrees of freedom in FeSe. Both degrees of freedom are considered to be classical 2D quantities, thus the Kosterlitz-Thouless theory of collective modes applies. We first study how small momenta fluctuations renormalize DFT bands and then we move on to topological large momentum excitations, vortexes. The DFT study revealed that we need to develop a Renormalization Group (RG) method which, simultaneously with a relevance of usual instabilities, can capture formation of vortex crystal (VC) phase. We achieve it by incorporating vortex-vortex interactions. This allows us to derive a phase diagram including VC together with effects of the standard instabilities. The theoretical method that we developed will be useful not only for the specific case of FeSe but also for other systems where large vortex fugacity regime is reachable. However advantage of FeSe is that here the RG results can be experimentally tested: they enabled us to build a bridge between our former DFT results (that works best assuming a presence of magnetic phase) and experimental measurements (which had revealed the presence of magnetism only for a finite pressure).