Magnetic Fluctuations in Single-Layer FeSe

The electronic structure of single-layer FeSe films on SrTiO 3 presents a quandary: experimentally there is no long-range magnetic order, but the observed bands are reasonably well described by density functional calculations assuming the checkerboard antiferromagnetic (CB-AFM) ordering despite this configuration not being the calculated ground state. Here we investigate the paramagnetic nature of this system via first-principles spin-spiral calculations. Fits of the spin-spiral dispersion to spin models place this S = 1 spin system in a region of parameter space where CB-AFM quantum fluctuations lead to a magnetically disordered paramagnetic state. Modeling the paramagnetic state as an incoherent superposition of spin-spiral states arising from thermal and/or quantum fluctuations, the resulting electronic bands around the Fermi level are found to closely resemble those of the ordered CB-AFM configuration, thus providing a consistent explanation of the angle-resolved photoemission observations. These results suggest that CB-AFM fluctuations play a more important role than previously thought.