Magnetic Fluctuations in Single-Layer FeSe*

The high-T_C superconductivity in monolayer FeSe films is revisited from the viewpoint of magnetic fluctuations. Using density-functional calculations, we derive the spin-spiral energy dispersion E(q) – including the energies of the checkerboard (CB), collinear stripe (CL), and staggered-dimer antiferromagnetic (AFM) configurations – and map it onto the S=1 square-lattice spin model. We find that (i) there is a plateau in E(q) extending around the CB state, (ii) the q states in the plateau share very similar electronic band structure, (iii) Heisenberg exchange interactions are in the frustrated regime J₂/J₁~0.5, and (iv) both biquadratic (K) and four-spin (F) exchange interactions are important: K~F~0.2 J₁. These results reveal that both quantum and entropic (thermal) effects favor the CB-AFM type fluctuations and naturally account for the CB-like features observed in the normal-state electronic structure[1] and the fully-gapped yet anisotropic superconducting order parameter[2].
[1] T. Shishidou et al., “Magnetic fluctuations in single-layer FeSe”, arXiv:1708.07868v2 (2017).
[2] D.F. Agterberg et al., “Resilient nodeless d-wave superconductivity in monolayer FeSe”, arXiv:1706.01978v2 (2017).