Tatsuya Shishidou
University of Wisconsin-Milwaukee, USA
Title: Magnetic fluctuations in single layer FeSe
Biography
Biography: Tatsuya Shishidou
Abstract
Single layer FeSe films grown on SrTiO3(001) (STO) have been an extensive focus because of their reported high superconducting critical temperature of 40-100K. Despite a lot of work done already, the nature of the paramagnetic state and the origin of superconductivity remains puzzling. Here we use density functional theory (DFT) spin-spiral calculations to address the paramagnetic and superconducting nature of the monolayer FeSe/STO. The spin-spiral energy dispersion E(q) is found to be extremely flat around the q=0 checkerboard (CB)-antiferromagnetic (AFM) configuration. Those q states in the plateau share similar electronic band structure. Mapping E(q) onto (extended) Heisenberg models places 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/quantum fluctuations, the electronic spectrum around the Fermi level closely resembles that observed by angle-resolved photoemission spectroscopy. A superconducting theory is developed within a symmetry-based k·p like method with the electrons coupled to CB-AFM type spin fluctuations; this model provides a robust prediction of nodeless d-wave superconductivity, and naturally explains the experimental finding of fully-gapped yet anisotropic order parameter