Distorted magnetic orders and electronic structures of tetragonal FeSe from first principles.

We use the state-of-the-art density-functional-theory method to study various magnetic orders and their effects on the electronic structures of FeSe. Our calculated results show that, for the spins of the single Fe layer, the striped antiferromagnetic orders with distortion are more favorable in total energy than the checkerboard antiferromagnetic orders with tetragonal symmetry, which is consistent with known experimental data, and the interlayer magnetic interaction is very weak. We investigate the electronic structures and magnetic property of the distorted phases. We also present our calculated spin coupling constants and discuss the reduction of the Fe magnetic moment by quantum many-body effects. These results are useful in understanding the structural, magnetic and electronic properties of FeSe, and may have some helpful implications for other FeAs-based materials.