Understanding the importance of local magnetic moment in monolayer FeSe

Abstract

Local magnetic moment (LMM) and antiferromagnetic (AFM) fluctuation play a critical role in affecting the properties of FeSe superconductor. By constraining the local magnetic moment on Fe atoms using density functional theory, we investigate how LMM in FeSe monolayer alters the total energy, heights of Se atoms, band structure, and the electronic properties, for three different AFM spin arrangements which consist of the checkerboard (CB), collinear (CL), and pair-checkerboard (PC) spin phases. We find that (i) the total energy decreases drastically in all three spin structures when LMM develops, showing that the existence of LMM significantly stabilizes the system. The optimal LMM is found to be 2.23, 2.54, and $2.47\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B}$, respectively, in the CB, CL, and PC spin phases. (ii) The heights of Se atoms increase markedly (and in a quadratic manner) with LMM, demonstrating a strong magnetostriction effect. Also intriguingly, we find that the Se heights are insensitive to spin ordering, displaying a rather universal dependence on LMM in three different AFM spin phases. (iii) LMM is shown to alter substantially the electron band structures and Fermi surfaces. Near their optimal LMM, while both CB and PC phases possess electron pockets and no hole pockets, the CL phase exhibits neither electron pockets nor hole pockets, and interestingly, it becomes a semiconductor of a small gap of 60 meV. These results reveal that there is a rich and interesting physics to be tuned by LMM in FeSe superconductor.