LDA + Negative U Solves a Puzzle of too Large Calculated Magnetic Moment in Iron-based Superconductor LaFeAsO_{1-x}F_x

TL;DR

This paper demonstrates that using a slightly negative U in LDA+U calculations resolves the discrepancy between theoretical and experimental magnetic moments in LaFeAsO_{1-x}F_x, also aligning with other measurements.

Contribution

Introducing a negative U parameter in LDA+U calculations provides a novel solution to the magnetic moment puzzle in iron-based superconductors.

Findings

Negative U drastically reduces the calculated magnetic moment.

Negative U explains experimental lattice constants and electronic DOS.

U dependence shows magnetic moment decrease with negative U.

Abstract

A puzzle in the iron-based superconductor LaFeAsO_{1-x}F_x is that the magnetic moment obtained by first-principle electronic structure calculations is unexpectedly much larger than the experimentally observed one. For example, the calculated value is ~ 2.0 \mu_B in the mother compound, while it is ~ 0.3 \mu_B in experiments. We find that the puzzle is solved within the framework LDA + U by expanding the U value into a slightly negative range. We show U dependence of the obtained magnetic moment in both the undoped x=0.0 and doped x = 0.125. These results reveal that the magnetic moment is drastically reduced when entering to the slightly negative range of U. Moreover, the negative U well explains other measurement data, e.g., lattice constants and electronic DOS at the Fermi level. We discuss possible origins of the negative U in these compounds.