LDA + Negative U Solves a Puzzle of too Large Calculated Magnetic Moment in Iron-based Superconductor LaFeAsO_1-xF_x

2008/06/30 by Nakamura, H., Hayashi, N., Nakai, N. +1
#FOS: Physical sciences #Superconductivity (cond-mat.supr-con)

paper · doi:10.48550/arxiv.0806.4804

Abstract

A puzzle in the iron-based superconductor LaFeAsO_1-xF_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 μ_B in the mother compound, while it is ~ 0.3 μ_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.

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