Evidence for a spin singlet state in the intermetallic semiconductorFeGa3

Abstract

${\text{FeGa}}_{3}$ is unusual for an intermetallic compound in displaying a semiconducting gap, on the order of 0.5 eV. Conventional density-functional based electronic-structure calculations in the local-density approximation (LDA) give a similar gap but it is expected that Fe, a $3d$ transition metal, is likely to display an on-site Coulomb repulsion $U$ that should be taken into account, particularly in an insulating compound with some narrow bands. We analyze first-principles LDA calculations for ${\text{FeGa}}_{3}$, and then include on-site Coulomb repulsion in a mean-field way ($\text{LDA}+U$ method) to show that, with the moderate value $U$ on the order of 2 eV, one obtains Fe moments in antialigned ${\text{Fe}}_{2}$ dimers (band theory ``singlets'') with a band gap that still coincides with the observed gap. Counterintuitively, increasing the value of $U$ gradually reduces the gap and finally produces an incorrect metallic state. We suggest that more experimental study should be done to distinguish between the ``${\text{Fe}}_{2}$ singlet'' and nonmagnetic descriptions and provide calculations of the optical properties for comparison with data.