Computational Design of Axion Insulators Based on 5d Spinel Compounds

Based on density functional calculation using the local density approximation + U method, we predict that osmium compounds such as CaOs2O4 and SrOs2O4 can be stabilized in the geometrically frustrated spinel crystal structure. They show ferromagnetic order in a reasonable range of the on-site Coulomb correlation U and exotic electronic properties, in particular, a large magnetoelectric coupling characteristic of axion electrodynamics. Depending on U, other electronic phases including a 3D Weyl semimetal and Mott insulator are also shown to occur.