Chern-Simons orbital magnetoelectric coupling in generic insulators

We present a Wannier-based method to calculate the Chern-Simons orbital magnetoelectric coupling in the framework of first-principles density-functional theory. In view of recent developments in connection with strong Z2 topological insulators, we anticipate that the Chern-Simons contribution to the magnetoelectric coupling could, in special cases, be as large or larger than the total magnetoelectric coupling in known magnetoelectrics like Cr2O3. The results of our calculations for the ordinary magnetoelectrics Cr2O3, BiFeO3 and GdAlO3 confirm that the Chern-Simons contribution is quite small in these cases. On the other hand, we show that if the spatial inversion and time-reversal symmetries of the Z2 topological insulator Bi2Se3 are broken by hand, large induced changes appear in the Chern-Simons magnetoelectric coupling.