Calculated interband optical transition spectra of GdN

The imaginary part of the dielectric function Im[epsilon(omega)] of rocksalt GdN is calculated using the linear muffin-tin orbital method in the atomic sphere approximation. The local spin-density approximation with added Hubbard-U terms (LSDA+U) is used to include orbital-dependent Coulomb interactions for the Gd 4f and 5d orbitals. The spin-orbit coupling is shown to affect the dominant spin character of the bands only negligibly so that we can analyze the optical interband transitions as being between bands of equal spin only. The separate band-to-band contributions to Im[epsilon(omega)] are analyzed as well as the origin of the main peaks in the Brillouin zone, allowing us to identify critical-point transitions. We find that the transitions from the upper valence band (doubly degenerate along some portions of the Brillouin zone) dominate the spectrum. The transitions to the minority-spin f bands occur as a double peak near 5.5-6 eV. The corresponding transitions of the filled f bands to the conduction band have negligible contribution. Many of the peaks correspond to complex avoided band crossings in the conduction band where local maxima and minima occur.