We report the valence-to-core resonant inelastic x-ray scattering (RIXS) of
EuS measured at the L3 edge of Eu. The obtained data reveal two sets of
excitations: one set is composed of a hole in the S 3p bands and an electron
excited to extended Eu 5d band states, the other is made up from a hole in the
Eu 4f states and an electron in localized Eu 5d states bound to the 4f hole by
its Coulomb potential. The delocalized excitations arise from the
dipole-allowed 5d to 2p emissions, whereas the localized excitations result
from the dipole-forbidden (quadrupole-allowed) 4f to 2p emissions. Both these
emission channels have a comparable intensity thanks to a small number of
occupied 5d states (approximately 0.6) combined with a large number of occupied
4f states (seven). We identify the localized electron-hole pairs with the
"magnetic excitons" suggested in the past as an interpretation of the sharp
features seen in the optical absorption spectra. Our observations provide a
direct experimental evidence of these excitons which has been missing up to
now.Comment: 7 pages, 4 figures, supplemental material (PDF, 8 pages) added as an
ancillary fil