  The rare-earth nitride ferromagnetic semiconductors owe their varying
magnetic properties to the progressive filling of 4f shell across the series.
Recent electrical transport measurements on samarium nitride, including the
observation of superconductivity, have been understood in terms of a
contribution from a 4f transport channel. Band structure calculations generally
locate an empty majority 4f-band within the conduction band although over a
wide range of possible energies. Here we report optical reflection and
transmission measurements on samarium nitride between 0.01 eV to 4 eV, that
demonstrate clearly that the 4f band forms the bottom of the conduction band.
Results at the lowest energies show no free carrier absorption, indicating a
semiconducting ground state, and supporting earlier conclusions based on
transport measurements.
