Optical spectroscopy of SmN: Locating the 4f conduction band

The rare-earth nitride ferromagnetic semiconductors owe their varying magnetic properties to the progressive filling of the 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-spin 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 from 0.01 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 support earlier conclusions based on transport measurements.