Optical conductivity of bcc transition metals: V, Nb, Ta, Cr, Mo, W

The optical constants $n$ and $k$ were determined for the bcc transition metals of the V and Cr groups from reflection and transmission measurements in the spectral range 0.5-6.5 eV. The optical measurements were made at room temperature on polycrystalline thin films, vacuum evaporated rapidly onto high-temperature (700-1000°C) fused-quartz substrates. The films were found to be well crystallized, and the reflectance of opaque films agreed with previous bulk values. For the thin-film results to be representative of bulk crystals, high substrate temperature was essential for the Cr group, and rapid evaporation rate for the V group. The values for Cr are a significant improvement over earlier results of Johnson and Christy, but those for V are only a slight improvement. Some recent calculations of the interband optical-conductivity agree reasonably well with our results for V, Cr, Nb, and Mo. Direct transitions between $d$ bands seem to account for the optical structure.