Optical investigation of niobium properties: Electrical- and physical constants

In this paper, we report optical (reflectance) measurements and investigations of optical properties of electropolished (EP), buffered chemical polished (BCP), and as-received (AR) from vendor niobium (Nb) samples typically used for fabrication of superconducting radio frequency (SCRF) cavities. Optical conductivity (σ(0), approximated near zero frequency) of EP (σ(0) ∼ 9 × 103 Ω−1 cm−1) sample is one order of magnitude higher than that of BCP (σ(0) ∼ 7 × 102 Ω−1 cm−1) and AR (σ(0) ∼ 3 × 102 Ω−1 cm−1) niobium samples. Furthermore, physical constants of electropolished Nb-SCRF materials such as concentration of conduction electrons ( ∼ 1.8 × 1022 electrons/cm3), average velocity ( ∼ 5.9 × 107 cm/s) of the electrons on the Fermi surface, and mean free path ( ∼ 0.53 nm) were also found to be considerably higher than that of the BCP and the AR samples. The depth of electric field penetration (in low frequency region) in the electropolished Nb sample ( ∼ 80 nm) is appreciably lesser than the BCP ( ∼ 450 nm) and the AR ( ∼ 400 nm) samples.