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 (sigma(0), approximated near zero frequency) of EP (sigma(0) similar to 9 x 10(3) Omega(-1) cm(-1)) sample is one order of magnitude higher than that of BCP (sigma(0) similar to 7 x 10(2) Omega(-1) cm(-1)) and AR (sigma(0) similar to 3 x 10(2) Omega(-1) cm(-1)) niobium samples. Furthermore, physical constants of electropolished Nb-SCRF materials such as concentration of conduction electrons (similar to 1.8 x 10(22) electrons/cm(3)), average velocity (similar to 5.9 x 10(7) cm/s) of the electrons on the Fermi surface, and mean free path (similar to 0.53nm) 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 (80nm) is appreciably lesser than the BCP (similar to 450nm) and the AR (similar to 400 nm) samples. (C) 2017 Elsevier B.V. All rights reserved.