We compare methods for estimating the residual resistivity ratio (RRR) of high-purity niobium samples and investigate the effects of using different functional models on the final value. RRR is typically defined as the ratio of the electrical resistances measured at 273 K (the ice point) and 4.2 K (the boiling point of helium at standard atmospheric pressure). However, pure niobium is superconducting below about 9.3 K, so the low-temperature resistance is defined as the normal-state (i.e., non-superconducting state) resistance extrapolated to 4.2 K and zero magnetic field. Thus, the estimated value of RRR depends significantly on the model used for extrapolation. We examine three models for extrapolation based on temperature versus resistance, two models for extrapolation based on magnetic field versus resistance, and a new model based on the Kohler relationship that can be applied to combined temperature and field data. We also investigate the possibility of re-defining RRR so that the quantity is not dependent on extrapolating an arbitrary model.
Citation: Journal of Research of the National Institute of Standards and Technology
Pub Type: Journals
cryogenic, electrical resistivity, Kohler s rule, magnetoresistance, residual resistivity ratio, superconductor