Magneto thermal conductivity of superconducting Nb with intermediate level of impurity

Abstract

Niobium materials with intermediate purity level are used for fabrication of superconducting radio frequency cavities (SCRF), and thermal conductivity is an important parameter influencing the performance of such SCRF cavities. We report here the temperature and magnetic field dependence of thermal conductivity κ for superconducting niobium (Nb) samples, for which the electron mean free path le, the phonon mean free path lg, and the vortex core diameter 2rC are of the same order of magnitude. The measured thermal conductivity is analyzed using the effective gap model (developed for le ≫ 2rC (Dubeck et al 1963 Phys. Rev. Lett. 10 98)) and the normal core model (developed for le ≪ 2rC (Ward and Dew-Hughes 1970 J. Phys. C: Solid St. Phys. 3 2245)). However, it is found that the effective gap model is not suitable for low temperatures when le ∼ 2rC. The normal core model, on the other hand, is able to describe κ(T,H) over the entire temperature range except in the field regime between HC1 and HC2 i.e. in the mixed state. It is shown that to understand the complete behavior of κ in the mixed state, the scattering of quasi-particles from the vortex cores and the intervortex quasi-particle tunneling are to be invoked. The quasi-particle scattering from vortices for the present system is understood in terms of the framework of Sergeenkov and Ausloos (1995 Phys. Rev. B 52 3614) extending their approach to the case of Nb. The intervortex tunneling is understood within the framework of Schmidbauer et al (1970 Z. Phys. 240 30). Analysis of the field dependence of thermal conductivity shows that while the quasi-particle scattering from vortices dominates in the low fields, the intervortex quasi-particle tunneling dominates in high fields. Analysis of the temperature dependence of thermal conductivity shows that while the quasi-particle scattering is dominant at low temperatures, the intervortex quasi-particle tunneling is dominant at high temperatures.