Thermal Conductivity of Superconducting Niobium

Thermal-conductivity measurements on niobium of intermediate purity (lξ₀) were performed in the normal, superconducting, and mixed states. The lattice and electronic components of the conductivity in the superconducting state are separated. The critical temperature and the energy gap are found to be Tc=(9±0.1^∘K and 2ε(0)=3.96kTc. The temperature dependence of the minimum in the thermal conductivity in the mixed state suggests a decrease in both the phonon and the electron conductivities just above Hc1. The comparative importance of each depends on a combination of temperature range and purity of the sample. The slope ∂Kₘ∂H and upper-critical-field parameter κ₁(t) are found to change with temperature much faster than predicted by relevant theories.