Investigation of thermal conductivity of Nb within the framework of strong coupling theory

Thermal conductivitites of Nb with resistivity ratios 6300, 2500, 1090 and 195 are investigated in the light of the strong coupling theory. The zero temperature energy gap function Δ (ω,0) and renormalization functions Z sub(s)(ω,0) and Z sub(n)(ω,0) for Nb are determined by Eliashberg equations. The impurity and phonon scattering functions, f=K sub(es)sup(i) / K sub(en) sup(i) and g=K sub(es) sup(p) / K sub(en) sup(p), are derived experimentally. The behaviors of f are explained well by the Bardeen-Rickayzen-Tewordt theory. The behaviors of g are described well by the Ambegaokar-Tewordt (AT) theory with the strong coupling effect rather than by the Kadanoff-Martin (KM) theory without it. The limiting values of the slope of g at t=T/T sub(c) = 1 are found to be 1.4, 3.0 and 3.8 for the KM, experimental and AT results, respectively. (auth.)