Thermal Conductivity of Clean Niobium in Mixed State

Thermal conductivity measurements on single crystal niobium specimens of resistivity ratios 6300 and 2500 are reported as a function of magnetic fields. The measured thermal conductivity varies linearly with H c2 – H in fields very close to H c2 and approximately with ( H c2 - H ) 1/2 in slightly lower fields, as predicted by the Houghton-Maki (HM) theory. Experimental results on Nb6300 in longitudinal fields agree with the HM theory using the density of states N (0)=2.3×10 36 cm -3 erg -1 at lower temperatures (\(T{\lesssim}3\) K) whereas at higher temperatures (\(T{\gtrsim}4\) K) they do not agree with the theory. Experimental data on Nb6300 in transverse fields cannot be explained by the theory with the same N (0) as in the longitudinal case, even at low temperatures. The field coefficients of the thermal conductivity do not show the purity dependence in our specimens investigated.