Low‐temperature thermal conductivity of pure and impure niobium and tantalum single crystals

The thermal conductivities of niobium and tantalum single crystals of different impurity contents are measured between 0.3 and 20 K. Below about 1 K the thermal conductivities of all specimens vary with temperatures as T 3 , but the magnitude of the thermal conductivity decreases with increasing impurity content. A careful investigation of the defect structure of the specimens leads to the conclusion that interstitial and substitutional impurities give rise to a thermal resistance even at low temperatures.