Anomalous Thermal Conductivity in Superconducting Niobium

Anomalously high thermal conductivity of superconducting Nb between 0.3 and 0.6°K is interpreted as evidence for a small second energy gap. The temperature dependence of the excess thermal conductivity is consistent with a BCS-like gap about $\frac{1}{25}$ of the dominant gap in both magnitude and transition temperature. The magnitude of the excess conductivity appears to justify identifying the small gap with the hole sheet of the Fermi surface centered about symmetry point $\Gamma$. Less readily explained on the basis of a two-gap model are the high phonon conductivity observed in this and other experiments and the very high $Q_S$ observed for Nb microwave cavities at low temperatures.