Case for a Second Energy Gap in Superconducting Niobium

Low-temperature tunneling studies of high-purity single crystals of niobium indicate the existence of two energy gaps ${\Delta }_s$ and ${\Delta }_d$, one for each of the two bands lying at the Fermi level. The value of ${\Delta }_s$ is approximately $\frac{{\Delta }_d}{10}$. The crystallographic dependence of ${\Delta }_s$ indicates that ${\Delta }_s$ does not appear in directions where there is no $s-d$ overlap at $E_F$. The behavior of ${\Delta }_s$ with temperature and magnetic field indicates that interband coupling is significant in niobium. The results are related to current theories of multiband super-conductivity.