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 Δ_s and ${\Delta }_d$, one for each of the two bands lying at the Fermi level. The value of Δ_s is approximately $\frac{{\Delta }_d}{10}$. The crystallographic dependence of Δ_s indicates that Δ_s does not appear in directions where there is no s-d overlap at $E_F$. The behavior of Δ_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.