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 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 superconductivity.