Ultrasonic Evidence against Multiple Energy Gaps in Superconducting Niobium

Abstract

Suhl et al.¹ extended the BCS theory of superconductivity to the case of two overlapping electron bands and suggested that this might apply to pure transition metals such as Nb. The principal feature of their results was that each band could have distinct superconducting energy gaps and possibly different transition temperatures. Heat capacity measurements on Nb by Shen et al.² were interpreted³ in terms of a d-band gap 2∆_d(0) = 3.5kT _c and an s-band gap 2∆_s(0) = 0.32kT _c. The ratio of the density of states in these bands N _s /N _d was impurity dependent and was 0.015 for a sample with a residual resistivity ratio (RRR) of 110. Hafstrom and MacVicar⁴ also found an energy gap 2∆_s(0) = 0.37kT _c by superconducting tunneling along a number of crystallographic directions in Nb single crystals. However, recent thermal conductivity measurements⁵ have provided no evidence for this small energy gap.

Work supported by the Science Research Council, London, England.