Ultrasonic Evidence against Multiple Energy Gaps in Superconducting Niobium
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.
KeywordsSuperconducting State Heat Capacity Measurement Residual Resistivity Ratio Pure Transition Metal Ultrasonic Attenuation Measurements6
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