Ultrasonic Evidence Against Multiple Energy Gaps in Superconducting Niobium

Abstract

The attenuation of longitudinal ultrasound along the [100] direction in Nb, resistivity ratio $\frac{R_{300}}{R_0}=5200\mathrm{}$, has been measured with precision at low temperatures. The total attenuation in the normal state can be accurately deduced by a BCS calculation, with a single gap $2\Delta \mathrm{}\left(0\right)=(3.52\mathrm{}\pm 0.02)k{T}_c$, from the measured superconducting attenuation. No experimental evidence has been found for the existence of either a smaller or a larger gap.