We have observed anomalous ultrasonic attenuation in pure superconducting Nb[100] for $q{l}_e\approx 1$. The anomalous behavior is believed to be associated with the high purity of the sample, which has a measured resistivity ratio of 6500. The attenuation data have been analyzed by assuming that the material has two energy gaps, one with a standard BCS value of $2\frac{{\Delta }_1\mathrm{}\left(0\right)\mathrm{}}{k{T}_c}=3.4\mathrm{}$ and the other with $2\frac{{\Delta }_2\mathrm{}\left(0\right)\mathrm{}}{k{T}_c}=10\mathrm{}$.

• Received 23 June 1971

DOI:https://doi.org/10.1103/PhysRevLett.27.1128