Thermoelastic Effect in Niobium at the Superconducting Transition

An abrupt decrease in the acoustic loss $Q^{-1\mathrm{}}$ has been observed in flexural modes of polycrystalline Nb disk samples on cooling through the superconducting transition temperature $T_c$. High-resolution measurements on a 1.6-kHz mode of one sample showed a 30% decrease in $Q^{-1\mathrm{}}$ in a 20-mK temperature interval. It is shown that these results are clearly identifiable with the reduction of the thermoelastic contribution to the acoustic loss in the superconducting state.