No Evidence for Spin Density Waves in Pb using Phonon Imaging*

Phonon-imaging in superconducting Pb has proven to be a sensitive probe of quasiparticle density due to the highly anisotropic absorption of ballistic phonons by quasiparticles. Slower than expected temperature dependences of quasiparticle density observed previously by Wolfe and Short (\textit{Physica B} \textbf{316,} 107 (2002)) are explained here by taking into account the effects of nonequilibrium quasiparticles. Minimizing the effects of nonequilibrium quasiparticles enables us to extract a value of the zero-temperature superconducting gap parameter. We measure $\Delta $=1.32 $\pm $0.07 meV consistent with tunneling measurements in Pb and the conventional BCS picture, and inconsistent with earlier specific heat data that motivated the proposal of a spin-density-wave ground state in Pb (Overhauser and Daemen, \textit{PRL} \textbf{61}, 1885 (1988)).