Probing the anisotropic phonon attenuation in superconducting lead.*

Phonon imaging of single crystals of superconducting Pb have uncovered a strong attenuation of phonons in (111) planes. Early data by Wolfe and Short$^{2}$ raised the possibility of a spin-density-wave ground state as proposed by Overhauser and Daemen.$^{3}$ This interpretation depends on the frequencies of the ballistic phonons, which previously were determined by a Planckian heater source and isotope scattering. We have designed and fabricated a low-pass filter in an attempt to select phonon energies below a characteristic activation energy, in order to clarify the source of the attenuation lines. We present new phonon-imaging data under various conditions and discuss the possible interpretations. $^{2}$ Physica B 316-317 (2002) 107-109 $^{3 }$ Phys. Rev. B 39, 6431 (1989)