Electron-phonon coupling plays a crucial role in conventional superconductors. Understanding the behavior of phonons can therefore advance the development and applications of these materials. Here, we utilize inelastic scanning tunneling spectroscopy (ISTS) at 45 mK to investigate phonons within crystallographic defects, i.e. stacking fault tetrahedra (SFT), in the strong-coupling superconductor Pb(111) [1]. We find that the local Eliashberg function [2] is significantly enhanced at the SFT, similar to Pb thin films [3]. Importantly, inelastic quasiparticle interference (QPI) by ISTS is observed on the SFT. By disentangling the electronic QPI from inelastic QPI, we confirm for the first time the presence of lateral phonon standing waves on an SFT. Moreover, we find that the SFT strongly confines transverse acoustic phonons, whereas longitudinal acoustic phonons are much less confined. Our findings pave the way for phonon engineering in strong-coupling superconductors.

[1] W. L. McMillan and J. M. Rowell, Phys. Rev. Lett. 14, 108 (1965).

[2] G. M. Eliashberg, Sov. Phys. JETP 11, 696 (1960).

[3] M. Schackert et al., Phys. Rev. Lett. 114, 047002 (2015).