  Ferroelectric domain walls (DWs) are nanoscale topological defects that can
be easily tailored to create nanoscale devices. Their excitations, recently
discovered to be responsible for DW GHz conductivity, hold promise for faster
signal transmission and processing speed compared to the existing technology.
Here we find that DW phonons disperse from GHz to THz frequencies, thus
explaining the origin of the surprisingly broad GHz signature in DW
conductivity. Puzzling activation of nominally silent DW sliding modes in
BiFeO3 is traced back to DW tilting and resulting asymmetry in wall-localized
phonons. The obtained phonon spectra and selection rules are used to simulate
scanning impedance microscopy, emerging as a powerful probe in nanophononics.
The results will guide experimental discovery of the predicted phonon branches
and design of DW-based nanodevices.
