  We study phonons generated by current in a Pt nanowire, by measuring
resistance of another nanowire separated from the first one by an insulating
spacer. For thin spacers, the resistance varies almost linearly with current at
cryogenic temperatures, while an additional quadratic contribution emerges for
thicker spacers. These observations suggest a non-thermal distribution of
current-generated phonons that relax via strongly nonlinear dynamical processes
rather than few-phonon scattering. Our results provide insight into the
nonequilibrium phonon dynamics at nanoscale, which may facilitate efficient
heat management in electronic nanodevices.
