  Long metallic nanowires combine crucial factors for non-conservative
current-driven atomic mo- tion. These systems have degenerate vibrational
frequencies, clustered about a Kohn anomaly in the dispersion relation, that
can couple under current to form non-equilibrium modes of motion growing
exponentially in time. Such motion is made possible by non-conservative
current-induced forces on atoms, and we refer to it generically as the
waterwheel effect. Here the connection be- tween the waterwheel effect and the
stimulated directional emission of phonons propagating along the electron flow
is discussed in an intuitive manner. Non-adiabatic molecular dynamics show that
waterwheel modes self-regulate by reducing the current and by populating modes
nearby in fre- quency, leading to a dynamical steady state in which
non-conservative forces are counter-balanced by the electronic friction. The
waterwheel effect can be described by an appropriate effective non- equilibrium
dynamical response matrix. We show that the current-induced parts of this
matrix in metallic systems are long-ranged, especially at low bias. This
non-locality is essential for the characterisation of non-conservative atomic
dynamics under current beyond the nanoscale.
