  Exploiting vibrational excitation for the dynamic control of material
properties is an attractive goal with wide-ranging technological potential.
Most metal-to-insulator transitions are mediated by few structural modes and
are thus ideal candidates for the selective driving towards a desired
electronic phase. Such targeted navigation within a generally multi-dimensional
potential energy landscape requires microscopic insight into the non
equilibrium pathway. However, the exact role of coherent inertial motion across
the transition state has remained elusive. Here, we demonstrate mode-selective
control over the metal-to-insulator phase transition of atomic indium wires on
the Si(111) surface, monitored by ultrafast low-energy electron diffraction. We
use tailored pulse sequences to individually enhance or suppress key phonon
modes and thereby steer the collective atomic motion within the potential
energy surface underlying the structural transformation. Ab initio molecular
dynamics simulations demonstrate the ballistic character of the structural
transition along the deformation vectors of the Peierls amplitude modes. Our
work illustrates that coherent excitation of collective modes via
exciton-phonon interactions evades entropic barriers and enables the dynamic
control of materials functionality.
