  Symmetry breaking and the emergence of order is one of the most fascinating
phenomena in condensed matter physics. It leads to a plethora of intriguing
ground states found in antiferromagnets, Mott insulators, superconductors, and
density-wave systems. Exploiting states of matter far from equilibrium can
provide even more striking routes to symmetry-lowered, ordered states. Here, we
demonstrate for the case of elemental chromium that moderate ultrafast
photo-excitation can transiently enhance the charge-density-wave (CDW)
amplitude by up to 30% above its equilibrium value, while strong excitations
lead to an oscillating, large-amplitude CDW state that persists above the
equilibrium transition temperature. Both effects result from dynamic
electron-phonon interactions, providing an efficient mechanism to selectively
transform a broad excitation of the electronic order into a well defined,
long-lived coherent lattice vibration. This mechanism may be exploited to
transiently enhance order parameters in other systems with coupled degrees of
freedom.
