  The simultaneous condensation of electronic and structural degrees of freedom
gives rise to new states of matter, including superconductivity and
charge-density-wave formation. When exciting such a condensed system, it is
commonly assumed that the ultrafast laser pulse disturbs primarily the
electronic order, which in turn destabilizes the atomic structure. Contrary to
this conception, we show here that structural destabilization of few atoms
causes melting of the macroscopic ordered charge-density wave in 1T-TiSe2.
Using ultrafast pump-probe non-resonant and resonant X-ray diffraction, we
observe full suppression of the Se 4p orbital order and the atomic structure at
excitation energies more than one order of magnitude below the suggested
excitonic binding energy. Complete melting of the charge-density wave occurs
4-5 times faster than expected from a purely electronic charge-screening
process, strongly suggesting a structurally assisted breakup of excitonic
correlations. Our experimental data clarifies several questions on the
intricate coupling between structural and electronic order in stabilizing the
charge-density-wave in 1T-TiSe2. The results further show that
electron-phonon-coupling can lead to different, energy dependent
phase-transition pathways in condensed matter systems, opening new
possibilities in the conception of non-equilibrium phenomena at the ultrafast
scale.
