  Charge density wave (CDW) materials can undergo an ultrafast phase transition
after an ultrashort laser pulse excitation, and the suggested underlying
mechanisms have always been associated with two main features: excitonic
interaction-induced CDW charge order and electron-phonon coupling-induced
periodic lattice distortion (PLD). Here, beyond these two mechanisms, we reveal
that photoexcitation induced CDW phase transition in the prototypic CDW example
1T-TiSe2 is similar to a puppet on a string: six Ti-Se bonds connected to each
distorted Ti atom acting as six strings controlling the PLD and in turn the CDW
orders. The photoexcitation induced modulation on charge population of the
Ti-Se bonds generates a laser-fluence-dependent interatomic-repulsive force
along each Ti-Se bond. The nonequal length of these six Ti-Se bonds gives rise
to a net force exerted on the central distorted Ti atom to push it toward the
suppressing of the PLD and thus the CDW orders. We further illustrate that the
dynamics of each distorted Ti atom behaves as though it attached to a spring in
a simple harmonic motion with a fluence-dependent oscillation frequency,
uniting two previously reported scaling laws for phase transition time. These
findings significantly advance the understanding of CDW instability and provide
new insights into how photoexcitation induced modulation on charge population
may lead to phase transitions by directly connecting interatomic forces with
reaction pathways.
