  Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial
applications such as nonvolatile memories, but reaction pathways for
crystalline-to-amorphous phase-change on picosecond timescales remain unknown.
Femtosecond laser excitation and an ultrashort x-ray probe is used to show the
temporal separation of electronic and thermal effects in a long-lived ($>$100
ps) transient metastable state of Ge$_{2}$Sb$_{2}$Te$_{5}$ with muted
interatomic interaction induced by a weakening of resonant bonding. Due to a
specific electronic state, the lattice undergoes a reversible nondestructive
modification over a nanoscale region, remaining cold for 4 ps. An independent
time-resolved x-ray absorption fine structure experiment confirms the existence
of an intermediate state with disordered bonds. This newly unveiled effect
allows the utilization of non-thermal ultra-fast pathways enabling artificial
manipulation of the switching process, ultimately leading to a redefined speed
limit, and improved energy efficiency and reliability of phase-change memory
technologies.
