  We propose that the driving force of an ultrafast crystalline-to-amorphous
transition in phase-change memory alloys are strained bonds existing in the
(metastable) crystalline phase. For the prototypical example of GST, we
demonstrate that upon breaking of long Ge-Te bond by photoexcitation Ge ion
shot from an octahedral crystalline to a tetrahedral amorphous position by the
uncompensated force of strained short bonds. Subsequent lattice relaxation
stabilizes the tetrahedral surroundings of the Ge atoms and ensures the
long-term stability of the optically induced phase.
