  The motion of line defects (dislocations) has been studied for over 60 years
but the maximum speed at which they can move is unresolved. Recent models and
atomistic simulations predict the existence of a limiting velocity of
dislocation motions between the transonic and subsonic ranges at which the
self-energy of dislocation diverges, though they do not deny the possibility of
the transonic dislocations. We use femtosecond x-ray radiography to track
ultrafast dislocation motion in shock-compressed single-crystal diamond. By
visualizing stacking faults extending faster than the slowest sound wave speed
of diamond, we show the evidence of partial dislocations at their leading edge
moving transonically. Understanding the upper limit of dislocation mobility in
crystals is essential to accurately model, predict, and control the mechanical
properties of materials under extreme conditions.
