  We report an unexpected characteristic of dislocation cores in silicon. Using
first-principles calculations, we show that all the stable core configurations
for a non-dissociated 60$^\circ$ dislocation are sessile. The only glissile
configuration, previously obtained by nucleation from surfaces, surprinsingly
corresponds to an unstable core. As a result, the 60$^\circ$ dislocation motion
is solely driven by stress, with no thermal activation. We predict that this
original feature could be relevant in situations for which large stresses
occur, such as mechanical deformation at room temperature. Our work also
suggests that post-mortem observations of stable dislocations could be
misleading, and that mobile unstable dislocation cores should be taken into
account in theoretical investigations.
