We report an unexpected characteristic of dislocation cores in silicon. Usingfirst-principles calculations, we show that all the stable core configurationsfor a non-dissociated 60$^\circ$ dislocation are sessile. The only glissileconfiguration, previously obtained by nucleation from surfaces, surprinsinglycorresponds to an unstable core. As a result, the 60$^\circ$ dislocation motionis solely driven by stress, with no thermal activation. We predict that thisoriginal feature could be relevant in situations for which large stressesoccur, such as mechanical deformation at room temperature. Our work alsosuggests that post-mortem observations of stable dislocations could bemisleading, and that mobile unstable dislocation cores should be taken intoaccount in theoretical investigations.
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