  Based on {\em ab initio} calculation, we propose a new structure for the
fundamental excitation of the reconstructed 30$^\circ$ partial dislocation in
silicon. This soliton has a rare structure involving a five-fold coordinated
atom near the dislocation core. The unique electronic structure of this defect
is consistent with the electron spin resonance signature of the hitherto
enigmatic thermally stable R center of plastically deformed silicon. We present
the first {\em ab initio} determination of the free energy of the soliton,
which is also in agreement with the experimental observation. This
identification suggests the possibility of an experimental determination of the
density of solitons, a key defect in understanding the plastic flow of the
material.
