  We suggest that the commonly-accepted core structure of the 90-degree partial
dislocation in Si may not be correct, and propose instead a period-doubled
structure. We present LDA, tight-binding, and classical Keating-model
calculations, all of which indicate that the period-doubled structure is lower
in energy. The new structure displays a broken mirror symmetry in addition to
the period doubling, leading to a wide variety of possible soliton-like defects
and kinks.
