Biatomic Steps on (001) Silicon Surfaces

The surprising tendency of clean (001) Si surfaces to become primitive upon annealing is explained by a $\pi$-bonded step reconstruction that lowers the relative enthalpy of reconstructed biatomic ($\frac{a_0}{2}$) steps on one particular fcc sublattice by 0.04 eV per step atom, and by correlation, which freezes out the step configurational entropy thereby suppressing the formation of other types of steps.