Nature of the step-height transition on vicinal Si(001) surfaces

The Si(001)2×{}1 surface is expected to undergo a phase transition from single to double-atonic-height steps with increasing angle of miscut. Here we show that this transition is quite different than previously believed, involving something like a ``devil's staircase'' of transitions in a mixed phase consisting of a complex sequence of single and double steps. Even at low angles, where only single steps occur, the areas of 2×{}1 and 1×{}2 regions are unequal, in agreement with recent experimental results.