Symmetry breaking and Si-based molecular flip-flop motion

We report on molecular binary switching by oxidation products on Si(111)-(7x7) surfaces. By using scanning tunneling microscopy, we show that the oscillator consists of a triad of center adatoms in the half-unit cell of a Si(111)-(7x7) surface. With two adatoms oxidated, each adatom of the triad plays a unique role. Two adatoms construct two mutually exclusive, flip-flopping bistable structures and the other a stationery structure that stabilizes the triad. Time series data of the flip-flop motion exhibits the Arrhenius behavior with the barrier height of about 0.7 eV. Decomposed triads show that a thermally activated mobile species, attributed to a molecular precursor, drives the flip-flip motion. The triad provides a novel example in which a symmetry breaking charge transfer provides the collective stability to the structure, and at the same time sheds new light on oxidation processes on silicon surfaces.