Evidence for Symmetry Breaking in the Ground State of the Si(111)‐7 × 7: The Origin of Its Metal–Insulator Transition

The perspective and top view of an ideal room-temperature unit cell of a new 5×5 honeycomb structure on Si(111), called the InterDigitated-Faulted-Adatom model (or DFA), are displayed on the frontispiece page. This 5×5 transforms at higher temperatures into a similar 7×7 DFA that is discussed by J. E. Demuth in article number 2000229. The 7×7 exhibits many paradoxical properties unaccounted for within current effective one-electron calculations. Evidence is presented for a Jahn–Teller distortion in its ground state and phonon-assisted adatom charge density distortions with increasing temperature. The symmetry properties of this new class of structures and the formation of resonance surface states are shown to account for this unusual behavior.