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

Abstract

Scanning tunneling microscopy (STM) of the Si(111)‐7 × 7 reconstructed surface at 5 K reveals that the highly symmetric STM image found at room temperature does not reflect the lowest energy, ground state of this system. Instead, the ground state has certain adatom charge densities distorted by ≈0.2–0.4 Å from the symmetric STM charge density positions observed at 300 K. This agrees with adatom ion core displacements in both a Patterson map and two additional phase‐reconstructed atom maps. Such ion core distortions are consistent with the Jahn–Teller effect associated with the threefold mirror symmetry of the Si(111)‐7 × 7 and its antiphase, faulted structure. The nodal properties of the electronic states of an underlying 7 × 7 honeycomb that arise from threefold symmetry are determined from solutions of the 2D inhomogeneous Helmholtz equation, and provide insight to the observed temperature‐dependent charge density changes. It is shown that an excited state of the honeycomb cells of a Si(111)‐7 × 7 support the higher symmetry charge densities observed at room temperature and the presence of strong electron–phonon coupling. The role of symmetry and anharmonicity in defining the structure and electronic interactions in the Si(111)‐7 × 7 are discussed.

A re-evaluation of diffraction from Si(111) 7 × 7: decoding the encoded phase information in the 7 × 7 diffraction pattern

“…As reported elsewhere 12 evidence has been presented that the side adatoms of the 7 Â 7 are slightly distorted. Interestingly, the type of distortion shown in Fig.…”

“…11(d) has a component that pulls these corner hole adatoms toward the central unit cell boundary, making them different than the side adatoms. This is opposite to the side atom distortions proposed from a Jahn-Teller effect in other work, 12 but has not been explored further due to additional interference effects expected in the complex 7 Â 7 adlayer as discussed in Section 6. An interesting aspect of the distortion evaluation performed here is that many arbitrary distortions can create 1/14 order beams or other coincident peaks which are not observed in any diffraction study of the clean 7 Â 7.…”

“…10,11,15 Further evaluation of several earlier measurements of the 7 Â 7 has also recently led to the discovery of a magnetic surface state 11 as well as an instability in 7 Â 7 that leads to a Jahn-Teller distortion of its atoms and an insulating ground state. 12 Additional charge density distortions are also found experimentally that occur with increasing temperature and appear related to the 7 Â 7's observed insulator to metal transition.…”

“…47 In addition, as discussed elsewhere, evidence suggests that electron phonon coupling in the 7 Â 7 system arises from an electron phonon instability that creates static lattice distortions in the 7 Â 7 itself. 12 Considering the averaged beams in Fig. 7(b-d), these show slightly different intensity patterns with significant differences in many of the weaker fractional order beams.…”

“…The mirror symmetry of the two sides of the 7 Â 7 unit cell was proposed to also be essential for the creation of its insulating ground state. 12 Here the extra degree of freedom allowed by electron spin provides a way to remove the degeneracy of the two symmetric sides of the unit cell and lower the system energy. Presumably the various atomic distortions facilitate this magnetic ground state.…”