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Register a new userTwo-speed phase dynamics in Si(111) (7x7)-(1x1) phase transition
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We propose a natural two-speed model for the phase dynamics of Si(111) 7$times$7 phase transition to high temperature unreconstructed phase. We formulate the phase dynamics by using phase-field method and adaptive mesh refinement. Our simulated results show that a 7$times$7 island decays with its shape kept unchanged, and its area decay rate is shown to be a constant increasing with its initial area. LEEM experiments concerned are explained, which confirms that the dimer chains and corner holes are broken first in the transition, and then the stacking fault is remedied slowly. This phase-field method is a reliable approach to phase dynamics of surface phase transitions.
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This paper has been withdrawn by first author KM Seemann.
We demonstrate that it is possible to mechanically exfoliate graphene under ultra high vacuum conditions on the atomically well defined surface of single crystalline silicon. The flakes are several hundred nanometers in lateral size and their optical contrast is very faint in agreement with calculated data. Single layer graphene is investigated by Raman mapping. The G and 2D peaks are shifted and narrowed compared to undoped graphene. With spatially resolved Kelvin probe measurements we show that this is due to p-type doping with hole densities of n_h simeq 6x10^{12} cm^{-2}. The in vacuo preparation technique presented here should open up new possibilities to influence the properties of graphene by introducing adsorbates in a controlled way.
We report scanning tunneling microscopy observations of Ge deposited on the Si(111)-7x7 surface for a sequence of sub-monolayer coverages. We demonstrate that Ge atoms replace so-called Si adatoms. Initially, the replacements are random, but distinct patterns emerge and evolve with increasing coverage, till small islands begin to form. Corner adatom sites in the faulted half unit cells are preferred. First-principles density functional calculations find that adatom substitution competes energetically with a high-coordination bridge site, but atoms occupying the latter sites are highly mobile. Thus, the observed structures are indeed more thermodynamically stable.
We report an experimental refinement of the local charge density at the Si (111) 7x7 surface utilizing a combination of x-ray and high energy electron diffraction. By perturbing about a bond-centered pseudoatom model, we find experimentally that the adatoms are in an anti-bonding state with the atoms directly below. We are also able to experimentally refine a charge transfer of 0.26(4) e- from each adatom site to the underlying layers. These results are compared with a full-potential all-electron density functional DFT calculation.
We propose a two-dimensional phase-field-crystal model for the (2$times$1)-(1$times$1) phase transitions of Si(001) and Ge(001) surfaces. The dimerization in the 2$times$1 phase is described with a phase-field-crystal variable which is determined by solving an evolution equation derived from the free energy. Simulated periodic arrays of dimerization variable is consistent with scanning-tunnelling-microscopy images of the two dimerized surfaces. Calculated temperature dependence of the dimerization parameter indicates that normal dimers and broken ones coexist between the temperatures describing the charactristic temperature width of the phase-transition, $T_L$ and $T_H$, and a first-order phase transition takes place at a temperature between them. The dimerization over the whole temperature is determined. These results are in agreement with experiment. This phase-field-crystal approach is applicable to phase-transitions of other reconstructed surface phases, especially semiconductor $ntimes$1 reconstructed surface phases.
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