Studies on the phase transition of √3 x√3-Ag structures formed on the Si(111) surface

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A sample manipulator, regulating the temperature from 50 K to 1400K, has been designed and fabricated for the measurements of surface X-ray diffraction planned at the beamline 15B2 of the Photon Factor in KEK.The Si(111)/3x/3-Ag surface has been formed by depositing a monolayer of silver on the clean Si(111) surface kept at 500K. The measurements of grazing incidence x-ray diffraction have been done both at a low temperature of 50K and room temperature. The integrated intensities obtained at both temperatures for 26 diffraction spots have shown different intensity distributions. Furthermore, at low temperatures, diffuse scattering has been observed, indicating the formation of twin structures in which Ag atoms forming a triangle rotate in opposite directions each other around its center. Thus the symmetry of the surface structure changes from pSm to pS.The least squares analysis of intensity data also showed that the surface takes an inequvalent Triangle structure with p3 symmetry proposed by first-principles calculations and STM studies at low temperatures, while the surface is well explained by the well-known Honeycomb Chained Triangle model if anisotropic thermal vibrations are taken into account at room temperature. The result also indicates that the phase transition belongs to a displacive one with structural changes, and doesn't support order-disorder phase transitions proposed on the basis of the Inequvalent Triangle structures.