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

设计并制作了一个温度从50 K调节到1400K的样品操纵器，用于KEK光子因子15B2光束线处的表面x射线衍射测量。在500K温度下，在干净的Si（111）表面沉积一层银，形成了Si(111)/3x/3-Ag表面。在50K低温和室温条件下对掠入射x射线衍射进行了测量。在两个温度下得到的26个衍射点的积分强度显示出不同的强度分布。此外，在低温下，已经观察到扩散散射，表明形成了双结构，其中银原子形成三角形，围绕其中心以相反的方向旋转。强度数据的最小二乘分析也表明，在低温条件下，表面呈现由第一原理计算和STM研究提出的p3对称的不等价三角形结构，而在室温条件下，如果考虑各向异性热振动，则可以用著名的蜂窝链三角模型很好地解释表面。结果还表明，相变属于有结构变化的位移相变，不支持基于不等三角结构提出的有序-无序相变。

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.