Electron Diffraction Methods for Systems Involving Very Large Unit Cells

9972958 tong该基金支持理论研究，旨在对表面和界面的结构进行定量理解，这是理解材料研究中许多重要科学问题的基础。该研究将使用多种散射理论来分析对材料的短程或远程顺序敏感的表面光谱数据。一个重要的研究推力是使用新的n2标度迭代多次散射理论，在上一个资助期发展，分析光电子衍射（PD）和低能电子衍射（LEED）数据。新方法比现有的“精确”方法快了几个数量级，但精确度同样高。通过能够在超级单体板计算中包含多达700个原子/单位细胞-目前精确方法的限制是大约50个原子/单位细胞-新方法可以定量地解决与层生长和外延有关的一系列有趣问题，或相应的不相称跃迁等。第二个重点领域是发展结构确定的直接方法。表面结构分析的一个重要目标是能够反演电子衍射数据，直接得到实际空间中的原子坐标。40多年来，这个目标一直没有得到解决。PI和其他人已经证明，反演测量的电子衍射光谱可以在表面区域产生直接的原子位置。与实验组密切合作，将全息反演方法应用于求解未知表面和界面的结构。最后，本研究的第三个重点是建立一个多重散射理论来准确分析反射高能电子衍射（RHEED）的强度振荡和摇摆曲线。RHEED是大多数MBE室研究生长模式和过渡或高温结构的首选技术。在10 KeV或更高的能量下，为LEED或PD开发的方法不再实用。RHEED的多重散射理论将能够处理包含100个或更多原子的单元胞。利用这种新方法，将对大型梯田大小的RHEED强度振荡进行模拟，并将其与RHEED测量结果和原位STM对岛屿大小的观测结果进行比较。目的是建立最佳散射条件，定量使用RHEED振荡作为层完成的指标。该基金支持对表面和界面结构进行定量理解的理论研究，这是理解材料研究中许多重要科学问题的基础研究领域。该研究将使用多种散射理论来分析对材料的短程或远程顺序敏感的表面光谱数据。这些新开发的技术将被应用于一些涉及表面结构及其生长和演化的物理问题

9972958TongThis grant supports theoretical research which pursues a quantitative understanding of the structure of surfaces and interfaces, a field of research which is fundamental to the understanding of a host of important scientific questions in materials research. The research will use multiple scattering theories to analyze data from surface spectroscopies sensitive to the short-range or long-range order of materials.An important research thrust is to use a new N2-scaling iterative multiple scattering theory, developed in the last grant period, to analyze photoelectron diffraction (PD) and low-energy electron diffraction (LEED) data. The new method is orders of magnitude faster than existing "exact" methods, but just as accurate. By being able to include up to 700 atoms/unit-cell in a supercell slab calculation - the current limit with exact methods is approximately 50 atoms/unit-cell - the new method can address quantitatively a host of interesting problems relating to layer growth and epitaxy, or commensurate-incommensurate transitions, etc. A second area of focus is the development of direct methods for structural determination. An important goal of surface structural analysis is to be able to invert electron diffraction data to directly produce atomic coordinates in real space. This goal has eluded solution for over 40 years. The PI and others have demonstrated that inversion of measured electron diffraction spectra can produce direct atomic positions in the surface region. In close collaboration with experimental groups, the holographic inversion method will be applied to solve structures of unknown surfaces and interfaces. Finally, a third focus of this research is to develop a multiple scattering theory to accurately analyze reflection high energy electron diffraction (RHEED) intensity oscillations and rocking curves. RHEED is the technique of choice in most MBE chambers to study growth modes and transitional or high-temperature structures. At energies of 10 KeV or above, the methods developed for LEED or PD are no longer practical. A multiple scattering theory of RHEED will be developed capable of handling unit cells containing 100 atoms or more. With this new method, simulations of large terrace-sized RHEED intensity oscillations will be undertaken and compared to RHEED measurements and observations of island sizes by in situ STM. The aim is to establish optimal scattering conditions for the quantitative usage of RHEED oscillations as an indicator of layer completion.%%%This grant supports theoretical research which pursues a quantitative understanding of the structure of surfaces and interfaces, a field of research which is fundamental to the understanding of a host of important scientific questions in materials research. The research will use multiple scattering theories to analyze data from surface spectroscopies sensitive to the short-range or long-range order of materials. These newly developed techniques will then be applied to a number of important problems involving the physics of surface structure and its growth and evolution.***