Electronic and vibration excitations of ordered adatom assemblies, studied by optical spectroscopy and electron scattering

金属原子（如Au，Sn，Pb）在半导体表面（Si，Ge）上的自组织有序亚单层组装引起了电子相关效应的许多表现，如Mott-Hubbard绝缘体行为，电荷密度波和Tomonaga-Luttinger液体物理。对于这些系统，丰富的实验和理论信息是可用的，其中主要包括它们的基态行为。对它们的激发的研究远没有那么深入。这个项目的目标是在这类材料系统的电子和振动激发的实验研究。电子激发研究的目的是表面轨道带的差距能量和带间跃迁几率，以及金属系统的等离子体色散。振动激发方面的声子模式的本征频率和它们的对称性和位移模式进行了探讨。这些研究的价值是双重的。首先，电子和振动激发特性的知识，例如相关诱导的能隙或软声子行为，对于充分理解相关物理是必不可少的。此外，应该注意的是，激发提供了基本基态的原子和轨道构型的基本信息，这对于许多系统来说还没有完全理解。不同的振动模式提供了一个轨道选择性访问的键强度，也反映了原子结构。由于它的轨道选择性和由于表面声子穿透深度的几个原子层，这种激发衍生的基态信息是补充STM和LEED。因此，它构成了完成基态构型的重要组成部分。电子激发的实验访问是通过电子能量损失谱（EELS）和光学反射各向异性谱（RAS）。振动激发主要通过非弹性光散射（拉曼光谱）来处理。由于电子相关的出现从根本上与结构有序的横向相关长度有关，因此将通过低能电子衍射斑点轮廓分析（SPA-LEED）定量研究这方面。我们所关注的材料系统是一维（1D）系统Au/Ge（001）和Au/Si（111）（5 × 2），以及Sn和Pb在Si（111）和Ge（111）上的二维（2D）（103 × 103）组装。Au/Ge（001）c（8 × 2）和Au/Si（111）（5 × 2）的一个重要方面是复杂链的原子结构。二维体系中的相关现象是Sn或Pb吸附原子组装体中的有序/无序转变。除了温度诱导的Mott-Hubbard跃迁，对于Sn/Si（111），从（113 x 113）-重构到（3 x 3）的有争议的相变的振动影响也是非常有趣的。

Zusammenfassung (deutsch)Self-organized ordered sub-monolayer assemblies of metal adatoms (e.g. Au, Sn, Pb) on semiconductor surfaces (Si, Ge) give rise to numerous manifestations of electron correlation effects, such as Mott-Hubbard insulator behavior, charge density waves and Tomonaga-Luttinger liquid physics. For these systems a wealth of experimental and theoretical information is available, which predominantly covers their ground state behavior. Their excitations have been studied far less intensively. The goal of this project is the experimental investigation of the electronic and vibrational excitations in this class of material systems. The electronic excitation studies aim at the gap energies of surface-orbital bands and the interband transition probabilities, as well as the plasmon dispersion of metallic systems. The vibrational excitations are explored with regard to the eigenfrequencies of the phonon modes and their symmetry and displacement patterns. The merit of these studies is twofold. First, the knowledge of electronic and vibrational excitation characteristics, such as e.g. correlation-induced gap energies or soft-phonon behavior, is essential for a full understanding of the correlation physics. Moreover, it should be noted that the excitations provide essential information on the atomic and orbital configuration of the underlying ground state, which is for many systems not yet fully understood. The various vibration modes offer an orbital-selective access to the bond strengths and also reflect the atomic structure. Due to its orbital selectivity and because of the surface-phonon penetration depth of a few atomic layers, this excitation-derived ground state information is complementary to STM and LEED. Thus, it constitutes a vital component for completing the picture of the ground state configuration. Experimental access to the electronic excitations is achieved by Electron Energy Loss Spectroscopy (EELS) and optically by Reflection Anisotropy Spectroscopy (RAS). The vibrational excitations are tackled primarily by inelastic light scattering (Raman Spectroscopy). Since the emergence of electron correlation is fundamentally connected with the lateral correlation length of the structural ordering, this aspect will be investigated quantitatively by Low-Energy Electron Diffraction with Spot Profile Analysis (SPA-LEED). The material systems in our focus are the one-dimensional (1D) systems Au/Ge(001) and Au/Si(111) (5 x 2), and the two-dimensional (2D) (√3 x √3) assemblies of Sn and Pb on Si(111) and Ge(111). An essential aspect of Au/Ge(001) c(8 x 2) and Au/Si(111) (5 x 2) is the atomic structure of the complex chains. Relevant phenomena in the 2D systems are the order / disorder transitions within the adatom assemblies of Sn or Pb. Beside the temperature induced Mott-Hubbard transitions, for Sn/Si(111) also the vibration impact of the disputed phase transition from the (√3 x √3)-reconstruction to (3 x 3) is highly interesting.