Kondo Effect at Molecule-Metal-Interfaces: Investigation by Angle-resolved Photoemission and Orbital Tomography

在某些情况下，在分子和金属表面之间的界面处观察到近藤效应的发生。通过局域态与导电电子池的相互作用，这种效应可以增加费米能级的态密度，从而对输运和接触性质产生重要影响。除了对相关模型系统进行了大量的STM和输运研究外，导致分子近藤效应发生的基本关系，特别是所涉及的单粒子态的性质和局域磁性，还没有完全弄清楚。在这方面，角度分辨光电子能谱(ARPES)提供了获取微观信息的途径，这是STM所不能或仅有的非常有限的。利用角度信息允许对所涉及的局域态的特征和对称性进行陈述。在理想情况下，有机吸附体系的分子单粒子轨道可以重构。通过研究来自入射光的偏振方向和光子能量的光电发射信号的依赖关系来提供附加信息。此外，相关的模型参数可以从ARPES实验中得到，在此基础上可以得到Kondo模型或单杂质Anderson模型(SIAM)中的微观关系的一致图像。在前人工作的基础上，本项目将利用ARPES系统地研究分子-金属界面的Kondo效应，特别是回答以下问题：(I)哪种对称性与许多粒子信号有关，例如Kondo共振？我们可以从对称性中了解到这些态的本质是什么？近藤效应所涉及的状态的性质尚不清楚，将从ARPES中的角度信息和可变光偏振的实验中得出。(2)根据近藤共振谱和核心能级谱的温度依赖性确定特征温度或能量标度。在很大的温度范围内光电子发射强度与温度的关系的实验测定提供了相关的特征能量标度。(3)相关模型参数能否在实践中系统地改变？在SIAM框架中对分子-底物界面系统的描述涉及模型参数，这些参数可以通过光谱或通过与相应的计算进行比较来确定。这些参数的外部控制允许操作接口属性。

In certain cases the occurrence of a Kondo effect is observed at interfaces between molecules and metallic surfaces. By the interaction of a localized state with the bath of conduction electrons this effect can increase the density of states at the Fermi level and thus crucially influence the transport and contact properties. Besides extensive STM and transport investigations on relevant model systems the fundamental relations, which lead to the occurrence of the molecular Kondo effect, in particular the nature of the involved single particle states and the local magnetic properties, are not completely clarified. In this respect angle-resolved photoelectron spectroscopy (ARPES) provides access to microscopic information, which is not or only very limited available by STM. Employing the angular information allows statements on the character and the symmetry of the involved localized states. In an ideal case molecular single particle orbitals can be reconstructed for organic adsorbate systems. Additional information is provided by investigating the dependence of the photoemission signal from the polarization direction and photon energy of the incoming light. Beyond this, the relevant model parameters can be derived from ARPES experiments, based on which a consistent picture of the microscopic relations in the Kondo model or Single Impurity Anderson Model (SIAM) can be obtained. Based on own previous work this project will systematically investigate the Kondo effect at molecule-metal interfaces by ARPES and in particular answer the following questions: (i) Which symmetry is associated with the many particle signals such as the Kondo resonance? What can we learn about the nature of these states from symmetries? The nature of the states involved in the occurrence of the Kondo effect is not yet clear and will be derived from the angular information in ARPES and from experiments with variable light polarization. (ii) Determination of the characteristic temperature- or energy scale from the temperature dependence of the Kondo resonance and core level spectra. The experimental determination of the temperature dependence of the photoemission intensities over a large temperature range provides access to the relevant characteristic energy scales. (iii) Can the relevant model parameters be systematically varied in practice? The description of molecule-substrate interface systems in the framework of the SIAM involves model parameters, which can be determined spectroscopically or by comparison to corresponding calculations. An external control of theses parameters allows manipulating the interface properties.