Lattice effects in quasi-2D Kondo systems

拟议的项目将侧重于包括大Z元素的三元重费米子化合物的电子结构，特别是铋和锑。本项目是本课题组第一阶段工作的直接继续，在第一阶段，我们用角度分辨光电子能谱(ARPES)研究了Ce基二元表面合金的电子结构。对于几种金属间化合物合金，大Z元素诱导了强烈的自旋-轨道相互作用，从而导致了电子表面态的巨大拉什巴分裂。我们将详细研究导带态的Rashba分裂如何影响Kondo晶格系统的性质。被提升的简并度直接影响Ce“杂质”的4f态与导带之间的相互作用。假设这些表面合金中的特殊情况会导致增强的近藤效应，并在某些情况下在费米水平上产生特征的杂化带隙，例如在近藤绝缘系统Ce3Bi4Pt3或CeRhSb中。最近有人提出，后一种材料代表了弱拓扑绝缘体，其中强自旋轨道分裂和奇宇称波函数一起产生了满足拓扑绝缘体(TI)标准的情况。我们小组建立的有序4f表面合金的制备将使我们能够通过高分辨率光电子能谱(ARPES)和其他表面敏感技术(例如扫描隧道显微镜)来研究这一新方面。此外，还原为表面体系具有各种重要的实验优势，例如通过避免光电发射终态效应以及通过解缠整体和表面性质。然而，Ce基近藤绝缘体很可能只是弱拓扑，即状态很不稳定，很难无序。因此，我们将在混合价区对其他材料的单晶体样品进行等效的高分辨率ARPES实验，以再次详细研究表面电子结构。将这一建议的表面合金与大块晶体4f和5f材料进行比较，与表面合金相比，这两种材料也可以用体敏方法进行表征，这将对获得表面和薄膜系统中结构、维度和电子关联的复杂相互作用的全面图景具有决定性意义。光电子发射数据的分析和解释需要对电子能带结构和依赖温度的多体效应进行理论计算，以便在研究股的框架内使用。这种合作对于深入了解特定的单粒子性质(例如，自旋轨道分裂、拓扑能带结构)至关重要，因为它存在导致近藤屏蔽和费米能级附近的重准粒子的强多体效应。

The proposed project shall focus on the electronic structure of ternary heavy-fermion compounds including large-Z elements, as in particular Bi and Sb. This project is a direct continuation of the work in the first period of the Research Unit in which we have investigated the electronic structure of binary surface alloys based on Ce by angle resolved photoemission spectroscopy (ARPES). As demonstrated for several intermetallic alloys, large-Z elements induce a strong spin-orbit interaction and therewith a huge Rashba splitting of the electronic surface state bands. It shall be investigated in detail how this Rashba splitting of the conduction band states influences the properties of the Kondo lattice system. The lifted degeneracy affects directly the interaction between the 4f states of the Ce “impurities” and the conduction bands. It is assumed that the peculiar situation in these surface alloys results in an enhanced Kondo effect and — under certain circumstances — in a characteristic hybridization band-gap at the Fermi level, as e.g. in the Kondo insulator systems Ce3Bi4Pt3 or CeRhSb. It was proposed recently that these latter materials represent weak topological insulators, where the strong spin-orbit splitting together with the odd-parity wavefunctions produces a situation where the criteria for a topological insulator (TI) are fulfilled. The preparation of well ordered 4f surface alloys, established in our group, will us allow to investigate this new aspect by high resolution photoemission spectroscopy (ARPES) and other surface sensitive techniques (e.g. scanning tunneling microscopy). Moreover, the reduction to a surface system has various crucial experimental advantages, as for example by avoiding photoemission final-state effects and by the disentanglement of bulk and surface properties. However, the Ce based Kondo insulators are most likely only weakly topological, i.e. the states are rather instable to disorder. Therefore, we shall have a look at equivalent highresolution ARPES experiments on single-crystalline bulk samples of other materials in the mixed-valence regime, to study again in detail the surface electronic structure. A comparison of the surface alloys of this proposal with bulk crystalline 4f and 5f materials, which — in contrast to the surface alloys—can be characterized by bulk sensitive methods as well, will be decisive to obtain a comprehensive picture of the complex interplay of structure, dimensionality, and electronic correlations in surface and thin film systems. The analysis and interpretation of the photoemission data requires theoretical calculations of the electronic band structure and temperature dependent many-body effects, as being accessible within the framework of the Research Unit. This collaboration is essential to give a detailed insight into the specific single-particle properties (e.g. spin-orbit splitting, topological band structure) in presence of strong many body effects leading to Kondo screening and heavy quasiparticles near the Fermi level.