Spin friction of single atoms investigated by scanning tunneling microscopy

利用扫描隧道显微镜（STM）尖端对原子进行操纵是一种广泛而直接的技术，可以以原子为单位构建复杂的纳米结构。在这个项目中，我想使用原子操纵作为成像工具来研究磁性表面上原子的静态和动态特性。与在标准（自旋极化）STM中测量表面以上几个原子距离的状态密度（自旋相关）不同，被操纵的adatom可以看作是在表面上的尖端探测（自旋相关）力的延伸。这个项目的核心问题是：当原子在磁性表面上移动时，哪些相互作用会影响它的路径？在这里，我对最近的海森堡交换之外的磁相互作用特别感兴趣。与这个问题密切相关的是能量耗散和摩擦方面，以及自旋自由度在这些过程中发挥作用的程度。为了解开不同的贡献，特别是磁性和电子的贡献，我将在各种表面自旋纹理上使用磁性和非磁性的针尖和附原子，如反铁磁体、自旋螺旋、斯基米子晶格和畴壁。由于操纵实验可以揭示附着原子的位点依赖结合强度，因此应该有可能估计磁性在多大程度上可以影响附着原子的扩散，从而影响一般的生长过程。计划进行更多的原子扩散实验来验证这一想法。此外，在静止或移动的吸附原子作用于局部表面自旋织构的情况下，我将尝试利用这种效应来通过操纵吸附原子的集合来控制磁态。

Atom manipulation with the tip of a scanning tunneling microscope (STM) is a widespread and straightforward technique to build complex nanostructures in an atom-by-atom fashion. In this project I want to use atom manipulation as an imaging tool to investigate the static and dynamic properties of adatoms on magnetic surfaces. Instead of measuring (spin-dependent) density of states a few atomic distances above the surface, as in standard (spin-polarized) STM, the manipulated adatom can be seen as an extension of the tip probing (spin-dependent) forces right at the surface. The central question of this project is the following: Which interactions influence an adatom's path when it is moved across a magnetic surface? Here, I am especially interested in magnetic interactions beyond the nearest neighbor Heisenberg exchange. Closely related to this question are the aspects of energy dissipation and friction, and to what extend the spin degree of freedom plays a role in these processes. To disentangle different contributions, especially the magnetic from the electronic ones, I will use magnetic as well as non-magnetic tips and adatoms on a variety of surface spin textures such as antiferromagnets, spin spirals, skyrmion lattices and domain walls. Since the manipulation experiments can reveal site-dependent binding strengths of adatoms, it should be possible to estimate to what extent magnetism can influence adatom diffusion and thus growth processes in general. Additional atom diffusion experiments are planned to verify this idea. Furthermore, in cases where the resting or moving adatom acts back onto the local surface spin texture, I will try and exploit this effect to control magnetic states with manipulated ensembles of adatoms.