Van-der-Waals magnets: Frustrated magnetism, magnetotransport, and optically driven excitations

低维凝聚态物质系统通常表现出与其三维整体类似物明显不同的行为和奇异现象。研究这些效应和设计具有所需特性的系统的一个特别有成效的平台是基于剥离的范德华材料的二维少层异质结构。这些新颖的器件允许对强关联电子进行前所未有的控制，例如通过改变层与层之间的相对位移(包括扭转)或电门控。直到2017年，才发现了单层和少层范德华(VDW)系统中存在磁性的第一个证据。从那时起，显著的实验进展已经发现了其他几种VDW材料，它们表现出不同的铁磁和反铁磁有序模式。这些系统中有许多具有强烈增强的磁光效应，新颖的隧道磁阻和磁电效应，以及磁和晶格自由度之间的强耦合。本项目的目标是深入了解几层范德华系统中的新现象，目的是识别新现象和提出实验方案，并从理论上模拟先前观察到的效应。为此，在本项目的第一部分，我们使用一个连续介质模型来研究强自旋-晶格耦合对最近提出的通过扭转双层磁层获得的受阻莫尔磁体中非共线基态和激发的影响。第二，我们利用连续介质模型研究了强自旋-晶格耦合对最近提出的通过扭转双层磁层获得的受阻莫尔磁体中的非共线基态和激发的影响我们将通过模拟任意晶格位移的层间磁振子隧穿来研究在这些扭曲的双层磁体中由磁场诱导的新的相，并计算相应的自旋波色散。本项目的第三部分涉及在一些半导体磁范德华少数层系统中观察到的新的与磁相关的层间电子输运，特别是CrI3。在这里，我们计划用有效质量模型来模拟低能电子激发，然后将这个模型耦合到几层体系中的磁学连续介质理论。最后，我们将探索超快光学方法在多大程度上可以用来激发磁性VDW系统中的磁激发，特别是利用自旋-晶格耦合，并研究这些异质结构中的非平衡动力学。

Condensed matter systems in low dimensions often show behavior and exotic phenomena markedly different from their three-dimensional bulk analogues. A particularly fruitful platform to study these effects and design systems with desired characteristics are two-dimensional few-layer heterostructures based on exfoliated van-der-Waals materials. These novel devices allow for an unprecedented degree of control of strongly correlated electrons by e.g. varying the relative displacement between layer (including twisting) or electric gating. Only in 2017, first evidence for magnetism in mono- and few-layer van-der-Waals (vdW) systems was found. Since then, remarkable experimental progress has uncovered several other vdW materials exhibiting various ferro- and antiferromagnetic ordering patterns. Many of these systems feature strongly enhanced magnetooptical effects, novel tunneling magnetoresistance and magnetoelectric effects, and strong coupling between magnetic and lattice degrees of freedom.This project has the goal of gaining a thorough understanding of novel phenomena in few-layer van-der-Waals systems, with the aim of both identifying novel phenomena and proposing experimental protocols, as well as theoretically modelling previously observed effects.To this end, in the first part of this project, we employ a continuum model to study the impact of strong spin-lattice coupling on the non-collinear ground states and excitations in the recently proposed frustrated Moiré magnets obtained by twisting a bilayer magnet.Second, we will investigate novel phases induced by a magnetic field in these twisted bilayer magnets and compute the corresponding spin-wave dispersions by modelling interlayer magnon tunneling for arbitrary lattice displacements.The third part of this project is concerned with the novel magnetism-dependent interlayer electronic transport observed in some semiconducting magnetic van-der-Waals few-layer systems, in particular CrI3. Here, we plan to model low-energy electronic excitations in an effective mass model and then couple this model to a continuum theory for magnetism in the few-layer systems.Lastly, we will explore to what extent ultrafast optical methods can be used to excite magnetic excitations in magnetic vdW systems, in particular also utilizing spin-lattice coupling, and study non-equilibrium dynamics in these heterostructures.