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

低维凝聚态系统通常表现出与三维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富有成效的平台是基于脱落范德华材料的二维少层异质结构。这些新颖的器件允许通过改变层之间的相对位移（包括扭曲）或电门来空前程度地控制强相关电子。直到2017年，才首次发现了单层和多层范德-瓦尔斯（vdW）体系中存在磁性的证据。从那时起，显著的实验进展已经发现了其他几种具有不同铁磁和反铁磁有序模式的vdW材料。许多这些系统具有强增强的磁光效应，新的隧道磁电阻和磁电效应，以及磁性和晶格自由度之间的强耦合。这个项目的目标是彻底理解在几层范德华体系中的新现象，目的是识别新现象，提出实验方案，以及对先前观察到的效应进行理论建模。为此，在本项目的第一部分中，我们采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曲双层磁体获得的受挫莫尔<s:1>磁体的非共线基态和激发的影响。其次，我们将研究磁场在这些扭曲的双层磁体中诱导的新相，并通过模拟任意晶格位移的层间磁振子隧道来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性范德瓦尔斯少层系统中观察到的新型磁性依赖层间电子输运，特别是CrI3。在这里，我们计划在有效质量模型中模拟低能电子激发，然后将该模型与少层系统中磁性的连续统理论相结合。最后，我们将探索在多大程度上使用超快光学方法来激发磁性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.