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

• Applicant: Dr. Urban Seifert
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449890867

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.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Leon Balents, Ph.D.