Control and Topology of Driven Magnets

驱动手征磁体构成了一个有吸引力的平台，发现新的物理学基本概念。这是因为它们可以承载大量令人兴奋的不同物理行为，包括（但不限于）涌现的电磁学，激活的戈德斯通模式，磁振子激光和拓扑相变，同时仍然很容易在实验上实现。在这个项目中，我的两个主要目标将是探索无序和拓扑结构在驱动磁系统中的作用。最近的实验中，飞秒激光脉冲驱动的skyrmion晶格的旋转（戈德斯通模式）运动的数量级大于理论上预期的干净的系统，我将看看如何无序可以帮助磁纹理的动力学。通过工程和随机无序的组合，分析计算和微磁模拟，我将试图揭示无序可以成为“朋友”而不是“敌人”的制度，并加速驱动场激活的戈德斯通模式。这种“无序辅助棘轮运动”虽然罕见，但在其他驱动耗散系统中已经提出，包括无序超导导线，生物布朗马达和无序介质中的量子棘轮。该项目的第二部分将致力于拓扑激光物理学。拓扑非平凡相，如skyrmion晶格可以托管手性边缘状态，局部化到样品的表面。由于磁振子是玻色子，在周期性驱动下，它们宏观上占据单个相干磁振子态，形成磁振子激光器。我的目标将是利用这些磁振子激光特性，直接泵浦到表面手征态。这将激活巨大的自旋电流，在样品表面几乎没有耗散，这有许多有趣的实验应用。我还将尝试寻找趋肤效应--一种在非厄米特物理学控制的系统中预测和观察到的奇异的新现象。趋肤效应预测，所有的本征态都局限在样品表面，因此，如果在我们的驱动磁体中观察到，我预计会产生更巨大的表面电流！

Driven chiral magnets constitute an attractive platform for discovering new fundamental concepts of physics. This is because they can host a wealth of exciting different physical behaviours, including (but not limited to) emergent electromagnetism, activated Goldstone modes, magnon lasers and topological phase transitions, while remaining very easy to realise experimentally. In this project, my two main aims will be to explore the role of disorder and topology in driven magnetic systems. Motivated by recent experiments where the rotational (Goldstone mode) motion of a skyrmion lattice driven by femtosecond laser pulses was orders of magnitude larger than expected theoretically for a clean system, I will look at how disorder can assist the dynamics of magnetic textures. By a combination of engineered and random disorder, analytical calculations and micromagnetic simulations, I will attempt to uncover regimes where disorder can be a "friend" rather than a "foe" and speed up the Goldstone modes activated by the driving field. Such "disorder-assisted ratchet motion", while rare, has been suggested in other driven dissipative systems, including disordered superconducting wires, biological Brownian motors and quantum ratchets in disordered media. The second part of the project will be dedicated to topological laser physics. Topologically non-trivial phases such as the skyrmion lattice can host chiral edge states, localised to the surface of the sample. As magnons are bosons, under periodic drive they macroscopically occupy a single coherent magnon state, forming a magnon laser. My goal will be to pump directly into the surface chiral states by exploiting these magnon laser properties. This should activate giant spin currents with little dissipation at the surface of the sample, which has many interesting experimental applications. I will also try to look for the skin effect – an exotic new phenomenon predicted and observed in systems governed by non-Hermitian physics. The skin effect predicts that all the eigenstates are localised at the sample surface thus if observed in our driven magnet I would expect even more gigantic surface currents to be generated!