磁性系统中拓扑磁振子态及拓扑自旋波电子学

In this project, we shall theoretically investigate magnetic topological states. The initial studies of topological states were exclusively for electronic systems, and topological states were believed to be a quantum phenomenon. It is now known that topological states can also exist in classical systems. Topological states of matter have been studied intensively in recent years because of their exotic properties such as topologically protected surface chiral or helical states. Despite active searches for topological states in many fields, studies of topological states in magnetism are rare although they are important and useful in magnonics such as spin wave transportation and manipulations based on the chiral nature of surface states. We will search for possible topological states in magnetic systems that are governed by either the quantum Hamiltonian or the Landau-Lifshitz-Gilbert (LLG) equation. Magnonics is about the creation, detection, and manipulation of magnons that are the quanta of spin waves. Magnons are spin-1 particles whose motion generates a spin current, free from Joule heating, that can be used to encode and transmit information. Various magnon-based nanodevices such as the magnon-drag thermopile, spin-wave multiplexer, and magnon transistor have already been proposed and studied on the basis of normal spin wave properties. We shall focus on magnonic topological insulators where spin waves have gapped bulk states and gapless topologically protected surface states and magnonic Weyl states the bulk spin waves are linearly dispersed and cross at so called Weyl nodes. It is anticipated that magnonic topological materials can enhance the performance of magnonic devices, and even open doors for new functional magnonic devices because of the topologically protected chiral/helical states. It is expected that the outcome of this study will significantly impacts on condensed matter physics in general and the field of spintronics in particular.

拓扑保护的手性表面态和螺旋表面态有很多新奇的性质。尽管在许多领域都已经兴起了对于拓扑性质和拓扑保护态的研究，磁学领域中自旋波的拓扑性质的研究却刚刚起步。本项目将从磁性系统的量子力学模型或经典朗道-栗弗西斯-吉尔伯特方程出发，重点探寻磁性系统中可能的拓扑非平凡自旋波态，如拓扑绝缘体在自旋波系统中的对应，外尔半金属在自旋波系统中的对应等。可以预见，拓扑保护的自旋波态由于不易被外部干扰散射，可以提升自旋波器件的效能，甚至由于拓扑非平凡的手性态/螺旋态可能具有的新奇性质可以设计出更多功能性器件。本项目的研究成果不仅可以加深对于磁性系统和拓扑性质的理解，更可以为自旋波电子学的器件设计开辟新的方向，对于自旋电子学领域乃至整个凝聚态物理领域都有积极影响。

物质的拓扑性在很多领域已经被广泛研究，并由此衍生出很多新奇的现象。在这个项目开始时，几乎没有关于磁学领域的拓扑学研究。自从我们在2017年至2018年期间发表了数篇关于拓扑自旋波的文章，澄清了拓扑相不是量子物理学的特权之后，该主题在磁学界引起了广泛关注，越来越多的学者投入相关领域的研究。我们的研究提出了“拓扑磁振子学”这一概念，并相继建立了自旋波系统中的其他与拓扑相关的概念。我们通过系统研究铁磁蜂窝晶格中的自旋波，总结并介绍了磁振子能谱及其拓扑不变量的计算方法，并设计出了一系列自旋波器件。我们对拓扑系统中存在的自旋波，以及具有对称性的三维二阶拓扑绝缘体中的无序相变展开了细致的研究，澄清了磁性中的拓扑可以是空间中的拓扑自旋结构。我们研究了反铁磁磁畴壁的动力学和量子行为，参数泵浦下斯格明子的运动，铁磁体和反铁磁体中无序对斯格明子动力学的影响，并对反铁磁动力学研究进展进行了综述。我们证明了斯格明子晶体中出现的条状结构也是一种斯格明子，研究了斯格明子从条状到圆形的转变过程，并从理论上分别给出了孤立和条状斯格明子的大小公式和自旋分布，解决了斯格明子的尺寸难题。我们发现一种在两个耦合的自旋扭矩纳米振荡器之间新奇的跃动同步现象，并证明这种跃动具有普遍性，从而为设计新器件提供了广阔的思路。通过本项目的研究，我们完善了拓扑性在磁学中的应用，为拓扑自旋波和拓扑磁结构在自旋电子学乃至信息存储计算通信等领域起到了重要的推动作用。