Magnetic and electronic excitations in low-dimensional and nanostructured materials

在亚微米范围内具有一个或多个长度的磁性材料的研究，如在低维纳米结构和阵列中，是由它们巨大的技术潜力驱动的（用于高频设备，磁存储器，快速开关等）。就理论而言，有必要在基本水平上理解这些长度尺度上的动态过程。这就提供了本文提出的磁集体激励（或波）与电子激励的理论研究的主要动机。考虑到由于表面和界面而产生的具有挑战性的影响是很重要的，因为纳米结构磁性材料可能会显示出在其大块组成材料中不存在的新特性。***我将研究磁性纳米元件阵列中磁自旋波的线性和非线性动力学，以及石墨烯等碳基纳米材料中相关的电子和磁效应。例如，磁流变学的扩展领域涉及人工介质，如具有磁性重复（或周期性）变化的图案薄膜。利用这种周期性和外加磁场可以控制自旋波的频率和方向，作为器件应用的基础。在自旋电子学中也有移动的电子电荷，在材料中产生电流；这些影响自旋波允许进一步的设备应用。在非线性动力学中，自旋波之间的相互作用起了作用，我将在小长度尺度上研究这些效应。我将重点关注的一些非线性效应包括自旋波衰减和不稳定性，特别是在强微波场中，以及磁性玻色-爱因斯坦凝聚。在其周期或准周期阵列中的磁性元素的选择为开发磁性器件应用的新特性提供了多种情况。***方法包括多体理论和量子场理论（图解法、格林函数、响应理论）。从以前的工作中，我在自旋系统的专门方法方面具有专业知识，我将适应非线性体系中的纳米材料，并应用微波抽运场。我将利用与实验家合作使用非弹性光散射，铁磁和电子自旋共振，以及微波光谱。我的研究对于在非常小的长度尺度上了解动态过程具有重要意义，并可能导致高频设备的新发展。了解这些系统在远离平衡状态（由于微波场）或在较高温度下（当非线性过程增强时）的量子统计将是这项工作的重要方面。更好地了解石墨烯纳米结构中的磁效应是在低维材料中的新应用的重要目标。

Research on magnetic materials with one or more lengths in the submicron range, as in low-dimensional nanostructures and arrays, is driven by their immense technological potential (for high-frequency devices, magnetic memories, fast switches, etc). For theory there is a need to understand at a fundamental level the dynamical processes on these length scales. This provides the primary motivation for this proposed theoretical research on the collective magnetic excitations (or waves) together with the electronic excitations. Taking into account the challenging effects due to surfaces and interfaces is important, since nanostructured magnetic materials may display novel characteristics that are absent in their bulk constituent materials.***I will study the linear and nonlinear dynamics of magnetic spin waves in arrays of magnetic nano-elements and the related electronic and magnetic effects in carbon-based nanomaterials like graphene. For example, the expanding field of magnonics concerns artificial media like patterned thin films with a repeating (or periodic) variation in their magnetic properties. The frequencies and directions of the spin waves can be controlled using this periodicity and an applied magnetic field, as the basis for device applications. In spintronics there are also moving electronic charges, giving electric currents in the materials; these influence the spin waves allowing further device applications. In the nonlinear dynamics the effects of interactions between the spin waves come into play, and I will study these on the small length scales. Some nonlinear effects that I will focus on include spin-wave decay and instability, particularly in a strong microwave field, and the magnetic Bose-Einstein condensation. The choices of magnetic elements in their periodic or quasiperiodic arrays provide a variety of situations for exploiting new properties for magnetic device applications. ***Methods include many-body and quantum-field theories (diagrammatic methods, Green's functions, response theory). From previous work I have expertise in specialized methods for spin systems, which I will adapt for nanomaterials in nonlinear regimes with a microwave pumping field being applied. I will utilize collaborations with experimentalists using inelastic light scattering, ferromagnetic and electron-spin resonance, and microwave spectroscopy. My research will be significant for insights to dynamical processes at very small length scales and may lead to new developments for high-frequency devices. Understanding the quantum statistics in these systems when they are far from equilibrium (due to the microwave fields) or at higher temperatures (when nonlinear processes are enhanced) will be important aspects of this work. A better knowledge of the magnetic effects in graphene nanostructures is an important goal for new applications in low-dimensional materials.