Magnetic and electronic excitations in low-dimensional and nanostructured materials

对一种或多种长度在亚微米范围内的磁性材料的研究，如在低维纳米结构和阵列中，受到其巨大的技术潜力(用于高频设备、磁存储器、快速开关等)的推动。从理论上讲，有必要在基本水平上理解这些长度尺度上的动力过程。这为我们提出的关于集体磁激发(或波)和电子激发的理论研究提供了主要的动机。考虑到表面和界面的挑战性效应是重要的，因为纳米结构磁性材料可能显示出其块体组成材料所不具备的新特性。 我将研究磁性纳米元素阵列中磁性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的相关电子和磁效应。例如，磁振子的扩展领域涉及人工介质，如具有重复(或周期性)磁性变化的图案化薄膜。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，也有运动的电子电荷，在材料中产生电流；这些影响自旋波，从而允许进一步的设备应用。在非线性动力学中，自旋波之间相互作用的影响开始发挥作用，我将在小长度尺度上研究这些影响。我将重点讨论一些非线性效应，包括自旋波的衰减和不稳定性，特别是在强微波场中，以及磁性玻色-爱因斯坦凝聚。磁性元件在其周期性或准周期性阵列中的选择为利用磁性设备应用的新特性提供了各种情况。 方法包括多体理论和量子场论(图解方法、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拥有专业知识，我将使其适用于应用微波泵浦磁场的非线性区域中的纳米材料。我将利用与实验者的合作，使用非弹性光散射、铁磁和电子自旋共振以及微波光谱学。我的研究将对深入了解极小长度尺度的动态过程具有重要意义，并可能导致高频设备的新发展。理解这些系统在远离平衡时(由于微波场的作用)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低维材料新应用的重要目标。

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.