Computational design and modeling of topological insulator-based heterostructures for spin-orbitronics and skyrmionics

自旋电子学探索自旋和电子携带的电荷相互缠绕的现象，它表现出了一种非凡的能力，可以在未来应用的基础研究中为基础研究提供肥沃的子领域。第一代自旋电子学导致了硬盘上存储的数字信息量的革命性增长，也关键地依赖于新材料和异质结构的发现。下一代自旋电子学预计将导致超低功耗存储器和逻辑器件，可以与传统的电子集成。本提案中的项目将为最近发现的拓扑绝缘体（TI）材料中的特殊相对论效应（称为自旋轨道耦合（SOC））所带来的进展探索新兴资源。TI是绝缘的，在他们的散装，但也主机金属表面与强大的SOC。最近的实验表明，当电流注入到异质结构中，TI是附着在铁磁（FM）层，FM层的磁化动态可以点燃潜在的耗散比目前可用的技术基础的磁性随机存取存储器。具有强SOC和破缺反转对称性的TI和FM层之间的界面也可以产生涡旋自旋织构，其特征在于纳米尺度的尺寸、对缺陷和杂质的拓扑稳定性以及类似于磁场下带电粒子的陀螺动力学。使用高性能计算（HPC）模拟来寻找TI和FM材料的最佳组合，可以显着缩短生产功能器件所需的时间。拟议的研究将产生更广泛的影响，包括培训研究生在非平衡量子统计力学，先进的科学计算技术和基本了解TI为基础的异质结构在非平衡条件下。学生将与国际合作者互动。该计划将为高密度数据存储和非易失性存储器开发新的建模软件和计算设计，并实现超低能耗操作。利用非平衡绿色函数理论（NEGF），非共线密度泛函理论（DFT）和半经典Langevin方程技术相结合， 本研究计画将发展下列基本的了解：SO转矩;自旋泵浦与自旋电荷转换;以及在TI所带来的强界面SOC存在下，吉尔伯特阻尼与杂讯对磁化动力学的影响。拟议的研究将从TI/FM异质结构的第一性原理筛选开始，以确定那些对界面周围的磁性原子具有最大SOC邻近效应或界面Dzyaloshinsky-Moriya相互作用的异质结构，这些相互作用可能在室温下产生skyrmions（目前已知的所有磁性skyrmions的例子都发生在室温以下）。在第二阶段，SO扭矩最有前途的异质结构将被计算。SO扭矩将被用作随机Landau-Lifshitz-吉尔伯特（LLG）方程的输入，以研究（经典描述）磁化或skyrmion自旋纹理在快速移动（量子力学描述）电子的存在下的缓慢动力学，包括阻尼和非平衡噪声效应。这项研究将为下一代基于拓扑绝缘体的超低功耗存储器和逻辑自旋电子器件奠定基础。

Spintronics, which explores phenomena intertwining spin and charge carried by an electron, has exhibited a remarkable ability to re-energize itself in directions that germinate fertile subfields for basic research aimed at future applications. The first generation spintronics, which has led to revolutionary increase in the amount of digital information stored on hard drives, has also crucially relied on the discovery of new materials and heterostructures. The next generation spintronics is expected to lead to ultralow power dissipation memory and logic devices that can be integrated with conventional electronics. The projects in this proposal will explore emerging resource for such advances brought about by the special relativistic effects, termed spin-orbit coupling (SOC), in recently discovered topological insulator (TI) materials. The TIs are insulating in their bulk, but also host metallic surfaces with strong SOC. The very recent experiments have demonstrated that when current is injected into heterostructures where TI is attached to a ferromagnetic (FM) layer, magnetization dynamics of the FM layer can be ignited with potentially much less dissipation than in presently available technologies underlying magnetic random access memory. The interface between TI and FM layers with strong SOC and broken inversion symmetry could also generate swirling spin texture characterized by nano-scale size, topological stability against defects and impurities, and gyro-dynamics analogous to that of a charged particle under magnetic field. Using high performance computing (HPC) simulations to search for optimal combination of TI and FM materials for these phenomena can significantly shorten time needed to produce functional devices. Broader impact of the proposed research will, include training for graduate students in nonequilibrium quantum statistical mechanics, advanced scientific computing techniques and fundamental understanding of TI-based heterostructures under nonequilibrium conditions. Students will interact with the international collaborators. New modeling software and computational design for high-density data-storage and nonvolatile memory with ultra-low energy cost manipulation developed under the program will be available for researchers in the field.Using combination of nonequilibrium Green function theory (NEGF), noncollinear density functional theory (DFT), and semiclassical Langevin equation techniques, this research program will develop fundamental understanding of: SO torque; spin pumping and spin-to-charge conversion; and Gilbert damping and noise effects on magnetization dynamics in the presence of strong interfacial SOC brought by TIs. The proposed research will commence with first principles screening of TI/FM heterostructures in order to identify those with the largest SOC proximity effect onto the magnetic atoms around the interface or interfacial Dzyaloshinsky-Moriya interaction that could give rise to skyrmions at room temperature (all presently known examples of magnetic skyrmions occur below room temperature). In the second stage, SO torque for most promising heterostructures will be computed. The SO torque will be used as an input for the stochastic Landau-Lifshitz-Gilbert (LLG) equation to study slow dynamics of (classically described) magnetization or skyrmion spin texture in the presence of fast moving (quantum-mechanically described) electrons, including damping and nonequilibrium noise effects generated by them. This research will form the basis for next generation of ultralow power dissipation memory and logic spintronic devices based on topological insulators.