Understanding Spin-Transfer Torques using Ab-initio Simulations

该项目的首要目标是使用第一性原理模拟来理解自旋轨道扭矩的微观物理，以及如何使用飞秒激光脉冲来控制它。自旋轨道扭矩（SOT）和相关的自旋转移扭矩（STT）可能会支撑所有未来的技术，因为它们提供了一种有效的方法来读取和写入数据。SOT和STT都利用电子自旋电流而不是电荷电流来产生局部磁矩上的扭矩，其方向对数据进行编码。因此，通过理解这些过程的微观、量子力学和物理学，我们可以设计出更好的设备来满足我们未来的技术需求。然而，这些现象的物理机制还没有完全被理解，在这种情况下，从头算模拟变得有用，如果不是必须的话。这些第一性原理模拟对基本物理没有任何假设，只是简单地计算材料对外部电场和磁场的响应。因此，我将模拟STT和SOT在不同情况下的行为（例如改变材料和几何形状），以分离和阐明所涉及的各种物理过程。这样的计算在计算上要求很高，因为它们必须解决许多相互作用电子的量子力学薛定谔方程。为了解决这个问题，我将使用计算效率高，但仍然从头算，含时密度泛函理论（TDDFT）的方法。在过去的5年里，我们的研究小组已经证明了TDDFT可以成功地解释和预测超快自旋动力学中的几个有趣的现象。STT/SOT所需的自旋电流可以通过在磁性材料中感应电流或利用非磁性材料中的自旋霍尔效应（SHE）来产生。本项目将研究这两种机制。此外，SHE本身是非常有趣的，特别是在超快的制度，许多目前的SHE模型是不适用的。基于这个原因，我将免费发布（在Gnu公共许可下）我开发和实现的用于计算线性响应和非线性区域中自旋霍尔效应的代码。最后，我将研究STT/SOT如何通过应用飞秒激光脉冲在超快时间尺度上表现。这些时间尺度比目前可用的设备快大约一百万倍。通过研究自旋电流和磁矩如何依赖于激光参数（例如频率，持续时间，强度），我可以理解STT/SOT如何进入这个新领域。

The overarching goal of this project is to use first-principles simulations tounderstand the microscopic physics of Spin-Orbit Torque, and how it can be controlled using femtosecond laser pulses.Spin-Orbit Torque (SOT) and the related Spin-Transfer Torque (STT) will likely underpin all future technology as they provide an efficient method to read and write data. Both SOT and STT make use of the electronic spin-current, rather than the charge current, to create torque on local magnetic moments, the direction of which encodes the data. Thus, by understanding the microscopic, quantum-mechanical, physics of these processes, we can design better devices to meet our future technological needs. However, the physics of these phenomena is not yet completely understood.In such a situation, it becomes useful, if not imperative, to perform ab-initio simulations. These first-principles simulations make no assumptions on the underlying physics and simply calculate the response of a material to external electric and magnetic fields. Thus, I will simulate the behavior of the STT and SOT in different situations (e.g. changing the materials and geometries) to isolate and elucidate the various physical processes involved. Such calculations can be computationally demanding as they must solve the Schrödinger equation of quantum mechanics for many interacting electrons. To remedy this problem, I will use the computationally efficient, but still ab-initio, method of time-dependent density functional theory (TDDFT). In the past 5 years, our research group has proven that TDDFT can successfully explain and predict several interesting phenomena in ultrafast spin dynamics. Thus, it is an effective tool to use in this study.The spin currents required for STT/SOT can be generated by inducing a current in a magnetic material or by using the Spin Hall effect (SHE) in non-magnetic materials. Both mechanisms will be studied in this project. Furthermore, the SHE is very interesting in its own right, particularly in the ultrafast regime where many current SHE models are inapplicable. For this reason, I will freely distribute (under the Gnu public license) the code I develop and implement to calculate the spin-Hall effect in the linear response and non-linear regimes.Finally, I will study how the STT/SOT behaves on ultrafast timescales by applying femtosecond laser pulses. These timescales are approximately a million times faster than devices currently available. By studying how the spin-currents and magnetic torques depend on the laser parameters (e.g. frequency, duration, intensity), I can understand how STT/SOT can be advanced into this new domain.