RUI: Investigating Spin Currents from Nuclear Field Gradients

该奖项支持与未来“自旋电子学”技术相关的自旋和电荷电流产生的研究，教育更广泛的学生群体关于自旋的量子力学，并向当地社区推广与量子科学有关的问题。熟悉的电子学是基于电流的。相反，自旋电子学的工作原理是磁自旋电流，人们可以把它想象成无限小的条形磁铁。有效地产生和测量自旋电流是许多提出的基于自旋的器件的条件。这项研究工作将从理论上研究自旋电流产生和测量的机制，该机制涉及嵌入半导体中的原子核产生的非均匀磁场。这项活动的另一个目标是将这种新机制与传统的自旋-轨道耦合在自旋电流产生和探测方面的优点数字进行比较。学生将参与项目的各个方面：学习和应用分析和数值方法，起草文章，并在会议上展示结果。首席研究员（PI）将为自旋量子力学的入门课程编写课程材料，该课程将面向广大学生：那些没有上过传统量子力学或许多高级物理课程的学生。课程将设计为化学和工程/计算机科学专业的学生，以努力使量子力学和量子计算概念成为主流。量子技术有望带来一场量子革命。PI将让校园和当地社区参与讨论（通过公开演讲）量子科学，量子技术将引领的方向，以及量子教育对国家的重要性。技术概述：该奖项支持在核领域中利用梯度方法进行自旋电流到电荷电流和电荷电流到自旋电流转换的理论研究。自旋霍尔效应及其逆效应是目前自旋/电荷电流转换的主要方法。自旋-霍尔转换需要具有强自旋-轨道相互作用的材料，这可以缩短自旋扩散长度。这里的研究重点是利用电荷/自旋电流转换的替代材料，在这些材料中，强自旋轨道相互作用是不必要的或不可取的；相反，我们依赖于强的超精细相互作用。核场或超细梯度提供了一种机制，通过这种机制可以产生自旋电流或电荷电流。通过这种核梯度通道的自旋分离让人想起斯特恩-格拉赫效应，但由于核场不是真正意义上的洛伦兹力，自旋能够比在真正的磁场梯度中更有效地分离（核场——在费米接触势的水平上——只作用于自旋而不作用于轨道自由度）。项目目标是建立和求解包含梯度效应的自旋和电荷漂移-扩散方程。对于某些构型，解析解是可能的，但通常耦合方程是非线性的，必须采用数值方法。此外，该效应的两种可能的实验实现将被建模：电自旋注入和光自旋注入。在这两种情况下，通过注入自旋极化载流子的非均匀动态核极化，实现核梯度是可行的。通过对这些场景的建模和评估，将为希望观察自旋和电荷分离效应的实验人员提供指导。最终，所开发的自旋/电荷漂移扩散形式将能够包含其他类型的场梯度，即自旋-轨道场，例如由于Rashba相互作用而产生的场。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARYThis award supports research into spin and charge current generation relevant to future "spintronics" technologies, educating wider student populations about the quantum mechanics of spin, and outreach to local communities on issues pertaining to quantum science. Familiar electronics is based on a current of charges. Spintronics works, instead, on a current of magnetic spins, which one can think of as infinitesimal bar magnets. Efficiently generating and measuring spin currents is a condition for many proposed spin-based devices. The research effort will theoretically investigate a mechanism of spin-current generation and measurement that involves non-uniform magnetic fields coming from atomic nuclei embedded in a semiconductor. A further goal of this activity will be to compare figures of merit of this new mechanism to traditional spin-orbit coupling for spin-current generation and detection. Students will take part in all aspects of the project: learning and applying analytic and numerical methods, drafting articles, and presenting results at meetings. The principal investigator (PI) will develop course materials for an introductory class in the quantum mechanics of spin that will be geared toward a wide swath of students: those who have not taken traditional quantum mechanics or even many upper-level physics courses. The curriculum will be designed to be accessible to chemistry and engineering/computer science students in an effort to mainstream quantum mechanical and quantum computing concepts. Quantum technologies are expected to bring about a quantum revolution. The PI will engage the campus and local community in discussions (through public presentations) of quantum science, the directions quantum technology will lead, and the importance of quantum education for the nation.TECHNICAL SUMMARYThis award supports theoretical investigations of spin-current-to-charge-current and charge-current-to-spin-current conversion by means of gradients in nuclear field. The spin-Hall effect and its inverse effect are presently the chief methods by which spin/charge currents are converted. Spin-Hall conversions require materials with strong spin-orbit interactions, which shorten spin diffusion lengths. The research thrust here is to tap into alternative materials for charge/spin current conversion where strong spin-orbit interactions are not necessary or desirable; instead, strong hyperfine interactions are relied upon. Nuclear field or hyperfine gradients offer a mechanism by which either spin or charge current can be generated. Spin separation via this nuclear gradient channel is reminiscent of the Stern-Gerlach effect, but since the nuclear field is not a true magnetic field in the sense of a Lorentz force being operational, spins are able to separate more effectively than they would in a true magnetic field gradient (the nuclear field - at the level of the Fermi contact potential - acts only on the spin and not on the orbital degree of freedom). The project goal is to develop and solve spin and charge drift-diffusion equations with the gradient effect included. Analytic solutions are possible for some configurations but in general the coupled equations are nonlinear and necessitate numerical methods. Furthermore, two possible experimental realizations of the effect will be modeled: electrical spin injection and optical spin injection. In both situations, a nuclear gradient is feasible through non-uniform dynamic nuclear polarization by injected spin polarized carriers. By modeling and evaluating these scenarios, a guide will be provided to experimentalists who wish to observe the spin and charge separating effects. Ultimately, the spin/charge drift diffusion formalism developed will be able to incorporate other types of field gradients as well, namely, spin-orbit fields such as those due to the Rashba interaction.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.