Experimental and theoretical studies of iron pnictides through zero field nuclear magnetic resonance

非技术描述：超导性是某些材料无电阻导电的能力，目前只能在非常低的温度下实现。如果这可以在室温下工作，它将彻底改变许多应用，可以在许多方面造福社会，如电能传输和量子计算。对医学成像、无噪音推进发动机等至关重要的大磁场的产生也依赖于超导性。为了取得进展以获得这些好处，重要的是要正确理解非常规超导的微观起源。这个项目旨在研究由铁制成的超导材料中的磁波动。这是因为这些波动被认为提供了必要的“胶水”，将电子结合成对，使铁基材料具有超导性。该项目训练研究生和本科生的实验和计算技能。有了这些技能，这些学生通常会在各个科学和技术部门找到工作，并为急需的量子劳动力做出贡献。此外，该项目还为代表性不足的K-12女孩提供外展活动，以吸引她们从事STEM职业。技术描述：研究的问题是磁性的作用，特别是向列波动，在铁基超导体中超导性的出现。目的是利用零场核磁共振对砷-75研究磁相、向列相和顺磁相的波动。在零场极限下，核哈密顿量由反映磁序的超精细场和反映轨道序的电场梯度主导。因此，共振频率和弛豫速率与波动直接相关。该项目通过将测量的场梯度与从头算密度泛函计算的固定交错磁化强度进行比较，建立了在特征共振时间内平均的向列自旋波动幅度。到目前为止，这些信息还无法获得，因为现有的方法要么探测瞬时磁矩，要么探测静态磁矩（物理上无限时间的平均值）。了解向列自旋涨落的频谱对于揭示铁基超导性的难以捉摸的机制是非常宝贵的。此外，学生将在两个前沿研究方面进行培训：通过密度函数理论执行的从头计算和超导单晶的零场核磁共振。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 Description: Superconductivity is the ability of certain materials to conduct electricity without resistance, which currently is only possible at very low temperatures. If this can be made to work at room temperature, it would revolutionize many applications that can benefit society in many ways, such as electrical energy transmission and quantum computation. The creation of large magnetic fields which is vital for medical imaging, noiseless propulsion engines etc., also rely on superconductivity. In order to make progress to reap these benefits, it is important to gain proper understanding of the microscopic origin of unconventional superconductivity. This project aims to investigate magnetic fluctuations in superconducting materials made with iron. This is because these fluctuations are believed to pro-vide the necessary “glue” that combines electrons into pairs to enable superconductivity in iron-based materials. The project trains graduate and undergraduate students in both experimental and computational skills. With such skills, these students typically find employment in various science and technology sectors and also contribute to the much-needed quantum workforce. Furthermore, the project provides outreach activities for underrepresented K-12 girls to attract them to STEM careers.Technical Description: The problem studied is the role of magnetic, and especially nematic fluctuations, in the emergence of superconductivity in iron-based superconductors. The goal is to study the fluctuations across the magnetic, nematic and paramagnetic phases using zero-field nuclear magnetic resonance on arsenic-75. In the limit of zero-field, the nuclear Hamiltonian is dominated by the hyperfine field, reflecting magnetic order, and the electric field gradient, reflecting orbital order. Therefore, the resonance frequencies and relaxation rates are directly tied to the fluctuations. The project establishes the amplitude of the nematic spin-fluctuations averaged over the characteristic resonance time by comparing the measured field gradients with ab initio density functional calculations with fixed staggered magnetization. This information has not been accessible so far, as existing methods probe either instantaneous or static (averaged over physically infinite time) magnetic moments. Understanding of the frequency spectrum of nematic spin fluctuations is invaluable for uncovering the yet elusive mechanism of Fe-based superconductivity. Further, students will be trained in cutting-edge research on two fronts: ab-initio calculations executed through density function theory and zero-field nuclear magnetic resonance of superconducting single crystals.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.