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

非技术描述：超导性是某些材料在没有电阻的情况下导电的能力，目前只有在非常低的温度下才能实现。如果这能在室温下工作，它将彻底改变许多可以在许多方面造福社会的应用，例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。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电性的微观起源。该项目旨在研究由铁制成的超导材料中的磁涨落。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电性。该项目培训研究生和本科生的实验和计算技能。有了这些技能，这些学生通常会在不同的科技部门找到工作，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。技术说明：所研究的问题是磁性，特别是向列相涨落在铁基超导体中出现超导电性方面的作用。目标是利用砷-75上的零场核磁共振研究磁相、向列相和顺磁相的波动。在零场极限下，核哈密顿量主要由反映磁序的超精细场和反映轨道序度的电场梯度决定。因此，共振频率和弛豫率直接与涨落有关。该项目通过比较测量的磁场梯度和具有固定交错磁化强度的从头算密度泛函计算，建立了特征共振时间内向列相自旋涨落的平均幅度。由于现有方法要么探测瞬时磁矩，要么探测静态磁矩(在物理上无限长的时间内求平均值)，因此到目前为止还不能获取这些信息。了解向列相自旋涨落的频谱对于揭示铁基超导电性仍然难以捉摸的机制是非常有价值的。此外，学生将在两个方面接受尖端研究方面的培训：通过密度泛函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NSF的法定使命，并通过使用基金会的智力优势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.