Unconventional metals in carrier-tuned spin-orbit Mott materials

摘要：本项目探索一类由绝缘体产生的金属特有的迷人和复杂的电子行为，这些金属既具有电子之间的强相互作用，也具有电子轨道运动与其固有磁矩或自旋之间的强耦合。这些“自旋轨道辅助莫特绝缘体”可以通过添加载流子变成金属，其中许多激发态，如高温超导和量子信息的电子态被预测。在该项目中，这些金属的超高纯度晶体得以生长，并进行了探索其不寻常金属状态的结构、磁性和电子特性的实验。特别是，研究活动针对一系列掺杂氧化铱材料，其母体绝缘状态最近被证明会坍塌成具有异常电荷和自旋性质的金属。这建立了评估这些材料在未来超导和量子信息应用中的可行性所需的知识库。该项目对研究生进行先进晶体生长技术和国家用户设施先进散射技术的培训，以满足国家对这两个领域训练有素人员的需求。此外，该项目还实施了多层次的教育和推广工作，旨在促进STEM学科特别是材料科学领域的多元化参与。这是通过本科生的研究机会来实现的，旨在吸引和留住材料科学领域代表性不足的人口。技术摘要：本项目实验探索了两类具有典型自旋轨道莫特态的铱氧化物——Srn+1IrnO3n+1的Ruddlesden-Popper系列和焦绿石Al2Ir2O7的稀土系列。探索这些化合物的关键变体，目的是了解Ruddlesden-Popper铱酸盐中非常规自旋和电荷密度波形成的起源，它们与这些系统中奇异金属行为和潜在超导性的关系，以及火绿石铱酸盐的磁性和金属-绝缘体跃迁之间的耦合。所有这些现象对于识别预测在这些材料中出现的奇异电子态具有相当大的兴趣，例如从高温超导到磁性Weyl半金属态。实验方法包括同步加速器x射线和中子散射测量，体电子特性（如电荷输运和磁化）测量，以及通过合作者网络进行的互补电子探针。研究中一个关键的使能工具是使用高压浮区晶体生长技术来创建用于研究的超高纯度样品。通过利用这一工具，该项目为发现载流子掺杂自旋轨道辅助Mott材料中预测的和其他不可预见的非常规电子态创造了新的途径。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。

Nontechnical Abstract: This project explores the fascinating and complex electronic behaviors endemic to a class of metals born from insulators with both strong interactions between electrons and strong coupling between the electron's orbital motion and its inherent magnetic moment or spin. These 'spin-orbit assisted Mott insulators' can be turned into metals by adding charge carriers, where many exciting states such as high temperature superconductivity and electronic states for quantum information are predicted. Ultrahigh purity crystals of these metals are grown in the project and experiments exploring the structural, magnetic, and electronic properties of their unusual metallic states performed. In particular, research activities target a series of doped iridium oxide materials whose parent insulating states have recently been shown to collapse into metals with anomalous charge and spin properties. This builds the knowledgebase needed for assessing the viability of these materials for potential future superconducting and quantum information applications. The project trains graduate students in advanced crystal growth techniques as well as in advanced scattering techniques at national user facilities, addressing the national need for trained personnel in both arenas. Additionally, the project executes a multitier education and outreach effort targeted at promoting diverse participation in STEM disciplines and within materials science in particular. This is achieved through undergraduate research opportunities targeted at attracting and retaining underrepresented demographics within materials science.Technical Abstract: This project experimentally explores two classes of iridium oxides where canonical spin-orbit Mott states appear--the Ruddlesden-Popper series of Srn+1IrnO3n+1 and the rare-earth series of pyrochlore Al2Ir2O7. Key variants of these compounds are explored with the goals of understanding the origins of unconventional spin and charge density wave formation in the Ruddlesden-Popper iridates, their relation to strange-metal behavior and potential superconductivity in these systems, and the coupling between magnetism and the metal-insulator transitions of the pyrochlore iridates. All of these phenomena are of considerable interest for identifying exotic electronic states predicted to manifest in these materials, with examples ranging from high-temperature superconductivity to magnetic Weyl semimetal states. Experimental approaches include synchrotron x-ray and neutron scattering measurements, bulk electronic properties (such as charge transport and magnetization) measurements, and complementary electronic probes performed via a network of collaborators. A key enabling tool in the research is the use high-pressure floating zone crystal growth techniques to create ultrahigh purity specimens for study. By leveraging this tool, the project creates new pathways for discovering predicted and other unforeseen unconventional electronic states in carrier-doped spin-orbit assisted Mott materials.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.