International Research Fellowship Program: Microscopic Role of Magnetism in High Temperature Superconductivity

该奖项由2009年美国复苏和再投资法案(公法111-5)资助。国际研究奖学金计划使美国科学家和工程师能够在国外进行9至24个月的研究。该计划的奖项提供了联合研究的机会，并利用国外独特或互补的设施、专业知识和实验条件。该奖项将支持Jerald J.Kavich博士与西班牙Catala de NanoTechnologia研究所的Pietro Gambardella博士合作的为期24个月的研究奖学金。该项目研究高温超导材料中替代磁性杂质的局部电子和磁性结构。在非常规超导中，磁性扮演着一个不平凡但难以捉摸的角色。以前的杂质研究集中在超导体的宏观性质上，留下了一个未知的领域，使得它们的电子结构和对称性在理论模型中被过度简化。为了解决这些问题，重点关注与杂质的真实描述有关的三个关键方面，包括确定电子基态构型、轨道占有率的空间相关性以及高温超导材料表现出的所有主要相(即超导相、伪能隙相和正常态)的磁序各向异性。对这些先进材料的研究正在使用可变偏振软X射线光谱学完成，这需要最先进的超亮同步加速器设备。正在用圆偏振和线偏振X射线进行与温度和场有关的研究，以绘制局部杂质矩的大小，并确定轨道占有率和磁性的各向异性。结合实验方法，正在利用多组态多重技术进行先进的计算，以获得对实验数据的定量分析，从而允许高温超导体的微观和宏观性质与杂质之间的关联。对Bi2Sr2Ca1Cu2O8等材料中的Mn、Fe、Co和Ni替代杂质的光谱研究有望提供对它们的价态和相关局域矩的基本认识，解决长期以来关于杂质态的杂化程度以及它们的矩与环境的筛选或耦合的争论。此外，首次将在掺杂超导体的超导转变温度范围内分别监测自旋和轨道杂质磁化的行为，从而阐明电子配对过程中的磁效应。最后，通过测量样品在不同磁场中的磁化强度，我们计划确定杂质选择磁化强度曲线。这些通常被标准磁学实验中测量到的大反磁信号掩盖在超导体的转变温度以下。这一提议不仅有望推动超导体物理学的发展，而且还将增加对强关联环境中孤立磁原子的基础知识。超导材料的发展和纳入新的和现有的技术预计将对全球环境、工业和经济因素产生深远影响。好处是广泛的，从提高电能无损运输和存储的可靠性，到利用超导量子比特进行量子计算的愿望。这项建议中的基础研究与美国长期科学研究目标的核心方面是一致的。

0853415KavichThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad.This award will support a twenty-four-month research fellowship by Dr. Jerald J. Kavich to work with Dr. Pietro Gambardella at the Institut Catala de Nanotechnologia in Spain.This project examines the local electronic and magnetic structure of substitutional magnetic impurities in high-temperature superconducting materials. Magnetism plays a non-trivial, but elusive role in unconventional superconductivity. Previous impurity studies have concentrated on the macroscopic properties of the superconductors, leaving the impurities themselves an unexplored area and permitting their electronic structure and symmetries to be oversimplified in theoretical models. To address these issues, focus is being directed toward three key aspects related to a realistic description of the impurity, including determination of the electronic ground state configuration, spatial dependence of orbital occupancies, and anisotropy of the magnetic order in all major phases exhibited by high-temperature superconducting materials, i.e. the superconducting phase, the pseudo-gap phase, and the normal state. The investigation of these advanced materials is being accomplished using variable polarization soft x-ray spectroscopy, requiring state-of-the-art, ultra-bright synchrotron facilities. Temperature and field dependent studies are being conducted with both circularly and linearly polarized x-rays to map the magnitude of the local impurity moment and to determine anisotropies in the orbital occupancies and magnetism. In combination with experimental methods, advanced calculations using multi-configurational multiplet techniques, are being carried out to obtain a quantitative analysis of experimental data allowing for correlations between micro- and macroscopic properties of high-temperature superconductors and the impurity. The spectroscopic study of Mn, Fe, Co, and Ni substitutional impurities in materials such as Bi2Sr2Ca1Cu2O8 is expected to provide fundamental insight into their valence state and related local moment, settling a long standing debate about the degree of hybridization of the impurity states, and screening or coupling of their moment with the environment. Also, for the first time, the behavior of the spin and orbital impurity magnetization will be separately monitored across the superconducting transition temperature of a doped superconductor, illuminating magnetic effects on electron paring processes. Finally, by measuring the sample in different fields, we plan to determine the impurity-selective magnetization curves. These are normally masked below the transition temperature in superconductors by the large diamagnetic signal measured in standard magnetometry experiments. This proposal promises not only to advance superconductor physics, but also to increase the fundamental knowledge of isolated magnetic atoms in a strongly correlated environment. The development and incorporation of superconducting materials into new and existing technologies is expected to have a profound impact on global environmental, industrial, and economic factors. The benefits are extensive, ranging from the improved reliability of the lossless transportation and storage of electrical energy, to aspirations of quantum computing with superconducting qubits. The fundamental research in this proposal is consistent with core aspects of long-term scientific research goals in the U.S.