FRG: Two-Gap Superconductivity in Magnesium Diboride and its Implications for Applications

超导体本质上是无摩擦的电导体。因此，它们避免了即使在非常高导电性的铜线中也会发生的热效应。高磁场器件，不可能用铜制造，因此成为一种可能。不幸的是，超导性只在相当低的温度下发生。MgB2是一种由廉价原料制成的看似简单的超导体，最近发现它的超导性是其他任何简单超导体的两倍，这让人们非常兴奋。MgB2的电子特性为探索改善材料性能的可能性提供了独特的机会，使其优于任何现有的超导体。该重点研究小组（FRG）奖为威斯康星-麦迪逊大学（UW）、亚利桑那州立大学（ASU）、宾夕法尼亚州立大学（PSU）和波多黎各-马亚圭斯大学（UPRM）的小组之间的机构间合作提供支持。该项目旨在了解如何使MgB2具有应用吸引力。该团队已经证明，在更高的磁场中，MgB2比基于铌的材料保持超导性。目前99%以上的超导磁体是由铌基材料制成的。超导性在许多技术方面都是至关重要的，特别是对磁共振成像（MRI）。通用电气是世界上最大的核磁共振成像仪制造商之一，将参与这项研究。这项工作将通过研究生开展的研究来实施，并通过与UPRM正在开发的材料科学项目的合作，以及在K-12阶段向亚利桑那州的美洲原住民和西班牙裔社区推广来扩大。该项目得到了材料研究部凝聚态物理、陶瓷和MRSEC项目以及多学科活动办公室的支持。***技术摘要***二硼化镁是最近发现的具有40K超导转变温度的六方层状化合物，几乎是任何其他电子-声子超导体的两倍。更有趣的是，MgB2包含两个截然不同的超导间隙，它们彼此之间只有弱耦合。较大的sigma间隙是由平面内sigma硼键形成的，而较小的π间隙是由Mg和B平面之间的π硼键形成的。这个跨机构重点研究小组（FRG）由威斯康星大学麦迪逊分校（UW）、亚利桑那州立大学（ASU）、宾夕法尼亚州立大学（PSU）和波多黎各-马亚圭斯大学（UPRM）的小组组成，将解决MgB2合金的基础物理和材料科学问题，重点关注MgB2技术具有巨大潜力的体状样品和损伤研究。该项目旨在了解MgB2的sigma和pi波段内和之间的散射如何影响上部临界场，以及当MgB2合金化或离子辐照时散射如何变化。散装样品将是西澳大学和upm研究的主要内容；透射电子显微镜将在华盛顿大学进行。PSU的研究人员将专注于合金化过程的建模。离子辐照和连通性效应将是亚利桑那州立大学的研究重点。更广泛的影响包括技术和教育。超导磁体用户社区对最近的证明感到兴奋，合金MgB2的Hc2可以超过基于nb的超导体的Hc2，目前几乎所有超导磁体都是由Hc2制成的。美国工业和国家实验室，以及国际学术合作者，将与FRG合作，探索MgB2在10-30K范围内的低温磁体的全部潜力，以及任何铌基材料无法达到的超高场磁体。通过最近在upm启动的UW-UPRM NSF-PREM的研究外展合作将进一步发展，K-12外展将在亚利桑那州立大学开始到亚利桑那州的西班牙裔和美洲原住民社区。该项目得到了材料研究部的几个项目以及多学科活动办公室的支持。

***NON-TECHNICAL ABSTRACT***Superconductors are essentially frictionless conductors of electricity. Thus, they avoid the heating effects that occur even in very high-conductivity copper wires. High magnetic field devices, impossible to make with copper, therefore become a possibility. Unfortunately superconductivity occurs only at rather low temperatures. There was great excitement when MgB2, an apparently simple superconductor made from inexpensive raw materials, was recently discovered to be superconducting at twice the temperature of any other simple superconductor. The electronic characteristics of MgB2 provide unique opportunities to explore the possibility of improving the material's properties to make them superior to those of any present superconductor. This Focused Research Group (FRG) award provides support for an inter-institutional collaboration between groups at the U. of Wisconsin-Madison (UW), Arizona State U. (ASU), Pennsylvania State U. (PSU), and the U. of Puerto Rico-Mayaguez (UPRM). The project seeks to understand how to make MgB2 attractive for applications. The team has already demonstrated that MgB2 remains superconducting in higher magnetic fields than materials based on Nb. More than 99% of all current superconducting magnets are made from Nb based materials. Superconductivity is vital to many aspects of technology, especially to Magnetic Resonance Imaging (MRI). General Electric, one of the world's largest manufacturers of MRI machines will collaborate on this study. This effort will be implemented by research carried out by graduate research students and amplified by collaboration with the developing materials science program at UPRM and by outreach at the K-12 level to Native American and Hispanic communities in Arizona. The project is supported by the Condensed Matter Physics, Ceramics, and MRSEC programs in the Division of Materials Research, as well as by the Office of Multidisciplinary Activities.***TECHNICAL ABSTRACT***Magnesium diboride is a hexagonal layered compound recently found to have a 40K superconducting transition temperature, almost twice as high as any other electron-phonon superconductor. Even more interesting is that MgB2 contains two distinct superconducting gaps that are only weakly coupled to each other. The larger sigma gap is formed by in-plane sigma boron bonds, whereas the smaller pi gap results from pi boron bonds between the Mg and B planes. This inter-institutional Focused Research Group (FRG), consisting of groups at the U. of Wisconsin-Madison (UW), Arizona State U. (ASU), Pennsylvania State U. (PSU), and the U. of Puerto Rico-Mayaguez (UPRM), will address fundamental physics and materials science issues of MgB2 alloys, concentrating on bulk-form samples and damage studies that have great potential for MgB2 technology. The project seeks to understand how the upper critical field is affected by scattering in and perhaps between the sigma and pi bands of MgB2 and how the scattering changes as MgB2 is alloyed or ion irradiated. Bulk form samples will be the primary thrust of the studies at UW and UPRM; transmission electron microscopy will be performed at UW. Researchers at PSU will concentrate on modeling of the alloying process. Ion irradiation and connectivity effects will be the focus of research at ASU. The broader impacts are both technological and educational. The superconducting magnet user community is excited by recent demonstrations that Hc2 of alloyed MgB2 can exceed Hc2 of the Nb-based superconductors, from which virtually all superconducting magnets are presently made. US industry and national laboratories, as well as international academic collaborators, will work with the FRG to explore the full potential of MgB2 for cryocooled magnets in the 10-30K range, as well as ultra high-field magnets beyond the reach of any Nb-based material. Outreach collaborations in research through a recently started UW-UPRM NSF-PREM at UPRM will be further developed and K-12 outreach will start at ASU to Hispanic and Native American communities in Arizona. The project is supported by several programs in the Division of Materials Research, as well as by the Office of Multidisciplinary Activities.