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

*非技术抽象*超导体本质上是无摩擦的电力导体。因此，它们避免了即使在非常高电导率的铜线中也会出现的加热效应。因此，不可能用铜制造的高磁场设备成为可能。不幸的是，超导只有在相当低的温度下才会发生。当由廉价原材料制成的看似简单的超导体MgB2最近被发现超导体的温度是其他任何简单超导体的两倍时，人们非常兴奋。硼化镁的电子特性提供了独特的机会，可以探索改善材料性能的可能性，使其优于任何现有的超导体。这一重点研究小组(FRG)奖为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)和波多黎各大学马亚圭兹分校(UPRM)的小组之间的机构间合作提供支持。该项目试图了解如何使mgB2对应用程序具有吸引力。该团队已经证明，与基于Nb的材料相比，MgB2在更高的磁场中仍然保持超导。目前超过99%的超导磁体是由Nb基材料制成的。超导对许多方面的技术都是至关重要的，特别是对磁共振成像(MRI)而言。世界上最大的核磁共振机制造商之一通用电气将在这项研究上进行合作。这一努力将通过研究生研究学生进行的研究来实施，并通过与UPRM正在开发的材料科学方案合作以及通过在亚利桑那州向美洲原住民和西班牙裔社区进行K-12级别的外联来扩大。该项目得到了材料研究部的凝聚态物理、陶瓷和MRSEC计划以及多学科活动办公室的支持。*技术摘要*二硼化镁是一种六方层状化合物，最近发现具有40K的超导转变温度，几乎是任何其他电子-声子超导体的两倍。更有趣的是，MgB_2包含两个不同的超导带隙，它们之间只有微弱的耦合。较大的西格玛带隙是由面内的西格玛硼键形成的，而较小的pi带隙是由镁和B面间的pi硼键形成的。这个跨机构的重点研究小组(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)和波多黎各大学马亚圭兹分校(UPRM)的小组组成，将致力于解决镁B2合金的基本物理和材料科学问题，专注于具有巨大潜力的镁B2技术的块状样品和损伤研究。该项目试图了解上临界场是如何受到散射的影响，以及当镁被合金化或离子辐照时，散射是如何改变的。大块样品将是UW和UPRM研究的主要内容；UW将进行透射电子显微镜检查。巴黎州立大学的研究人员将专注于合金化过程的建模。离子辐照和连接效应将是亚利桑那州立大学研究的重点。更广泛的影响既有技术上的，也有教育上的。令超导磁体用户群体感到兴奋的是，最近有证据表明，合金化镁B2的Hc2可以超过Nb超导体的Hc2，目前几乎所有超导磁体都是由Nb基超导体制成的。美国工业界和国家实验室以及国际学术合作伙伴将与FRG合作，探索在10-30K范围内用于低温冷却磁体以及任何Nb基材料无法触及的超高场磁体的全部潜力。通过最近在UPRM启动的UW-UPRM NSF-PREM在研究方面的外联合作将得到进一步发展，K-12将在亚利桑那州亚利桑那州的亚利桑那州开始向拉美裔和美洲原住民社区进行K-12外联。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.