Study of gap symmetry and gap structures in iron pnictides using Josephson junctions

****技术摘要****本项目将利用电子掺杂和空穴掺杂铁离子超导体之间的异质结进行变符号s波对称性的相敏测试，并利用高质量的隧道结测量这些超导体的多重能隙的值和分布。铁衍生物和硫族化合物是一类具有非常规机理和特殊性能的新型高温超导体，其Tc可达55 K。铁衍生物中分离的电子和空穴费米表面产生了非常规的配对对称性和间隙结构，在几种可能性中，人们普遍认为改变符号的s波配对是正确的。用相敏测量明确地证明这种配对对称性是铁离子研究的“圣杯”。电子掺杂和空穴掺杂的铁化合物之间的约瑟夫森效应被认为是最有前途的研究间隙对称性和间隙结构的技术。本项目将通过将电子掺杂pnictide的外延薄膜沉积在空穴掺杂pnictide的单晶上来制备电子/空穴异质结，并利用它们进行相敏测量。这种对铁衍生物的基础研究将有助于理解高温超导的一般机理，并有助于发现更多具有更高Tc和更好性能的新型超导体。该项目将为一名博士生提供多学科培训，并为本科生提供研究经验。****非技术摘要****含铁超导体，包括含砷的嘌呤和含硒的硫族化合物，在2008年被发现具有高达55开尔文的超导转变温度。它们是一类新的高温超导体，具有不同寻常的机制和不同寻常的性质，其中之一就是在动量空间中特征超导序参量的独特对称性。人们普遍认为，含铁超导体中存在正序参数和负序参数，但符号在动量空间中不随方向变化，但尚未得到明确的实验证明。这个项目试图用电子掺杂和空穴掺杂之间的约瑟夫森结来证明这种对称性；该结将通过在空穴掺杂pnictide单晶上生长电子掺杂pnictide薄膜而形成。了解含铁超导体中超导序参量的对称性有助于理解高温超导的一般机制，进而可能导致发现更多具有更高转变温度和更好性能的新超导体。该项目将为一名研究生提供多学科的博士学位培训，并为天普大学的本科生提供研究经验，天普大学是一所拥有多元化学生群体的城市大学。

****Technical Abstract****This project will use heterojunctions between the electron-doped and hole-doped iron pnictide superconductors to perform the phase-sensitive test of the sign-changing s-wave symmetry and use high quality tunnel junctions to measure the values and distributions of the multiple energy gaps of these superconductors. The iron pnictides and chalcogenides, with Tc up to 55 K, are a new family of high temperature superconductors with unconventional mechanism and unusual properties. The disconnected electron and hole Fermi surfaces in the iron pnictides give rise to unconventional pairing symmetries and gap structures, with the sign-changing s-wave pairing widely believed to be the correct one among several possibilities. Proving this pairing symmetry unambiguously with phase-sensitive measurements is the "Holy Grail" of the iron pnictide research. Josephson effect between the electron- and hole-doped iron pnictides has been suggested as the most promising technique to probe the gap symmetry and gap structures. This project will fabricate the electron/hole heterojunctions by depositing epitaxial thin films of the electron-doped pnictide on the single crystals of the hole-doped pnictide and use them to perform the phase-sensitive measurement. This basic research on iron pnictides could lead to the understanding of the mechanism of high-temperature superconductivity in general and lead to the discovery of more new superconductors with even higher Tc and better properties. This project will provide multidisciplinary training for a PhD student and provide research experiences for undergraduate students. ****Non-Technical Abstract****The iron-containing superconductors, including the pnictides that contain arsenic and the chalcogenides that contain selenium, were discovered in 2008 to have superconducting transition temperatures up to 55 degrees Kelvin. They are a new family of high temperature superconductors with unconventional mechanism and unusual properties, one of which is the unique symmetry of the characteristic superconducting order parameter in the momentum space. It is widely believed, but not yet unambiguously proven experimentally, that both positive and negative order parameters exist in the iron-containing superconductors, but the signs do not change with respect to the directions in the momentum space. This project attempts to prove this symmetry using Josephson junctions between an electron-doped and a hole-doped pnictide; the junction will be formed by growing an electron-doped pnictide thin film on a hole-doped pnictide single crystal. Understanding the symmetry of the superconducting order parameter in the iron-containing superconductors can help to understand the mechanism of high-temperature superconductivity in general, which in turn could lead to the discovery of more new superconductors with even higher transition temperatures and better properties. This project will provide multidisciplinary training for one graduate student for his/her PhD degree, and provide research experiences for undergraduate students at Temple University, an urban university with a diverse student population.