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磷灰石超导体之间的异质结来进行变符号S波对称性的相敏测试，并使用高质量的隧道结来测量这些超导体的多重能隙的值和分布。铁镍矿和硫系化合物是一类新的高温超导体，具有非常规的机制和特殊的性质，其T_c可达55K。铁粒子中不连通的电子和空穴费米面产生了非传统的配对对称性和能隙结构，在几种可能性中，变号S波配对被广泛认为是正确的。用相敏测量明确地证明这种配对对称性是铁小球研究的“圣杯”。电子和空穴掺杂铁离子之间的约瑟夫森效应被认为是探测带隙对称性和带隙结构的最有前途的技术。本项目将通过在空穴掺杂的磷灰石单晶上沉积电子掺杂磷灰石的外延薄膜来制备电子/空穴异质结，并用它们进行相敏测量。对铁硫化物的基础研究有助于从总体上理解高温超导的机制，并有助于发现更多具有更高T_c和更好性能的新型超导体。该项目将为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，包括含砷的硫化物和含硒的硫系化合物，于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，具有非常规的机制和特殊的性质，其中之一是特征超导序参量在动量空间中的独特对称性。人们普遍认为，含铁超导体中既存在正序参数，也存在负序参数，但符号不随动量空间中的方向变化，这一点尚未得到明确的实验证明。这个项目试图用约瑟夫森结来证明这种对称性，这种结是通过在空穴掺杂的锂辉石单晶上生长掺电子的锂辉石薄膜来实现的。了解含铁超导体中超导序参数的对称性有助于从总体上理解高温超导的机制，从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博士学位的多学科培训，并为坦普尔大学的本科生提供研究经验，坦普尔大学是一所学生人口多样化的城市大学。

****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.