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

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****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.