May 17, 2011

Orbital-Independent Superconducting Gaps in Iron Pnictides

Journal Science - Orbital-Independent Superconducting Gaps in Iron Pnictides

A potentially path to room temperature superconductors comes from a theoretical view of how electron orbital pairing in a class of materials known as ferropnictides which provide a new road to high transition temperature superconductivity. A high transition temperature superconductivity of up to 55K (-218C) in ferropnictides was first observed in 2008. However, this behavior is not predicted from standard electron pairing based on lattice vibrations. Therefore, an alternate explanation was needed. If other materials are discovered to have several entangled orbitals near Fermi level, they may have the potential to show even higher transition temperature superconductivity due to orbital pairing. “For example,” he adds, “if the transition temperature approaches room temperature, superconducting wires for lossless electronic transportation and storage will quickly be deployed worldwide.”

The origin of superconductivity in the iron pnictides has been attributed to antiferromagnetic spin ordering that occurs in close combination with a structural transition, but there are also proposals that link superconductivity to orbital ordering. We used bulk-sensitive laser angle–resolved photoemission spectroscopy on BaFe2(As0.65P0.35)2 and Ba0.6K0.4Fe2As2 to elucidate the role of orbital degrees of freedom on the electron-pairing mechanism. In strong contrast to previous studies, an orbital-independent superconducting gap magnitude was found for the hole Fermi surfaces. Our result is not expected from the superconductivity associated with spin fluctuations and nesting, but it could be better explained invoking magnetism-induced interorbital pairing, orbital fluctuations, or a combination of orbital and spin fluctuations. Regardless of the interpretation, our results impose severe constraints on theories of iron pnictides.

16 pages of supplemental material

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