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In Cu-based HTS materials, the undoped phase is a robust Mott insulator while, in Fe-based HTS materials, the undoped phase is generally not an insulator at all. Thus, proximity to a Mott insulator appears neither indispensable nor universal to HTS. However, theory predicts that Fe-based materials could still be governed by strong electronic correlations proximate to a Mott insulator if an orbital selective Mott phase (OSMP) exists. A key signature of OSMP would be orbital selective Cooper pairing wherein electrons of a specific orbital character predominantly form the Cooper pairs.
We used Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry and the corresponding superconducting energy gaps of the famous iron-based superconductor FeSe. We show that both gaps are extremely anisotropic but nodeless, and they exhibit gap maxima oriented orthogonally in momentum space. By introducing a novel technique, we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing that, in FeSe, is based preferentially on electrons from the dyz orbitals of the iron atoms.
These results were published in Science 357, 75 (7 Jul 2017).