Discovery of Orbital Selective Cooper Pairing in FeSe Article Swipe
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· 2017
· Open Access
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· DOI: https://doi.org/10.7298/x4tb153z
· OA: W3098182177
FeSe is the focus of intense research interest because of its unusual non-magnetic nematic state and because it forms the basis for achieving the highest critical temperatures of any iron-based superconductor. However, its Cooper pairing mechanism has not been determined because an accurate knowledge of the momentum-space structure of superconducting energy gaps $\\Delta_i(\\vec{k})$ on the different electron-bands $E_i(\\vec{k})$ does not exist. Here we use Bogoliubov quasiparticle interference (BQPI) imaging to determine the coherent Fermi surface geometry of the $\\alpha$- and $\\varepsilon$-bands surrounding the $\\Gamma = (0, 0)$ and $X = (\\pi / a_{Fe}, 0)$ points of FeSe, and to measure their superconducting energy gaps $\\Delta_{\\alpha}(\\vec{k})$ and $\\Delta_{\\varepsilon}(\\vec{k})$. We show directly that both gaps are extremely anisotropic but nodeless, and are aligned along orthogonal crystal axes. Moreover, by implementing a novel technique we demonstrate the sign change between $\\Delta_{\\alpha}(\\vec{k})$ and $\\Delta_{\\varepsilon}(\\vec{k})$. This complex configuration of $\\Delta_{\\alpha}(\\vec{k})$ and $\\Delta_{\\varepsilon}(\\vec{k})$, which was unanticipated within pairing theories for FeSe, reveals a unique form of superconductivity based on orbital selective Cooper pairing of electrons from the $d_{yz}$ orbitals of iron atoms. This new paradigm of orbital selectivity may be pivotal to understanding the microscopic interplay of quantum paramagnetism, nematicity and high temperature superconductivity.
