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dc.contributor.authorSprau, Peter Oliver
dc.identifier.otherbibid: 10361432
dc.description.abstractFeSe 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.
dc.subjectCondensed matter physics
dc.subjectorbital selectivity
dc.subjectScanning Tunneling Microscopy
dc.subjectstrong correlations
dc.titleDiscovery of Orbital Selective Cooper Pairing in FeSe
dc.typedissertation or thesis University of Philosophy D., Physics
dc.contributor.chairDavis, James C.
dc.contributor.committeeMemberMcEuen, Paul L.
dc.contributor.committeeMemberKim, Eun-Ah

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