Superconductivity by hidden spin fluctuations in electron-doped iron selenide

Abstract

Berg, Metlitski, and Sachdev [Science 338, 1606 (2012)] have shown that the exchange of hidden spin fluctuations by conduction electrons with two orbitals can result in high-temperature superconductivity in copper-oxide materials. We introduce a similar model for high-temperature iron-selenide superconductors that are electron-doped. Conduction electrons carry the minimal 3dxz and 3dyz iron-atom orbitals. Low-energy hidden spin fluctuations at the checkerboard wave vector QAF result from nested Fermi surfaces at the center and at the corner of the unfolded (one-iron) Brillouin zone. Magnetic frustration from superexchange interactions via the selenium atoms stabilize hidden spin fluctuations at QAF versus true spin fluctuations. At half-filling, Eliashberg theory based purely on the exchange of hidden spin fluctuations reveals a Lifshitz transition to electron/hole Fermi-surface pockets at the corner of the folded (two-iron) Brillouin zone, but with vanishing spectral weights. The underlying hidden spin-density-wave ground state is therefore a Mott insulator. Upon electron doping, Eliashberg theory finds that the spectral weights of the hole Fermi-surface pockets remain vanishingly small, while the spectral weights of the larger electron Fermi-surface pockets become appreciable. This prediction is therefore consistent with the observation of electron Fermi-surface pockets alone in electron-doped iron selenide by angle-resolved photoemission spectroscopy (ARPES). Eliashberg theory also finds an instability to S+− superconductivity at electron doping, with isotropic Cooper pairs that alternate in sign between the visible electron Fermi-surface pockets and the faint hole Fermi-surface pockets. Comparison with the isotropic energy gaps observed in electron-doped iron selenide by ARPES and by scanning tunneling microscopy is consistent with short-range hidden magnetic order.

Source-reported materials — not catalogue approval

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