  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 3d xz and 3d yz
iron-atom orbitals. Low-energy hidden spin fluctuations at the checkerboard
wavevector Q_AF result from nested Fermi surfaces at the center and at the
corner of the unfolded (one-iron) Brillouin zone. Magnetic frustration from
super-exchange interactions via the selenium atoms stabilize hidden spin
fluctuations at Q_AF 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 groundstate 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 (STM) is consistent with short-range hidden magnetic order.
