Abstract
Observations of robust superconductivity in some of the iron based superconductors in the vicinity of a Lifshitz point where a spin density wave instability is suppressed as the hole band drops below the Fermi energy raise questions for spin-fluctuation theories. Here we discuss spin-fluctuation pairing for a bilayer Hubbard model, which goes through such a Lifshitz transition. We find s± pairing with a transition temperature that peaks beyond the Lifshitz point and a gap function that has essentially the same magnitude but opposite sign on the incipient hole band as it does on the electron band that has a Fermi surface.
Highlights
Observations of robust superconductivity in some of the iron based superconductors in the vicinity of a Lifshitz point where a spin density wave instability is suppressed as the hole band drops below the Fermi energy raise questions for spin-fluctuation theories
As the hole band drops below the Fermi energy, a gap opens in the low energy qz =π spin fluctuation spectrum and spectral weight is transfered to higher energies as shown in Fig. 5, which leads to stronger pairing[20]
We have studied a two-layer Hubbard model with parameters chosen so that a spin density wave (SDW) instability is suppressed by a Lifshitz transition in which the hole band at the Γpoint drops below the Fermi energy
Summary
Observations of robust superconductivity in some of the iron based superconductors in the vicinity of a Lifshitz point where a spin density wave instability is suppressed as the hole band drops below the Fermi energy raise questions for spin-fluctuation theories. It appears that superconductivity is induced in the FeSe mono-layer when the SDW order is suppressed by a Lifshitz transition arising from electron doping or strain[11].
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