Pairing mechanism of heavily electron doped FeSe systems: dynamical tuning of the pairing cutoff energy

Abstract

We studied the pairing mechanism of the heavily electron doped FeSe (HEDIS) systems, which commonly have one incipient hole band—a band top below the Fermi level by a finite energy distance ε _b —at Γ point and ordinary electron bands at M points in Brillouin zone (BZ). We found that the system allows two degenerate superconducting solutions with the exactly same T _c in clean limit: the incipient ${s}_{{he}}^{\pm }$ -gap ( ${{\rm{\Delta }}}_{h}^{-}\ne 0$ , ${{\rm{\Delta }}}_{e}^{+}\ne 0$ ) and ${s}_{{ee}}^{++}$ -gap (Δ _h = 0, ${{\rm{\Delta }}}_{e}^{+}\ne 0$ ) solutions with different pairing cutoffs, Λ _sf (spin fluctuation energy) and ε _b , respectively. The ${s}_{{ee}}^{++}$ -gap solution, in which the system dynamically renormalizes the original pairing cutoff Λ _sf to Λ _phys = ε _b (<Λ _sf ), therefore actively eliminates the incipient hole band from forming Cooper pairs, but without loss of T _c , becomes immune to the impurity pair-breaking. As a result, the HEDIS systems, by dynamically tuning the pairing cutoff and therefore selecting the ${s}_{{ee}}^{++}$ -pairing state, can always achieve the maximum T _c —the T _c of the degenerate ${s}_{{he}}^{\pm }$ solution in the ideal clean limit—latent in the original pairing interactions, even in the dirty limit.

Keywords

• Iron-based superconductors
• heavily electron doped fese systems
• high-tc superconductivity
• renormalization of cutoff energy
• 74.20.-z
• 74.20.Rp