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 εb—at Γ point and ordinary electron bands at Mpoints in Brillouin zone (BZ).We found that the system allows two degenerate superconducting solutions with the exactly same Tc in clean limit: the incipient she-gap (D-h ¹ 0,D+e ¹ 0) and se+e+-gap (Δh=0,D+e ¹ 0) solutions with different pairing cutoffs,Λsf (spin fluctuation energy) and εb, respectively. The se+e+-gap solution, in which the system dynamically renormalizes the original pairing cutoffΛsf toΛphys=εb (<Λsf), therefore actively eliminates the incipient hole band from forming Cooper pairs, but without loss of Tc, becomes immune to the impurity pair-breaking. As a result, the HEDIS systems, by dynamically tuning the pairing cutoff and therefore selecting the se+e+-pairing state, can always achieve themaximumTc—the Tc of the degenerate she solution in the ideal clean limit—latent in the original pairing interactions, even in the dirty limit.