Superconductivity vs bound state formation in a two-band superconductor with small Fermi energy -- applications to Fe-pnictides/chalcogenides and doped SrTiO3

TL;DR

This paper investigates the relationship between superconductivity and bound state formation in two-band 2D fermion systems with small Fermi energy, revealing different behaviors depending on the Fermi energy relative to bound state energy, with applications to Fe-based superconductors and doped SrTiO3.

Contribution

The study extends the analysis of BCS-BEC crossover to two-band models with small Fermi energy, deriving key temperature scales and applying results to real materials.

Findings

At large Fermi energy, behavior is BCS-like with Tc close to Tins.

At small Fermi energy, the models show distinct behaviors: one with Tins ~ E0/log(E0/EF), the other with Tins ~ Tc.

Preformed pair behavior exists between Tins and Tc in both models.

Abstract

We analyze the interplay between superconductivity and the formation of bound pairs of fermions (BCS-BEC crossover) in 2D models of interacting fermions with small Fermi energy EF and weak attractive interaction, which extends to energies well above EF. The 2D case is special because one has to distinguish between bound state formation and superconductivity already at weak coupling. We briefly review the situation in the one-band model and then consider two different two-band models: the one with a hole and an electron band, and the one with two electron bands. In each case we obtain the bound state energy 2 E0 for two fermions in a vacuum and solve the set of coupled equations for the pairing gaps and the chemical potentials to obtain the onset temperature of the pairing, Tins and the quasiparticle dispersion. We then compute the superfluid stiffness and obtain the actual Tc. We show that, at EF >> E0 the behavior of both two-band models is BCS-like in the sense that Tc almost coincides with Tins and both are much smaller than EF. At EF < E0, the two models behave differently: in the model with two electron bands, Tins ~E0/log(E0/EF) and Tc ~EF << Tins. In between Tins and Tc the system displays preformed pair behavior. In the model with a hole and an electron band, Tins ~ Tc and both remain finite at EF=0 The preformed pair behavior still exists in this model because Tc is numerically smaller than Tins. For both models we re-express Tins in terms of the fully renormalized two-particle scattering amplitude by extending to two-band case the method pioneered by Gorkov and Melik-Barkhudarov back in 1961. We apply our results to to Nb-doped SrTiO3 and to Fe-pnictides and Fe-chalcogenides, particularly to FeSe, in which superconducting gap is comparable to the Fermi energy.